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Title: Electricity
Author: McCormick, W. H.
Language: English
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[Illustration: (cover)]
“ROMANCE OF REALITY” SERIES
Edited by ELLISON HAWKS
ELECTRICITY
_VOLUMES ALREADY ISSUED_
1. THE AEROPLANE. By GRAHAME WHITE and HARRY HARPER.
2. THE MAN-OF-WAR. By Commander E. H. CURREY, R.N.
3. MODERN INVENTIONS. By V. E. JOHNSON, M.A.
4. ELECTRICITY. By W. H. MCCORMICK.
5. ENGINEERING. By GORDON D. KNOX.
[Illustration: THE MARCONI TRANSATLANTIC WIRELESS STATION AT GLACE BAY,
NOVA SCOTIA
Drawing by Irene Sutcliffe]
_“ROMANCE OF REALITY” SERIES_
ELECTRICITY
BY
W. H. McCORMICK
[Illustration]
NEW YORK
FREDERICK A. STOKES COMPANY
PUBLISHERS
_Printed in Great Britain_
PREFACE
I gladly take this opportunity of acknowledging the generous assistance
I have received in the preparation of this book.
I am indebted to the following firms for much useful information
regarding their various specialities:--
Chloride Electrical Storage Co. Ltd.; General Electric Co. Ltd.; Union
Electric Co. Ltd.; Automatic Electric Co., Chicago; Westinghouse
Cooper-Hewitt Co. Ltd.; Creed, Bille & Co. Ltd.; India Rubber, Gutta
Percha, and Telegraph Works Co. Ltd.; W. Canning & Co.; C. H. F.
Muller; Ozonair Ltd.; Universal Electric Supply Co., Manchester; and
the Agricultural Electric Discharge Co. Ltd.
For illustrations my thanks are due to:--
Marconi’s Wireless Telegraph Co. Ltd.; Chloride Electrical Storage Co.
Ltd.; Harry W. Cox & Co. Ltd.; C. H. F. Muller; W. Canning & Co.; Union
Electric Co. Ltd.; Creed, Bille & Co. Ltd.; Ozonair Ltd.; Kodak Ltd.;
C. A. Parsons & Co.; Lancashire Dynamo and Motor Co. Ltd.; Dick, Kerr &
Co. Ltd.; Siemens Brothers Dynamo Works Ltd.; Vickers Ltd.; and Craven
Brothers Ltd.
Mr. Edward Maude and Mr. J. A. Robson have most kindly prepared for me
a number of the diagrams, and I am indebted to Dr. Myer Coplans for
particulars and a diagram of the heat-compensated salinometer.
I acknowledge also many important suggestions from Miss E. C. Dudgeon
on Electro-Culture, and from Mr. R. Baxter and Mr. G. Clark on
Telegraphy and Telephony.
Amongst the many books I have consulted I am indebted specially to
_Electricity in Modern Medicine_, by Alfred C. Norman, M.D.; _Growing
Crops and Plants by Electricity_, by Miss E. C. Dudgeon; and _Wireless
Telegraphy_ (Cambridge Manuals), by Prof. C. L. Fortescue. I have
derived great assistance also from the _Wireless World_.
Finally, I have to thank Mr. Albert Innes, A.I.E.E., of Leeds, for a
number of most valuable suggestions, and for his kindness in reading
through the proofs.
W. H. McC.
LEEDS, 1915
CONTENTS
CHAPTER PAGE
I. THE BIRTH OF THE SCIENCE OF ELECTRICITY 1
II. ELECTRICAL MACHINES AND THE LEYDEN JAR 9
III. ELECTRICITY IN THE ATMOSPHERE 18
IV. THE ELECTRIC CURRENT 27
V. THE ACCUMULATOR 38
VI. MAGNETS AND MAGNETISM 44
VII. THE PRODUCTION OF MAGNETISM BY ELECTRICITY 56
VIII. THE INDUCTION COIL 61
IX. THE DYNAMO AND THE ELECTRIC MOTOR 66
X. ELECTRIC POWER STATIONS 75
XI. ELECTRICITY IN LOCOMOTION 83
XII. ELECTRIC LIGHTING 93
XIII. ELECTRIC HEATING 109
XIV. ELECTRIC BELLS AND ALARMS 116
XV. ELECTRIC CLOCKS 124
XVI. THE TELEGRAPH 128
XVII. SUBMARINE TELEGRAPHY 144
XVIII. THE TELEPHONE 154
XIX. SOME TELEGRAPHIC AND TELEPHONIC INVENTIONS 171
XX. WIRELESS TELEGRAPHY AND TELEPHONY--PRINCIPLES AND
APPARATUS 179
XXI. WIRELESS TELEGRAPHY--PRACTICAL APPLICATIONS 203
XXII. ELECTROPLATING AND ELECTROTYPING 213
XXIII. INDUSTRIAL ELECTROLYSIS 224
XXIV. THE RÖNTGEN RAYS 228
XXV. ELECTRICITY IN MEDICINE 241
XXVI. OZONE 247
XXVII. ELECTRIC IGNITION 253
XXVIII. ELECTRO-CULTURE 258
XXIX. SOME RECENT APPLICATIONS OF ELECTRICITY--AN ELECTRIC
PIPE LOCATOR, ETC. 266
XXX. ELECTRICITY IN WAR 274
XXXI. WHAT IS ELECTRICITY? 287
INDEX 295
LIST OF PLATES
PLATE IN COLOUR: THE MARCONI TRANSATLANTIC WIRELESS STATION AT
GLACE BAY, NOVA SCOTIA _Frontispiece_
FACING PAGE
HYDRO-ELECTRIC POWER STATION 30
(_a_) EXPERIMENT TO SHOW MAGNETIC INDUCTION 48
(_b_) EXPERIMENT TO SHOW THE PRODUCTION OF MAGNETISM BY AN
ELECTRIC CURRENT 48
(_a_) LINES OF MAGNETIC FORCE OF TWO OPPOSITE POLES 50
(_b_) LINES OF MAGNETIC FORCE OF TWO SIMILAR POLES 50
A TYPICAL DYNAMO AND ITS PARTS 70
LOTS ROAD ELECTRIC POWER STATION, CHELSEA 76
POWER STATION BATTERY OF ACCUMULATORS 80
ELECTRIC COLLIERY RAILWAY 86
TYPICAL ELECTRIC LOCOMOTIVES 90
NIGHT PHOTOGRAPHS, TAKEN BY THE LIGHT OF THE ARC LAMPS 96
WHERE ELECTRICAL MACHINERY IS MADE 120
SPECIMEN OF THE WORK OF THE CREED HIGH-SPEED PRINTING TELEGRAPH 140
LARGE ELECTRIC TRAVELLING CRANE AT A RAILWAY WORKS 164
(_a_) MARCONI OPERATOR RECEIVING A MESSAGE 188
(_b_) MARCONI MAGNETIC DETECTOR 188
RÖNTGEN RAY PHOTOGRAPH OF BRITISH AND FOREIGN FOUNTAIN PENS 240
BACHELET “FLYING TRAIN” AND ITS INVENTOR 272
(_a_) CAVALRY PORTABLE WIRELESS CART SET 280
(_b_) AEROPLANE FITTED WITH WIRELESS TELEGRAPHY 280
ELECTRICITY
CHAPTER I
THE BIRTH OF THE SCIENCE OF ELECTRICITY
Although the science of electricity is of comparatively recent date,
electricity itself has existed from the beginning of the world. There
can be no doubt that man’s introduction to electricity was brought
about through the medium of the thunderstorm, and from very early
times come down to us records of the terror inspired by thunder and
lightning, and of the ways in which the ancients tried to account for
the phenomena. Even to-day, although we know what lightning is and how
it is produced, a severe thunderstorm fills us with a certain amount of
awe, if not fear; and we can understand what a terrifying experience it
must have been to the ancients, who had none of our knowledge.
These early people had simple minds, and from our point of view they
had little intelligence; but they possessed a great deal of curiosity.
They were just as anxious to explain things as we are, and so they
were not content until they had invented an explanation of lightning
and thunder. Their favourite way of accounting for anything they
did not understand was to make up a sort of romance about it. They
believed that the heavens were inhabited by various gods, who showed
their pleasure or anger by signs, and so they naturally concluded
that thunder was the voice of angry gods, and lightning the weapon
with which they struck down those who had displeased them. Prayers and
sacrifices were therefore offered to the gods, in the hope of appeasing
their wrath.
Greek and Roman mythology contains many references to thunder and
lightning. For instance, we read about the great god Zeus, who wielded
thunder-bolts which had been forged in underground furnaces by the
giant Cyclops. There was no doubt that the thunder-bolts were made
in this way, because one only had to visit a volcano in order to see
the smoke from the furnace, and hear the rumbling echo of the far-off
hammering. Then we are told the tragic story of Phaeton, son of the
Sun-god. This youth, like many others since his time, was daring and
venturesome, and imagined that he could do things quite as well as
his father. On one occasion he tried to drive his father’s chariot,
and, as might have been expected, it got beyond his control, and came
dangerously near the Earth. The land was scorched, the oceans were
dried up, and the whole Earth was threatened with utter destruction.
In order to prevent such a frightful catastrophe, Jupiter, the mighty
lord of the heavens, hurled a thunder-bolt at Phaeton, and struck him
from the chariot into the river Po. A whole book could be written about
these ancient legends concerning the thunderstorm, but, interesting as
they are, they have no scientific value, and many centuries were to
elapse before the real nature of lightning was understood.
In order to trace the first glimmerings of electrical knowledge we
must leave the thunderstorm and pass on to more trivial matters. On
certain sea-coasts the ancients found a transparent yellow substance
capable of taking a high polish, and much to be desired as an ornament;
and about 600 years B.C. it was discovered that this substance, when
rubbed, gained the power of drawing to it bits of straw, feathers,
and other light bodies. This discovery is generally credited to a
Greek philosopher named Thales, 941–563 B.C., and it must be regarded
as the first step towards the foundation of electrical science.
The yellow substance was amber. We now know it to be simply a sort
of fossilized resin, but the Greeks gave it a much more romantic
origin. When Phaeton’s rashness brought him to an untimely end, his
sorrowing sisters, the Heliades, were changed into poplar trees, and
their tears into amber. Amongst the names given to the Sun-god was
Alector, which means the shining one, and so the tears of the Heliades
came to have the name Electron, or the shining thing. Unlike most
of the old legends, this story of the fate of the Sun-maidens is of
great importance to us, for from the word “electron” we get the name
Electricity.
Thales and his contemporaries seem to have made no serious attempts
to explain the attraction of the rubbed amber, and indeed so little
importance was attached to the discovery that it was completely
forgotten. About 321 B.C. one Theophrastus found that a certain mineral
called “lyncurium” gained attractive powers when rubbed, but again
little attention was paid to the matter, and astonishing as it may
seem, no further progress worth mention was made until towards the
close of the sixteenth century, when Doctor Gilbert of Colchester
began to experiment seriously. This man was born about 1543, and took
his degree of doctor of medicine at Cambridge in 1569. He was very
successful in his medical work, and became President of the College
of Physicians, and later on physician to Queen Elizabeth. He had a
true instinct for scientific research, and was not content to accept
statements on the authority of others, but tested everything for
himself. He found that sulphur, resin, sealing-wax, and many other
substances behaved like amber when rubbed, but he failed to get any
results from certain other substances, such as the metals. He therefore
called the former substances “electrics,” and the latter “anelectrics,”
or non-electrics. His researches were continued by other investigators,
and from him dates the science of electricity.
[Illustration: FIG. 1.--Suspended pith ball for showing electric
attraction.]
Leaving historical matters for the present, we will examine the curious
power which is gained by substances as the result of rubbing. Amber
is not always obtainable, and so we will use in its place a glass rod
and a stick of sealing-wax. If the glass rod is rubbed briskly with a
dry silk handkerchief, and then held close to a number of very small
bits of paper, the bits are immediately drawn to the rod, and the same
thing occurs if the stick of sealing-wax is substituted for the glass.
This power of attraction is due to the presence of a small charge of
electricity on the rubbed glass and sealing-wax, or in other words,
the two substances are said to be electrified. Bits of paper are
unsatisfactory for careful experimenting, and instead of them we will
use the simple piece of apparatus shown in Fig. 1. This consists of a
ball of elder pith, suspended from a glass support by means of a silk
thread. If now we repeat our experiments with the electrified glass or
sealing-wax we find that the little ball is attracted in the same way
as the bits of paper. But if we look carefully we shall notice that
attraction is not the only effect, for as soon as the ball touches the
electrified body it is driven away or repelled. Now let us suspend, by
means of a thread, a glass rod which has been electrified by rubbing
it with silk, and bring near it in turn another silk-rubbed glass rod
and a stick of sealing-wax rubbed with flannel. The two glass rods are
found to repel one another, whereas the sealing-wax attracts the glass.
If the experiment is repeated with a suspended stick of sealing-wax
rubbed with flannel, the glass and the sealing-wax attract each other,
but the two sticks of wax repel one another. Both glass and sealing-wax
are electrified, as may be seen by bringing them near the pith ball,
but there must be some difference between them as we get attraction in
one case and repulsion in the other.
The explanation is that the electric charges on the silk-rubbed
glass and on the flannel-rubbed sealing-wax are of different
kinds, the former being called positive, and the latter negative.
Bodies with similar charges, such as the two glass rods, repel one
another; while bodies with unlike charges, such as the glass and the
sealing-wax, attract each other. We can now see why the pith ball was
first attracted and then repelled. To start with, the ball was not
electrified, and was attracted when the rubbed glass or sealing-wax was
brought near it. When however the ball touched the electrified body it
received a share of the latter’s electricity, and as similar charges
repel one another, the ball was driven away.
The kind of electricity produced depends not only on the substance
rubbed, but also on the material used as the rubber. For instance,
we can give glass a negative charge by rubbing it with flannel, and
sealing-wax becomes positively charged when rubbed with silk. The
important point to remember is that there are only two kinds of
electricity, and that every substance electrified by rubbing is charged
either positively, like the silk-rubbed glass, or negatively, like the
flannel-rubbed sealing-wax.
If we try to electrify a metal rod by holding it in the hand and
rubbing it, we get no result, but if we fasten to the metal a handle
of glass, and hold it by this while rubbing, we find that it becomes
electrified in the same way as the glass rod or the sealing-wax.
Substances such as glass do not allow electricity to pass along them,
so that in rubbing a glass rod the part rubbed becomes charged, and
the electricity stays there, being unable to spread to the other
parts of the rod. Substances such as metals allow electricity to pass
easily, so that when a metal rod is rubbed electricity is produced,
but it immediately spreads over the whole rod, reaches the hand, and
escapes. If we wish the metal to retain its charge we must provide
it with a handle of glass or of some other material which does not
allow electricity to pass. Dr. Gilbert did not know this, and so he
came to the conclusion that metals were non-electrics, or could not be
electrified.
Substances which allow electricity to pass freely are called
conductors, and those which do not are called non-conductors; while
between the two extremes are many substances which are called partial
conductors. It may be said here that no substance is quite perfect in
either respect, for all conductors offer some resistance to the passage
of electricity, while all non-conductors possess some conducting
power. Amongst conductors are metals, acids, water, and the human
body; cotton, linen, and paper are partial conductors; and air, resin,
silk, glass, sealing-wax, and gutta-percha are non-conductors. When a
conductor is guarded by a non-conductor so that its electricity cannot
escape, it is said to be insulated, from Latin, _insula_, an island;
and non-conductors are also called “insulators.”
So far we have mentioned only the electric charge produced on the
substance rubbed, but the material used as rubber also becomes
electrified. The two charges, however, are not alike, but one is always
positive and the other negative. For instance, if glass is rubbed with
silk, the glass receives a positive, and the silk a negative charge.
It also can be shown that the two opposite charges are always equal in
quantity.
The two kinds of electricity are generally represented by the signs +
and -, the former standing for positive and the latter for negative
electricity.
The electricity produced by rubbing, or friction, is known as Static
Electricity; that is, electricity in a state of rest, as distinguished
from electricity in motion, or current electricity. The word static is
derived from a Greek word meaning to stand. At the same time it must
be understood that this electricity of friction is at rest only in the
sense that it is a prisoner, unable to move. When we produce a charge
of static electricity on a glass rod, by rubbing it, the electricity
would escape fast enough if it could. It has only two possible ways
of escape, along the rod and through the air, and as both glass and
air are non-conductors it is obliged to remain at rest where it was
produced. On the other hand, as we have seen, the electricity produced
by rubbing a metal rod which is not protected by an insulating handle
escapes instantly, because the metal is a good conductor.
When static electricity collects in sufficient quantities it discharges
itself in the form of a bright spark, and we shall speak of these
sparks in Chapter III. Static electricity is of no use for doing useful
work, such as ringing bells or driving motors, and in fact, except
for scientific purposes, it is more of a nuisance than a help. It
collects almost everywhere, and its power of attraction makes it very
troublesome at times. In the processes of textile manufacture static
electricity is produced in considerable quantities, and it makes its
presence known by causing the threads to stick together in the most
annoying fashion. In printing rooms too it plays pranks, making the
sheets of paper stick together so that the printing presses have to be
stopped.
Curiously enough, static electricity has been detected in the act of
interfering with the work of its twin brother, current electricity. A
little while ago it was noticed that the electric incandescent lamps in
a certain building were lasting only a very short time, the filaments
being found broken after comparatively little use. Investigations
showed that the boy was in the habit of dusting the lamp globes with
a feather duster. The friction set up in this way produced charges
of electricity on the glass, and this had the effect of breaking the
filaments. When this method of dusting was discontinued the trouble
ceased, and the lamps lasted their proper number of hours.
CHAPTER II
ELECTRICAL MACHINES AND THE LEYDEN JAR
The amount of electricity produced by the rubbing of glass or
sealing-wax rods is very small, and experimenters soon felt the need of
apparatus to produce larger quantities. In 1675 the first electrical
machine was made by Otto von Guericke, the inventor of the air-pump.
His machine consisted of a globe of sulphur fixed on a spindle,
and rotated while the hands were pressed against it to provide the
necessary friction. Globes and cylinders of glass soon replaced the
sulphur globe, and the friction was produced by cushions instead of
by the hands. Still later, revolving plates of glass were employed.
These machines worked well enough in a dry atmosphere, but were very
troublesome in wet weather, and they are now almost entirely superseded
by what are known as _influence_ machines.
In order to understand the working of influence machines, it is
necessary to have a clear idea of what is meant by the word influence
as used in an electrical sense. In the previous chapter we saw that a
pith ball was attracted by an electrified body, and that when the ball
touched that body it received a charge of electricity. We now have to
learn that one body can receive a charge from another body without
actual contact, by what is called “influence,” or electro-static
induction. In Fig. 2 is seen a simple arrangement for showing this
influence or induction. A is a glass ball, and BC a piece of metal,
either solid or hollow, made somewhat in the shape of a sausage, and
insulated by means of its glass support. Three pairs of pith balls are
suspended from BC as shown. If A is electrified positively, and brought
near BC, the pith balls at B and C repel one another, showing that
the ends of BC are electrified. No repulsion takes place between the
two pith balls at the middle, indicating that this part of BC is not
electrified. If the charges at B and C are tested they are found to be
of opposite kinds, that at B being negative, and that at C positive.
Thus it appears that the positive charge on A has attracted a negative
charge to B, and repelled a positive one to C. If A is taken away, the
two opposite charges on BC unite and neutralise one another, and BC
is left in its original uncharged condition, while A is found to have
lost none of its own charge. If BC is made in two parts, and if these
are separated while under the influence of A, the two charges cannot
unite when A is removed, but remain each on its own half of BC. In this
experiment A is said to have induced electrification on BC. Induction
will take place across a considerable distance, and it is not stopped
by the interposition of obstacles such as a sheet of glass.
[Illustration: FIG. 2.--Diagram to illustrate Electro-static
Induction.]
We can now understand why an electrified body attracts an unelectrified
body, as in our pith ball experiments. If we bring a positively
charged glass rod near a pith ball, the latter becomes electrified
by induction, the side nearer the rod receiving a negative, and the
farther side a positive charge. One half of the ball is therefore
attracted and the other half repelled, but as the attracted half is the
nearer, the attraction is stronger than the repulsion, and so the ball
moves towards the rod.
[Illustration: FIG. 3.--The Electrophorus.]
Fig. 3 shows an appliance for obtaining strong charges of electricity
by influence or induction. It is called the _electrophorus_, the name
coming from two Greek words, _electron_, amber, and _phero_, I yield
or bear; and it was devised in 1775 by Volta, an Italian professor of
physics. The apparatus consists of a round cake, A, of some resinous
material contained in a metal dish, and a round disc of metal, B, of
slightly smaller diameter, fitted with an insulating handle. A simple
electrophorus may be made by filling with melted sealing-wax the lid
of a round tin, the disc being made of a circular piece of copper or
brass, a little smaller than the lid, fastened to the end of a stick of
sealing-wax. To use the electrophorus, the sealing-wax is electrified
negatively by rubbing it with flannel. The metal disc is then placed
on the sealing-wax, touched for an instant with the finger, and lifted
away. The disc is now found to be electrified positively, and it may be
discharged and the process repeated many times without recharging the
sealing-wax. The charge on the latter is not used up in the process,
but it gradually leaks away, and after a time it has to be renewed.
The theory of the electrophorus is easy to understand from what we
have already learnt about influence. When the disc B is placed on the
charged cake A, the two surfaces are really in contact at only three
or four points, because neither of them is a true plane; so that on
the whole the disc and the cake are like A and BC in Fig. 2, only much
closer together. The negative charge on A acts by induction on the
disc B, attracting a positive charge to the under side, and repelling
a negative charge to the upper side. When the disc is touched, the
negative charge on the upper side escapes, but the positive charge
remains, being as it were held fast by the attraction of the negative
charge on A. If the disc is now raised, the positive charge is no
longer bound on the under side, and it therefore spreads over both
surfaces, remaining there because its escape is cut off by the
insulating handle.
[Illustration: FIG. 4.--Wimshurst Machine.]
We may now try to understand the working of influence machines, which
are really mechanically worked electrophori. There are various types of
such machines, but the one in most general use in this country is that
known as the Wimshurst machine, Fig. 4, and we will therefore confine
ourselves to this. It consists of two circular plates of varnished
glass or of ebonite, placed close together and so geared that they
rotate in opposite directions. On the outer surfaces of the plates
are cemented sectors of metal foil, at equal distances apart. Each
plate has the same number of sectors, so that at any given moment the
sectors on one plate are exactly opposite those on the other. Across
the outer surface of each plate is fixed a rod of metal carrying at
its ends light tinsel brushes, which are adjusted to touch the sectors
as they pass when the plates are rotated. These rods are placed at an
angle to each other of from sixty to ninety degrees, and the brushes
are called neutralizing brushes. The machine is now complete for
generating purposes, but in order to collect the electricity two pairs
of insulated metal combs are provided, one pair at each end of the
horizontal diameter, with the teeth pointing inward towards the plates,
but not touching them. The collecting combs are fitted with adjustable
discharging rods terminating in round knobs.
The principle upon which the machine works will be best understood
by reference to Fig. 5. In this diagram the inner circle represents
the front plate, with neutralizing brushes A and B, and the outer one
represents the back plate, with brushes C and D. The sectors are shown
heavily shaded. E and F are the pairs of combs, and the plates rotate
in the direction of the arrows. Let us suppose one of the sectors at
the top of the back plate to have a slight positive charge. As the
plates rotate this sector will come opposite to a front plate sector
touched by brush A, and it will induce a slight negative charge on the
latter sector, at the same time repelling a positive charge along the
rod to the sector touched by brush B. The two sectors carrying the
induced charges now move on until opposite back plate sectors touched
by brushes C and D, and these back sectors will receive by induction
positive and negative charges respectively. This process continues
as the plates rotate, and finally all the sectors moving towards
comb E will be positively charged, while those approaching comb F
will be negatively charged. The combs collect these charges, and the
discharging rods K and L become highly electrified, K positively and
L negatively, and if they are near enough together sparks will pass
between them.
[Illustration: FIG. 5.--Diagram to illustrate working of a Wimshurst
Machine.]
At the commencement we supposed one of the sectors to have a positive
charge, but it is not necessary to charge a sector specially, for the
machine is self-starting. Why this is the case is not yet thoroughly
understood, but probably the explanation is that at any particular
moment no two places in the atmosphere are in exactly the same
electro-static condition, so that an uneven state of charge exists
permanently amongst the sectors.
The Wimshurst machine provides us with a plentiful supply of
electricity, and the question naturally arises, “Can this electricity
be stored up in any way?” In 1745, long before the days of influence
machines, a certain Bishop of Pomerania, Von Kleist by name, got the
idea that if he could persuade a charge of electricity to go into
a glass bottle he would be able to capture it, because glass was a
non-conductor. So he partly filled a bottle with water, led a wire down
into the water, and while holding the bottle in one hand connected
the wire to a primitive form of electric machine. When he thought he
had got enough electricity he tried to remove his bottle in order
to examine the contents, and in so doing he received a shock which
scared him considerably. He had succeeded in storing electricity in
his bottle. Shortly afterwards the bishop’s experiment was repeated by
Professor Muschenbrock of Leyden, and by his pupil Cuneus, the former
being so startled by the shock that he wrote, “I would not take a
second shock for the kingdom of France.” But in spite of shocks the end
was achieved; it was proved that electricity could be collected and
stored up, and the bottle became known as the Leyden jar. The original
idea was soon improved upon, water being replaced by a coating of
tinfoil, and it was found that better results were obtained by coating
the outside of the bottle as well as the inside.
As now used the Leyden jar consists of a glass jar covered inside and
outside with tinfoil up to about two-thirds of its height. A wooden
lid is fitted, through which passes a brass rod terminating above in a
brass knob, and below in a piece of brass chain long enough to touch
the foil lining. A Leyden jar is charged by holding it in one hand with
its knob presented to the discharging ball of a Wimshurst machine,
and even if the machine is small and feeble the jar will accumulate
electricity until it is very highly charged. It may now be put down on
the table, and if it is clean and quite dry it will hold its charge for
some time. If the outer and inner coatings of the jar are connected by
means of a piece of metal, the electricity will be discharged in the
form of a bright spark. A Leyden jar is usually discharged by means
of discharging tongs, consisting of a jointed brass rod with brass
terminal knobs and glass handles. One knob is placed in contact with
the outer coating of foil, and the other brought near to the knob of
the jar, which of course is connected with the inner coating.
The electrical capacity of even a small Leyden jar is surprisingly
great, and this is due to the mutual attraction between opposite
kinds of electricity. If we stick a piece of tinfoil on the centre of
each face of a pane of glass, and charge one positively and the other
negatively, the two charges attract each other through the glass;
and in fact they hold on to each other so strongly that we can get
very little electricity by touching either piece of foil. This mutual
attraction enables us to charge the two pieces of foil much more
strongly than if they were each on a separate pane, and this is the
secret of the working of the Leyden jar. If the knob of the jar is
held to the positive ball of a Wimshurst, the inside coating receives
a positive charge, which acts inductively on the outside coating,
attracting a negative charge to the inner face of the latter, and
repelling a positive charge to its outer face, and thence away through
the hand. The electricity is entirely confined to the sides of the jar,
the interior having no charge whatever.
Leyden jars are very often fitted to a Wimshurst machine as shown
at A, A, Fig. 4, and arranged so that they can be connected or
disconnected to the collecting combs as desired. When the jars are
disconnected the machine gives a rapid succession of thin sparks, but
when the jars are connected to the combs they accumulate a number of
charges before the discharge takes place, with the result that the
sparks are thicker, but occur at less frequent intervals.
It will have been noticed that the rod of a Leyden jar and the
discharging rods of a Wimshurst machine are made to terminate not in
points, but in rounded knobs or balls. The reason of this is that
electricity rapidly leaks away from points. If we electrify a conductor
shaped like a cone with a sharp point, the density of the electricity
is greatest at that point, and when it becomes sufficiently great the
particles of air near the point become electrified and repelled. Other
particles take their place, and are electrified and repelled in the
same way, and so a constant loss of electricity takes place. This may
be shown in an interesting way by fastening with wax a needle to the
knob of a Wimshurst. If a lighted taper is held to the point of the
needle while the machine is in action, the flame is blown aside by the
streams of repelled air, which form a sort of electric wind.
CHAPTER III
ELECTRICITY IN THE ATMOSPHERE
If the Leyden jars of a Wimshurst machine are connected up and the
discharging balls placed at a suitable distance apart, the electricity
produced by rotating the plates is discharged in the form of a
brilliant zigzag spark between the balls, accompanied by a sharp crack.
The resemblance between this spark and forked lightning is at once
evident, and in fact it is lightning in miniature. The discharging
balls are charged, as we have seen, with opposite kinds of electricity,
and these charges are constantly trying to reach one another across
the intervening air, which, being an insulator, vigorously opposes
their passage. There is thus a kind of struggle going on between the
air and the two charges of electricity, and this keeps the air in
a state of constant strain. But the resisting power of the air is
limited, and when the charges reach a certain strength the electricity
violently forces its way across, literally rupturing or splitting the
air. The particles of air along the path of the discharge are rendered
incandescent by the heat produced by the passage of the electricity,
and so the brilliant flash is produced. Just as a river winds about
seeking the easiest course, so the electricity takes the path of least
resistance, which probably is determined by the particles of dust in
the air, and also by the density of the air, which becomes compressed
in front, leaving less dense air and therefore an easier path on each
side.
The connexion between lightning and the sparks from electrified bodies
and electrical machines was suspected by many early observers, but it
remained for Benjamin Franklin to prove that lightning was simply a
tremendous electric discharge, by actually obtaining electricity from a
thunder-cloud. Franklin was an American, born at Boston in 1706. He was
a remarkable man in every way, and quite apart from his investigations
in electricity, will always be remembered for the great public
services he rendered to his country in general and to Philadelphia
in particular. He founded the Philadelphia Library, the American
Philosophical Society, and the University of Pennsylvania.
Franklin noticed many similarities between electricity and lightning.
For instance, both produced zigzag sparks, both were conducted by
metals, both set fire to inflammable materials, and both were capable
of killing animals. These resemblances appeared to him so striking that
he was convinced that the two were the same, and he resolved to put the
matter to the test. For this purpose he hit upon the idea of using a
kite, to the top of which was fixed a pointed wire. At the lower end of
the flying string was tied a key, insulated by a piece of silk ribbon.
In June 1752, Franklin flew his kite, and after waiting a while he
was rewarded by finding that when he brought his knuckle near to the
key a little spark made its appearance. This spark was exactly like
the sparks obtained from electrified bodies, but to make things quite
certain a Leyden jar was charged from the key. Various experiments were
then performed with the jar, and it was proved beyond all doubt that
lightning and electricity were one and the same.
Lightning is then an enormous electric spark between a cloud and the
Earth, or between two clouds, produced when opposite charges become so
strong that they are able to break down the intervening non-conducting
layer of air. The surface of the Earth is negatively electrified,
the electrification varying at different times and places; while the
electricity of the air is usually positive, but frequently changes to
negative in rainy weather and on other occasions. As the clouds float
about they collect the electricity from the air, and thus they may be
either positively or negatively electrified, so that a discharge may
take place between one cloud and another, as well as between a cloud
and the Earth.
Lightning flashes take different forms, the commonest being forked or
zigzag lightning, and sheet lightning. The zigzag form is due to the
discharge taking the easiest path, as in the case of the spark from
a Wimshurst machine. Sheet lightning is probably the reflection of a
flash taking place at a distance. It may be unaccompanied by thunder,
as in the so-called “summer lightning,” seen on the horizon at night,
which is the reflection of a storm too far off for the thunder to be
heard. A much rarer form is globular or ball lightning, in which the
discharge takes the shape of a ball of light, which moves slowly along
and finally disappears with a sudden explosion. The cause of this form
of lightning is not yet understood, but it is possible that the ball of
light consists of intensely heated and extremely minute fragments of
ordinary matter, torn off by the violence of the lightning discharge.
Another uncommon form is multiple lightning, which consists of a number
of separate parallel discharges having the appearance of a ribbon.
A lightning flash probably lasts from about 1/100,000 to 1/1,000,000 of a
second, and in the majority of cases the discharge is oscillatory; that
is to say, it passes several times backwards and forwards between two
clouds or between a cloud and the Earth. At times it appears as though
we could see the lightning start downwards from the cloud or upwards
from the Earth, but this is an optical illusion, and it is really
quite impossible to tell at which end the flash starts.
Death by lightning is instantaneous, and therefore quite painless.
We are apt to think that pain is felt at the moment when a wound is
inflicted. This is not the case however, for no pain is felt until the
impression reaches the brain by way of the nerves, and this takes an
appreciable time. The nerves transmit sensations at a speed of only
about one hundred feet per second, so that in the case of a man killed
by a bullet through the brain, no pain would be felt, because the brain
would be deprived of sensibility before the sensation could reach it.
Lightning is infinitely swifter than any bullet, so life would be
destroyed by it before any pain could be felt.
On one occasion Professor Tyndall, the famous physicist, received
accidentally a very severe shock from a large battery of Leyden jars
while giving a public lecture. His account of his sensations is very
interesting. “Life was absolutely blotted out for a very sensible
interval, without a trace of pain. In a second or so consciousness
returned; I saw myself in the presence of the audience and apparatus,
and, by the help of these external appearances, immediately concluded
that I had received the battery discharge. The intellectual
consciousness of my position was restored with exceeding rapidity,
but not so the optical consciousness. To prevent the audience from
being alarmed, I observed that it had often been my desire to receive
accidentally such a shock, and that my wish had at length been
fulfilled. But, while making this remark, the appearance which my body
presented to myself was that of a number of separate pieces. The arms,
for example, were detached from the trunk, and seemed suspended in the
air. In fact, memory and the power of reasoning appeared to be complete
long before the optic nerve was restored to healthy action. But what
I wish chiefly to dwell upon here is, the absolute painlessness of the
shock; and there cannot be a doubt that, to a person struck dead by
lightning, the passage from life to death occurs without consciousness
being in the least degree implicated. It is an abrupt stoppage of
sensation, unaccompanied by a pang.”
Occasionally branched markings are found on the bodies of those struck
by lightning, and these are often taken to be photographic impressions
of trees under which the persons may have been standing at the time of
the flash. The markings however are nothing of the kind, but are merely
physiological effects due to the passage of the discharge.
During a thunderstorm it is safer to be in the house than out in the
open. It is probable that draughts are a source of some danger, and
the windows and doors of the room ought to be shut. Animals are more
liable to be struck by lightning than men, and a shed containing horses
or cows is a dangerous place in which to take shelter; in fact it is
better to remain in the open. If one is caught in a storm while out of
reach of a house or other building free from draughts and containing
no animals, the safest plan is to lie down, not minding the rain.
Umbrellas are distinctly dangerous, and never should be used during a
storm. Wire fences, hedges, and still or running water should be given
a wide berth, and it is safer to be alone than in company with a crowd
of people. It is extremely foolish to take shelter under an isolated
tree, for such trees are very liable to be struck. Isolated beech trees
appear to have considerable immunity from lightning, but any tree
standing alone should be avoided, the oak being particularly dangerous.
On the other hand, a fairly thick wood is comparatively safe, and
failing a house, should be chosen before all other places of refuge.
Horses are liable to be struck, and if a storm comes on while one is
out driving it is safer to keep quite clear of the animals.
When a Wimshurst machine has been in action for a little time a
peculiar odour is noticed. This is due to the formation of a modified
and chemically more active form of oxygen, called _ozone_, the
name being derived from the Greek _ozein_, “to smell.” Ozone has
very invigorating effects when breathed, and it is also a powerful
germicide, capable of killing the germs which give rise to contagious
diseases. During a thunderstorm ozone is produced in large quantities
by the electric discharges, and thus the air receives as it were a new
lease of life, and we feel the refreshing effects when the storm is
over. We shall speak again of ozone in Chapter XXV.
Thunder probably is caused by the heating and sudden expansion of
the air in the path of the discharge, which creates a partial vacuum
into which the surrounding air rushes violently. Light travels at the
rate of 186,000 miles per second, and therefore the flash reaches us
practically instantaneously; but sound travels at the rate of only
about 1115 feet per second, so that the thunder takes an appreciable
time to reach us, and the farther away the discharge the greater the
interval between the flash and the thunder. Thus by multiplying the
number of seconds which elapse between the flash and the thunder by
1115, we may calculate roughly the distance in feet of the discharge. A
lightning flash may be several miles in length, the greatest recorded
length being about ten miles. The sounds produced at different points
along its path reach us at different times, producing the familiar
sharp rattle, and the following rolling and rumbling is produced by the
echoes from other clouds. The noise of a thunder-clap is so tremendous
that it seems as though the sound would be heard far and wide, but
the greatest distance at which thunder has been heard is about fifteen
miles. In this respect it is interesting to compare the loudest
thunder-clap we ever heard with the noise of the famous eruption of
Krakatoa, in 1883, which was heard at the enormous distance of nearly
three thousand miles.
When Franklin had demonstrated the nature of lightning, he began to
consider the possibility of protecting buildings from the disastrous
effects of the lightning stroke. At that time the amount of damage
caused by lightning was very great. Cathedrals, churches, public
buildings, and in fact all tall edifices were in danger every time
a severe thunderstorm took place in their neighbourhood, for there
was absolutely nothing to prevent their destruction if the lightning
chanced to strike them. Ships at sea, too, were damaged very frequently
by lightning, and often some of the crew were killed or disabled.
To-day, thanks to the lightning conductor, it is an unusual occurrence
for ships or large buildings to be damaged by lightning. The lightning
strikes them as before, but in the great majority of cases it is led
away harmlessly to earth.
Franklin was the first to suggest the possibility of protecting
buildings by means of a rod of some conducting material terminating in
a point at the highest part of the building, and leading down, outside
the building, into the earth. Lightning conductors at the present
day are similar to Franklin’s rod, but many improvements have been
made from time to time as our knowledge of the nature and action of
the lightning discharge has increased. A modern lightning conductor
generally consists of one or more pointed rods fixed to the highest
parts of the building, and connected to a cable running directly to
earth. This cable is kept as straight as possible, because turns
and bends offer a very high resistance to the rapidly oscillating
discharge; and it is connected to large copper plates buried in
permanently moist ground or in water, or to water or gas mains. Copper
is generally used for the cable, but iron also may be employed. In
any case, the cable must be of sufficient thickness to prevent the
possibility of its being deflagrated by the discharge. In ships the
arrangements are similar, except that the cable is connected to the
copper sheathing of the bottom.
The fixing of lightning conductors must be carried out with great care,
for an improperly fixed conductor is not only useless, but may be a
source of actual danger. Lightning flashes vary greatly in character,
and while a carefully erected lightning conductor is capable of dealing
with most of them, there are unfortunately certain kinds of discharge
with which it now and then is unable to deal. The only absolutely
certain way of protecting a building is to surround it completely by a
sort of cage of metal, but except for buildings in which explosives are
stored this plan is usually impracticable.
The electricity of the atmosphere manifests itself in other forms
beside the lightning. The most remarkable of these manifestations is
the beautiful phenomenon known in the Northern Hemisphere as the Aurora
Borealis, and in the Southern Hemisphere as the Aurora Australis.
Aurora means the morning hour or dawn, and the phenomenon is so called
from its resemblance to the dawn of day. The aurora is seen in its full
glory only in high latitudes, and it is quite unknown at the equator.
It assumes various forms, sometimes appearing as an arch of light with
rapidly moving streamers of different colours, and sometimes taking
the form of a luminous curtain extending across the sky. The light
of the aurora is never very strong, and as a rule stars can be seen
through it. Auroras are sometimes accompanied by rustling or crackling
sounds, but the sounds are always extremely faint. Some authorities
assert that these sounds do not exist, and that they are the result
of imagination, but other equally reliable observers have heard the
sounds quite plainly on several occasions. Probably the explanation of
this confliction of evidence is that the great majority of auroras are
silent, so that an observer might witness many of them without hearing
any sounds. The height at which auroras occur is a disputed point, and
one which it is difficult to determine accurately; but most observers
agree that it is generally from 60 to 125 miles above the Earth’s
surface.
There is little doubt that the aurora is caused by the passage of
electric discharges through the higher regions of the atmosphere,
where the air is so rarefied as to act as a partial conductor; and
its effects can be imitated in some degree by passing powerful
discharges through tubes from which the air has been exhausted
to a partial vacuum. Auroral displays are usually accompanied by
magnetic disturbances, which sometimes completely upset telegraphic
communication. Auroras and magnetic storms appear to be connected in
some way with solar disturbances, for they are frequently simultaneous
with an unusual number of sunspots, and all three run in cycles of
about eleven and a half years.
CHAPTER IV
THE ELECTRIC CURRENT
In the previous chapters we have dealt with electricity in charged
bodies, or static electricity, and now we must turn to electricity in
motion, or current electricity. In Chapter I. we saw that if a metal
rod is held in the hand and rubbed, electricity is produced, but it
immediately escapes along the rod to the hand, and so to the earth.
In other words, the electricity flows away along the conducting path
provided by the rod and the hand. When we see the word “flow” we at
once think of a fluid of some kind, and we often hear people speak of
the “electric fluid.” Now, whatever electricity may be it certainly is
not a fluid, and we use the word “flow” in connexion with electricity
simply because it is the most convenient word we can find for the
purpose. Just in the same way we might say that when we hold a poker
with its point in the fire, heat flows along it towards our hand,
although we know quite well that heat is not a fluid. In the experiment
with the metal rod referred to above, the electricity flows away
instantly, leaving the rod unelectrified; but if we arrange matters so
that the electricity is renewed as fast as it flows away, then we get a
continuous flow, or current.
Somewhere about the year 1780 an Italian anatomist, Luigi Galvani,
was studying the effects of electricity upon animal organisms, using
for the purpose the legs of freshly killed frogs. In the course of
his experiments he happened to hang against an iron window rail a
bundle of frogs’ legs fastened together with a piece of copper wire,
and he noticed that the legs began to twitch in a peculiar manner. He
knew that a frog’s leg would twitch when electricity was applied to
it, and he concluded that the twitchings in this case were caused in
the same way. So far he was quite right, but then came the problem
of how any electricity could be produced in these circumstances, and
here he went astray. It never occurred to him that the source of the
electricity might be found in something quite apart from the legs, and
so he came to the conclusion that the phenomenon was due to electricity
produced in some mysterious way in the tissues of the animal itself.
He therefore announced that he had discovered the existence of a kind
of animal electricity, and it was left for his fellow-countryman,
Alessandro Volta, to prove that the twitchings were due to electricity
produced by the contact of the two metals, the iron of the window rail
and the copper wire.
[Illustration: FIG. 6.--Voltaic Pile.]
Volta found that when two different metals were placed in contact
in air, one became positively charged, and the other negatively.
These charges however were extremely feeble, and in his endeavours
to obtain stronger results he hit upon the idea of using a number of
pairs of metals, and he constructed the apparatus known as the Voltaic
pile, Fig. 6. This consists of a number of pairs of zinc and copper
discs, each pair being separated from the next pair by a disc of
cloth moistened with salt water. These are piled up and placed in a
frame, as shown in the figure. One end of the pile thus terminates in
a zinc disc, and the other in a copper disc, and as soon as the two
are connected by a wire or other conductor a continuous current of
electricity is produced. The cause of the electricity produced by the
voltaic pile was the subject of a long and heated controversy. There
were two main theories; that of Volta himself, which attributed the
electricity to the mere contact of unlike metals, and the chemical
theory, which ascribed it to chemical action. The chemical theory is
now generally accepted, but certain points, into which we need not
enter, are still in dispute.
There is a curious experiment which some of my readers may like to try.
Place a copper coin on a sheet of zinc, and set an ordinary garden
snail to crawl across the zinc towards the coin. As soon as the snail
comes in contact with the copper it shrinks back, and shows every sign
of having received a shock. One can well imagine that an enthusiastic
gardener pestered with snails would watch this experiment with great
glee.
[Illustration: FIG. 7.--Simple Voltaic Cell.]
Volta soon found that it was not necessary to have his pairs of metals
in actual metallic contact, and that better results were got by
placing them in a vessel filled with dilute acid. Fig. 7 is a diagram
of a simple voltaic cell of this kind, and it shows the direction of
the current when the zinc and the copper are connected by the wire.
In order to get some idea of the reason why a current flows we must
understand the meaning of electric potential. If water is poured into
a vessel, a certain water pressure is produced. The amount of this
pressure depends upon the level of the water, and this in turn depends
upon the quantity of water and the capacity of the vessel, for a given
quantity of water will reach a higher level in a small vessel than
in a larger one. In the same way, if electricity is imparted to a
conductor an electric pressure is produced, its amount depending upon
the quantity of electricity and the electric capacity of the conductor,
for conductors vary in capacity just as water vessels do.
This electric pressure is called “potential,” and electricity tends to
flow from a conductor of higher to one of lower potential. When we say
that a place is so many feet above or below sea-level we are using the
level of the sea as a zero level, and in estimating electric potential
we take the potential of the earth’s surface as zero; and we regard a
positively electrified body as one at a positive or relatively high
potential, and a negatively electrified body as one at a negative
or relatively low potential. This may be clearer if we think of
temperature and the thermometer. Temperatures above zero are positive
and represented by the sign +, and those below zero are negative and
represented by the sign -. Thus we assume that an electric current
flows from a positive to a negative conductor.
[Illustration: PLATE I.
_By permission of_ _Dick, Kerr & Co. Ltd._
HYDRO-ELECTRIC POWER STATION.]
In a voltaic cell the plates are at different potentials, so that
when they are connected by a wire a current flows, and we say that
the current leaves the cell at the positive terminal, and enters it
again at the negative terminal. As shown in Fig. 7, the current moves
in opposite directions inside and outside the cell, making a complete
round called a _circuit_, and if the circuit is broken anywhere the
current ceases to flow. If the circuit is complete the current keeps on
flowing, trying to equalize the electric pressure or potential, but it
is unable to do this because the chemical action between the acid and
the zinc maintains the difference of potential between the plates. This
chemical action results in wasting of the zinc and weakening of the
acid, and as long as it continues the current keeps on flowing. When we
wish to stop the current we break the circuit by disconnecting the wire
joining the terminals, and the cell then should be at rest; but owing
to the impurities in ordinary commercial zinc chemical action still
continues. In order to prevent wasting when the current is not required
the surface of the zinc is coated with a thin film of mercury. The zinc
is then said to be amalgamated, and it is not acted upon by the acid so
long as the circuit remains broken.
The current from a simple voltaic cell does not remain at a constant
strength, but after a short time it begins to weaken rapidly. The
cell is then said to be polarized, and this polarization is caused by
bubbles of hydrogen gas which accumulate on the surface of the copper
plate during the chemical action. These bubbles of gas weaken the
current partly by resisting its flow, for they are bad conductors,
and still more by trying to set up another current in the opposite
direction. For this reason the simple voltaic cell is unsuitable for
long spells of work, and many cells have been devised to avoid the
polarization trouble. One of the most successful of these is the
Daniell cell. It consists of an outer vessel of copper, which serves
as the copper plate, and an inner porous pot containing a zinc rod.
Dilute sulphuric acid is put into the porous pot and a strong solution
of copper sulphate into the outer jar. When the circuit is closed,
the hydrogen liberated by the action of the zinc on the acid passes
through the porous pot, and splits up the copper sulphate into copper
and sulphuric acid. In this way pure copper, instead of hydrogen, is
deposited on the copper plate, no polarization takes place, and the
current is constant.
Other cells have different combinations of metals, such as silver-zinc,
or platinum-zinc, and carbon is also largely used in place of one
metal, as in the familiar carbon-zinc Leclanché cell, used for ringing
electric bells. This cell consists of an inner porous pot containing
a carbon plate packed round with a mixture of crushed carbon and
manganese dioxide, and an outer glass jar containing a zinc rod and a
solution of sal-ammoniac. Polarization is checked by the oxygen in the
manganese dioxide, which seizes the hydrogen on its way to the carbon
plate, and combines with it. If the cell is used continuously however
this action cannot keep pace with the rate at which the hydrogen is
produced, and so the cell becomes polarized; but it soon recovers after
a short rest.
The so-called “dry” cells so much used at the present time are not
really dry at all; if they were they would give no current. They are in
fact Leclanché cells, in which the containing vessel is made of zinc
to take the place of a zinc rod; and they are dry only in the sense
that the liquid is taken up by an absorbent material, so as to form
a moist paste. Dry cells are placed inside closely fitting cardboard
tubes, and are sealed up at the top. Their chief advantage lies in
their portability, for as there is no free liquid to spill they can be
carried about and placed in any position.
We have seen that the continuance of the current from a voltaic cell
depends upon the keeping up of a difference of potential between the
plates. The force which serves to maintain this difference is called
the electro-motive force, and it is measured in volts. The actual flow
of electricity is measured in amperes. Probably all my readers are
familiar with the terms volt and ampere, but perhaps some may not be
quite clear about the distinction between the two. When water flows
along a pipe we know that it is being forced to do so by pressure
resulting from a difference of level. That is to say, a difference
of level produces a water-moving or water-motive force; and in a
similar way a difference of potential produces an electricity-moving
or electro-motive force, which is measured in volts. If we wish to
describe the rate of flow of water we state it in gallons per second,
and the rate of flow of electricity is stated in amperes. Volts thus
represent the pressure at which a current is supplied, while the
current itself is measured in amperes.
We may take this opportunity of speaking of electric resistance. A
current of water flowing through a pipe is resisted by friction against
the inner surface of the pipe; and a current of electricity flowing
through a circuit also meets with a resistance, though this is not due
to friction. In a good conductor this resistance is small, but in a
bad conductor or non-conductor it is very great. The resistance also
depends upon length and area of cross-section; so that a long wire
offers more resistance than a short one, and a thin wire more than a
thick one. Before any current can flow in a circuit the electro-motive
force must overcome the resistance, and we might say that the volts
drive the amperes through the resistance. The unit of resistance is the
ohm, and the definition of a volt is that electro-motive force which
will cause a current of one ampere to flow through a conductor having a
resistance of one ohm. These units of measurement are named after three
famous scientists, Volta, Ampère, and Ohm.
[Illustration: FIG. 8.--Cells connected in Parallel.]
A number of cells coupled together form a battery, and different
methods of coupling are used to get different results. In addition to
the resistance of the circuit outside the cell, the cell itself offers
an internal resistance, and part of the electro-motive force is used
up in overcoming this resistance. If we can decrease this internal
resistance we shall have a larger current at our disposal, and one way
of doing this is to increase the size of the plates. This of course
means making the cell larger, and very large cells take up a lot of
room and are troublesome to move about. We can get the same effect
however by coupling. If we connect together all the positive terminals
and all the negative terminals of several cells, that is, copper to
copper and zinc to zinc in Daniell cells, we get the same result as
if we had one very large cell. The current is much larger, but the
electro-motive force remains the same as if only one cell were used, or
in other words we have more amperes but no more volts. This is called
connecting in “parallel,” and the method is shown in Fig. 8. On the
other hand, if, as is usually the case, we want a larger electro-motive
force, we connect the positive terminal of one cell to the negative
terminal of the next, or copper to zinc all through. In this way we add
together the electro-motive forces of all the cells, but the amount
of current remains that of a single cell; that is, we get more volts
but no more amperes. This is called connecting in “series,” and the
arrangement is shown in Fig. 9. We can also increase both volts and
amperes by combining the two methods.
[Illustration: FIG. 9.--Cells connected in Series.]
A voltaic cell gives us a considerable quantity of electricity at low
pressure, the electro-motive force of a Leclanché cell being about 1½
volts, and that of a Daniell cell about 1 volt. We may perhaps get some
idea of the electrical conditions existing during a thunderstorm from
the fact that to produce a spark one mile long through air at ordinary
pressure we should require a battery of more than a thousand million
Daniell cells. Cells such as we have described in this chapter are
called primary cells, as distinguished from accumulators, which are
called secondary cells. Some of the practical applications of primary
cells will be described in later chapters.
Besides the voltaic cell, in which the current is produced by chemical
action, there is the thermo-electric battery, or thermopile, which
produces current directly from heat energy. About 1822 Seebeck was
experimenting with voltaic pairs of metals, and he found that a
current could be produced in a complete metallic circuit consisting
of different metals joined together, by keeping these joinings at
different temperatures. Fig. 10 shows a simple arrangement for
demonstrating this effect, which is known as the “Seebeck effect.” A
slab of bismuth, BB, has placed upon it a bent strip of copper, C. If
one of the junctions of the two metals is heated as shown, a current
flows; and the same effect is produced by cooling one of the junctions.
This current continues to flow as long as the two junctions are kept at
different temperatures. In 1834 another scientist, Peltier, discovered
that if a current was passed across a junction of two different metals,
this junction was either heated or cooled, according to the direction
in which the current flowed. In Fig. 10 the current across the heated
junction tends to cool the junction, while the Bunsen burner opposes
this cooling, and keeps up the temperature. A certain amount of the
heat energy is thus transformed into electrical energy. At the other
junction the current produces a heating effect, so that some of the
electrical energy is retransformed into heat.
[Illustration: FIG. 10.--Diagram to illustrate the Seebeck effect.]
[Illustration: FIG. 11.--Diagram to show arrangement of two different
metals in Thermopile.]
A thermopile consists of a number of alternate bars or strips of two
unlike metals, joined together as shown diagrammatically in Fig. 11.
The arrangement is such that the odd junctions are at one side, and the
even ones at the other. The odd junctions are heated, and the even
ones cooled, and a current flows when the circuit is completed. By
using a larger number of junctions, and by increasing the difference of
temperature between them, the voltage of the current may be increased.
Thermopiles are nothing like so efficient as voltaic cells, and they
are more costly. They are used to a limited extent for purposes
requiring a very small and constant current, but for generating
considerable quantities of current at high pressure they are quite
useless. The only really important practical use of the thermopile
is in the detection and measurement of very minute differences of
temperature, which are beyond the capabilities of the ordinary
thermometer. Within certain limits, the electro-motive force of a
thermopile is exactly proportionate to the difference of temperature.
The very slightest difference of temperature produces a current,
and by connecting the wires from a specially constructed thermopile
to a delicate instrument for measuring the strength of the current,
temperature differences of less than one-millionth of a degree can be
detected.
CHAPTER V
THE ACCUMULATOR
If we had two large water tanks, one of which could be emptied only
by allowing the bottom to fall completely out, and the other by means
of a narrow pipe, it is easy to see which would be the more useful to
us as a source of water supply. If both tanks were filled, then from
the first we could get only a sudden uncontrollable rush of water,
but from the other we could get a steady stream extending over a long
period, and easily controlled. The Leyden jar stores electricity,
but in yielding up its store it acts like the first tank, giving a
sudden discharge in the form of a bright spark. We cannot control the
discharge, and therefore we cannot make it do useful work for us. For
practical purposes we require a storing arrangement that will act like
the second tank, giving us a steady current of electricity for a long
period, and this we have in the accumulator or storage cell.
A current of electricity has the power of decomposing certain liquids.
If we pass a current through water, the water is split up into its
two constituent gases, hydrogen and oxygen, and this may be shown by
the apparatus seen in Fig. 12. It consists of a glass vessel with two
strips of platinum to which the current is led. The vessel contains
water to which has been added a little sulphuric acid to increase its
conducting power, and over the strips are inverted two test-tubes
filled with the acidulated water. The platinum strips, which are
called _electrodes_, are connected to a battery of Daniell cells.
When the current passes, the water is decomposed, and oxygen collects
at the electrode connected to the positive terminal of the battery,
and hydrogen at the other electrode. The two gases rise up into the
test-tubes and displace the water in them, and the whole process is
called the electrolysis of water. If now we disconnect the battery and
join the two electrodes by a wire, we find that a current flows from
the apparatus as from a voltaic cell, but in the opposite direction
from the original battery current.
[Illustration: FIG. 12.--Diagram showing Electrolysis of Water.]
It will be remembered that one of the troubles with a simple voltaic
cell was polarization, caused by the accumulation of hydrogen; and that
this weakened the current by setting up an opposing electro-motive
force tending to produce another current in the opposite direction.
In the present case a similar opposing or back electro-motive force
is produced, and as soon as the battery current is stopped and the
electrodes are connected, we get a current in the reverse direction,
and this current continues to flow until the two gases have recombined,
and the electrodes have regained their original condition. Consequently
we can see that in order to electrolyze water, our battery must have an
electro-motive force greater than that set up in opposition to it, and
at least two Daniell cells are required.
This apparatus thus may be made to serve to some extent as an
accumulator or storage cell, and it also serves to show that an
accumulator does not store up or accumulate electricity. In a voltaic
cell we have chemical energy converted into electrical energy, and here
we have first electrical energy converted into chemical energy, and
then the chemical energy converted back again into electrical energy.
This is a rough-and-ready way of putting the matter, but it is good
enough for practical purposes, and at any rate it makes it quite clear
that what an accumulator really stores up is not electricity, but
energy, which is given out in the form of electricity.
The apparatus just described is of little use as a source of current,
and the first really practical accumulator was made in 1878 by Gaston
Planté. The electrodes were two strips of sheet lead placed one upon
the other, but separated by some insulating material, and made into a
roll. This roll was placed in dilute sulphuric acid, and one strip or
plate connected to the positive, and the other to the negative terminal
of the source of current. The current was passed for a certain length
of time, and then the accumulator partly discharged; after which
current was passed again, but in the reverse direction, followed by
another period of discharge. This process, which is called _forming_,
was continued for several days, and its effect was to change one plate
into a spongy condition, and to form a coating of peroxide of lead on
the other. When the plates were properly formed the accumulator was
ready to be fully charged and put into use. The effect of charging was
to rob one plate of its oxygen, and to transfer this oxygen to the
other plate, which thus received an overcharge of the gas. During the
discharge of the accumulator the excess of oxygen went back to the
place from which it had been taken, and the current continued until the
surfaces of both plates were reduced to a chemically inactive state.
The accumulator could be charged and discharged over and over again as
long as the plates remained in good order.
In 1881, Faure hit upon the idea of coating the plates with a paste of
red-lead, and this greatly shortened the time of forming. At first it
was found difficult to make the paste stick to the plates, but this
trouble was got rid of by making the plates in the form of grids, and
pressing the paste into the perforations. Many further improvements
have been made from time to time, but instead of tracing these we will
go on at once to the description of a present-day accumulator. There
are now many excellent accumulators made, but we have not space to
consider more than one, and we will select that known as the “Chloride”
accumulator.
The positive plate of this accumulator is of the Planté type, but it is
not simply a casting of pure lead, but is made by a building-up process
which allows of the use of a lead-antimony mixture for the grids. This
gives greater strength, and the grids themselves are unaffected by the
chemical changes which take place during the charging and discharging
of the cell. The active material, that is the material which undergoes
chemical change, is pure lead tape coiled up into rosettes, which are
so designed that the acid can circulate through the plates. These
rosettes are driven into the perforations of the grid by a hydraulic
press, and during the process of forming they expand and thus become
very firmly fixed. The negative plate has a frame made in two parts,
which are riveted together after the insertion of the active material,
which is thus contained in a number of small cages. The plate is
covered outside with a finely perforated sheet of lead, which prevents
the active material from falling out. It is of the utmost importance
that the positive and negative plates should be kept apart when in the
cell, and in the Chloride accumulator this is ensured by the use of a
patent separator made of a thin sheet of wood the size of the plates.
Before being used the wood undergoes a special treatment to remove
all substances which might be harmful, and it then remains unchanged
either in appearance or composition. Other insulating substances, such
as glass rods or ebonite forks, can be used as separators, but it is
claimed that the wood separator is not only more satisfactory, but that
in some unexplained way it actually helps to keep up the capacity of
the cell. The plates are placed in glass, or lead-lined wood or metal
boxes, and are suspended from above the dilute sulphuric acid with
which the cells are filled. A space is left below the plates for the
sediment which accumulates during the working of the cell.
In all but the smallest cells several pairs of plates are used, all the
positive plates being connected together and all the negative plates.
This gives the same effect as two very large plates, on the principle
of connecting in parallel, spoken of in Chapter IV. A single cell, of
whatever size, gives current at about two volts, and to get higher
voltages many cells are connected in series, as with primary cells.
The capacity is generally measured in ampere-hours. For instance, an
accumulator that will give a current of eight amperes for one hour, or
of four amperes for two hours, or one ampere for eight hours, is said
to have a capacity of eight ampere-hours.
Accumulators are usually charged from a dynamo or from the public
mains, and the electro-motive force of the charging current must be
not less than 2½ volts for each cell, in order to overcome the back
electro-motive force of the cells themselves. It is possible to charge
accumulators from primary cells, but except on a very small scale the
process is comparatively expensive. Non-polarizing cells, such as the
Daniell, must be used for this purpose.
The practical applications of accumulators are almost innumerable,
and year by year they increase. As the most important of these are
connected with the use of electricity for power and light, it will be
more convenient to speak of them in the chapters dealing with this
subject. Minor uses of accumulators will be referred to briefly from
time to time in other chapters.
CHAPTER VI
MAGNETS AND MAGNETISM
In many parts of the world there is to be found a kind of iron ore,
some specimens of which have the peculiar power of attracting iron,
and of turning to the north if suspended freely. This is called the
_lodestone_, and it has been known from very remote times. The name
Magnetism has been given to this strange property of the lodestone, but
the origin of the name is not definitely known. There is an old story
about a shepherd named Magnes, who lived in Phrygia in Asia Minor. One
day, while tending his sheep on Mount Ida, he happened to touch a dark
coloured rock with the iron end of his crook, and he was astonished and
alarmed to find that the rock was apparently alive, for it gripped his
crook so firmly that he could not pull it away. This rock is said to
have been a mass of lodestone, and some people believe that the name
magnet comes from the shepherd Magnes. Others think that the name is
derived from Magnesia, in Asia Minor, where the lodestone was found in
large quantities; while a third theory finds the origin in the Latin
word _magnus_, heavy, on account of the heavy nature of the lodestone.
The word lodestone itself comes from the Saxon _laeden_, meaning to
lead.
It is fairly certain that the Chinese knew of the lodestone long before
Greek and Roman times, and according to ancient Chinese records this
knowledge extends as far back as 2600 B.C. Humboldt, in his _Cosmos_,
states that a miniature figure of a man which always turned to the
south was used by the Chinese to guide their caravans across the plains
of Tartary as early as 1000 B.C. The ancient Greek and Roman writers
frequently refer to the lodestone. Thales, of whom we spoke in Chapter
I., believed that its mysterious power was due to the possession of a
soul, and the Roman poet Claudian imagined that iron was a food for
which the lodestone was hungry. Our limited space will not allow of an
account of the many curious speculations to which the lodestone has
given rise, but the following suggestion of one Famianus Strada, quoted
from Houston’s _Electricity in Every-Day Life_, is really too good to
be omitted.
“Let there be two needles provided of an equal Length and Bigness,
being both of them touched by the same lodestone; let the Letters of
the Alphabet be placed on the Circles on which they are moved, as the
Points of the Compass under the needle of the Mariner’s Chart. Let the
Friend that is to travel take one of these with him, first agreeing
upon the Days and Hours wherein they should confer together; at which
times, if one of them move the Needle, the other Needle, by Sympathy,
will move unto the same letter in the other instantly, though they are
never so far distant; and thus, by several Motions of the Needle to the
Letters, they may easily make up any Words or Sense which they have a
mind to express.” This is wireless telegraphy in good earnest!
The lodestone is a natural magnet. If we rub a piece of steel with a
lodestone we find that it acquires the same properties as the latter,
and in this way we are able to make any number of magnets, for the
lodestone does not lose any of its own magnetism in the process. Such
magnets are called artificial magnets. Iron is easier to magnetize than
steel, but it soon loses its magnetism, whereas steel retains it; and
the harder the steel the better it keeps its magnetism. Artificial
magnets, therefore, are made of specially hardened steel. In this
chapter we shall refer only to steel magnets, as they are much more
convenient to use than the lodestone, but it should be remembered that
both act in exactly the same way. We will suppose that we have a pair
of bar magnets, and a horse-shoe magnet, as shown in Fig. 13.
[Illustration: FIG. 13.--Horse-shoe and Bar Magnets, with Keepers.]
If we roll a bar magnet amongst iron filings we find that the filings
remain clinging to it in two tufts, one at each end, and that few or
none adhere to the middle. These two points towards which the filings
are attracted are called the poles of the magnet. Each pole attracts
filings or ordinary needles, and one or two experiments will show that
the attraction becomes evident while the magnet is still some little
distance away. If, however, we test our magnet with other substances,
such as wood, glass, paper, brass, etc., we see that there is no
attraction whatever.
If one of our bar magnets is suspended in a sort of stirrup of copper
wire attached to a thread, it comes to rest in a north and south
direction, and it will be noticed that the end which points to the
north is marked, either with a letter N or in some other way. This is
the north pole of the magnet, and of course the other is the south
pole. If now we take our other magnet and bring its north pole near
each pole of the suspended magnet in turn, we find that it repels the
other north pole, but attracts the south pole. Similarly, if we present
the south pole, it repels the other south pole, but attracts the north
pole. From these experiments we learn that both poles of a magnet
attract filings or needles, and that in the case of two magnets unlike
poles attract, but similar poles repel one another. It will be noticed
that this corresponds closely with the results of our experiments
in Chapter I., which showed that an electrified body attracts
unelectrified bodies, such as bits of paper or pith balls, and that
unlike charges attract, and similar charges repel each other. So far as
we have seen, however, a magnet attracts only iron or steel, whereas
an electrified body attracts any light substance. As a matter of fact,
certain other substances, such as nickel and cobalt, are attracted by a
magnet, but not so readily as iron and steel; while bismuth, antimony,
phosphorus, and a few other substances are feebly repelled.
The simplest method of magnetizing a piece of steel by means of one of
our bar magnets is the following: Lay the steel on the table, and draw
one pole of the magnet along it from end to end; lift the magnet clear
of the steel, and repeat the process several times, always starting at
the same end and treating each surface of the steel in turn. A thin,
flat bar of steel is the best for the purpose, but steel knitting
needles may be made in this way into useful experimental magnets.
We have seen that a magnet has two poles or points where the magnetism
is strongest. It might be thought that by breaking a bar magnet in the
middle we should get two small bars each with a single pole, but this
is not the case, for the two poles are inseparable. However many pieces
we break a magnet into, each piece is a perfect magnet having a north
and south pole. Thus while we can isolate a positive or a negative
charge of electricity, we cannot isolate north or south magnetism.
If we place the north pole of a bar magnet near to, but not touching, a
bar of soft iron, as in Plate II. _a_, we find that the latter becomes
a magnet, as shown by its ability to support filings; and that as
soon as the magnet is removed the filings drop off, showing that the
iron has lost its magnetism. If the iron is tested while the magnet
is in position it is found to have a south pole at the end nearer the
magnet, and a north pole at the end farther away; and if the magnet
is reversed, so as to bring its south pole nearer the iron, the poles
of the latter are found to reverse also. The iron has gained its new
properties by magnetic induction, and we cannot fail to notice the
similarity between this experiment and that in Fig. 2, Chapter II.,
which showed electro-static induction. A positively or a negatively
electrified body induces an opposite charge at the nearer end, and a
similar charge at the further end of a conductor, and a north or a
south pole of a magnet induces opposite polarity at the nearer end, and
a similar polarity at the further end of a bar of iron. In Chapter II.
we showed that the attraction of a pith ball by an electrified body
was due to induction, and from what we have just learnt about magnetic
induction the reader will have no difficulty in understanding why a
magnet attracts filings or needles.
[Illustration: PLATE II.
(_a_) EXPERIMENT TO SHOW MAGNETIC INDUCTION.]
[Illustration: (_b_) EXPERIMENT TO SHOW THE PRODUCTION OF MAGNETISM BY
AN ELECTRIC CURRENT.]
Any one who experiments with magnets must be struck with the distance
at which one magnet can influence filings or another magnet. If a layer
of iron filings is spread on a sheet of paper, and a magnet brought
gradually nearer from above, the filings soon begin to move about
restlessly, and when the magnet comes close enough they fly up to it
as if pulled by invisible strings. A still more striking experiment
consists in spreading filings thinly over a sheet of cardboard and
moving a magnet to and fro underneath the sheet. The result is most
amusing. The filings seem to stand up on their hind legs, and they
march about like regiments of soldiers. Here again invisible strings
are suggested, and we might wonder whether there really is anything of
the kind. Yes, there is. To put the matter in the simplest way, the
magnet acts by means of strings or lines of force, which emerge from it
in definite directions, and in a most interesting way we can see some
of these lines of force actually at work.
Place a magnet, or any arrangement of magnets, underneath a sheet of
glass, and sprinkle iron filings from a muslin bag thinly and evenly
all over the glass. Then tap the glass gently with a pencil, and the
filings at once arrange themselves in a most remarkable manner. All the
filings become magnetized by induction, and when the tap sets them free
for an instant from the friction of the glass they take up definite
positions under the influence of the force acting upon them. In this
way we get a map of the general direction of the magnetic lines of
force, which are our invisible strings.
Many different maps may be made in this way, but we have space for only
two. Plate III. _a_ shows the lines of two opposite poles. Notice how
they appear to stream across from one pole to the other. It is believed
that there is a tension along the lines of force not unlike that in
stretched elastic bands, and if this is so it is easy to see from the
figure why opposite poles attract each other.
Plate III. _b_ shows the lines of force of two similar poles. In
this case they do not stream from pole to pole, but turn aside as
if repelling one another, and from this figure we see why there is
repulsion between two similar poles. It can be shown, although in a
much less simple manner, that lines of electric force proceed from
electrified bodies, and in electric attraction and repulsion between
two charged bodies the lines of force take paths which closely resemble
those in our two figures. A space filled with lines of magnetic force
is called a _magnetic field_, and one filled with lines of electric
force is called an _electric field_.
A horse-shoe magnet, which is simply a bar of steel bent into the shape
of a horse-shoe before being magnetized, gradually loses its magnetism
if left with its poles unprotected, but this loss is prevented if the
poles are connected by a piece of soft iron. The same loss occurs with
a bar magnet, but as the two poles cannot be connected in this way it
is customary to keep two bar magnets side by side, separated by a strip
of wood; with opposite poles together and a piece of soft iron across
the ends. Such pieces of iron are called _keepers_, and Fig. 13 shows a
horse-shoe magnet and a pair of bar magnets with their keepers. It may
be remarked that a magnet never should be knocked or allowed to fall,
as rough usage of this kind causes it to lose a considerable amount of
its magnetism. A magnet is injured also by allowing the keeper to slam
on to it; but pulling the keeper off vigorously does good instead of
harm.
If a magnetized needle is suspended so that it is free to swing either
horizontally or vertically, it not only comes to rest in a north
and south direction, but also it tilts with its north-pointing end
downwards. If the needle were taken to a place south of the equator it
would still tilt, but the south-pointing end would be downwards. In
both cases the angle the needle makes with the horizontal is called the
_magnetic dip_.
[Illustration: PLATE III.
(_a_) LINES OF MAGNETIC FORCE OF TWO OPPOSITE POLES.
(_b_) LINES OF MAGNETIC FORCE OF TWO SIMILAR POLES.]
It is evident that a suspended magnetized needle would not invariably
come to rest pointing north and south unless it were compelled to do
so, and a little consideration shows that the needle acts as if it
were under the influence of a magnet. Dr. Gilbert of Colchester, of
whom we spoke in Chapter I., gave a great deal of time to the study
of magnetic phenomena, and in 1600 he announced what may be regarded
as his greatest discovery: _The terrestrial globe itself is a great
magnet_. Here, then, is the explanation of the behaviour of the
magnetized needle. The Earth itself is a great magnet, having its poles
near to the geographical north and south poles. But a question at once
suggests itself: “Since similar poles repel one another, how is it that
the north pole of a magnet turns towards the north magnetic pole of the
earth?” This apparent difficulty is caused by a confusion in terms. If
the Earth’s north magnetic pole really has north magnetism, then the
north-pointing end of a magnet must be a south pole; and on the other
hand, if the north-pointing end of a magnet has north magnetism, then
the Earth’s north magnetic pole must be really a south pole. It is a
troublesome matter to settle, but it is now customary to regard the
Earth’s north magnetic pole as possessing south magnetism, and the
south magnetic pole as possessing north magnetism. In this way the
north-pointing pole of a magnet may be looked upon as a true north
pole, and the south-pointing pole as a true south pole.
Magnetic dip also is seen to be a natural result of the Earth’s
magnetic influence. Here in England, for instance, the north magnetic
pole is much nearer than the south magnetic pole, and consequently its
influence is the stronger. Therefore a magnetized needle, if free to
do so, dips downwards towards the north. At any place where the south
magnetic pole is the nearer the direction of the dip of course is
reversed. If placed immediately over either magnetic pole the needle
would take up a vertical position, and at the magnetic equator it would
not dip at all, for the influence of the two magnetic poles would be
equal. A little study of Fig. 14, which represents a dipping needle
at different parts of the earth, will make this matter clearer. N and
S represent the Earth’s north and south magnetic poles, and the arrow
heads are the north poles of the needles.
[Illustration: FIG. 14.--Diagram to illustrate Magnetic Dip.]
Since the Earth is a magnet, we should expect it to be able to induce
magnetism in a bar of iron, just as our artificial magnets do, and
we can show that this is actually the case. If a steel poker is held
pointing to and dipping down towards the north, and struck sharply
with a piece of wood while in this position, it acquires magnetic
properties which can be tested by means of a small compass needle. It
is an interesting fact that iron pillars and railings which have been
standing for a long time in one position are found to be magnetized.
In the northern hemisphere the bases of upright iron pillars are north
poles, and their upper ends south poles, and in the southern hemisphere
the polarity is reversed.
The most valuable application of the magnetic needle is in the compass.
An ordinary pocket compass for inland use consists simply of a single
magnetized needle pivoted so as to swing freely over a card on which
are marked the thirty-two points of the compass. Ships’ compasses are
much more elaborate. As a rule a compound needle is used, consisting of
eight slender strips of steel, magnetized separately, and suspended
side by side. A compound needle of this kind is very much more reliable
than a single needle. The material of which the card is made depends
upon whether the illumination for night work is to come from above or
below. If the latter, the card must be transparent, and it is often
made of thin sheet mica; but if the light comes from above, the card is
made of some opaque material, such as very stout paper. The needle and
card are contained in a sort of bowl made of copper. In order to keep
this bowl in a horizontal position, however the ship may be pitching
and rolling, it is supported on gimbals, which are two concentric rings
attached to horizontal pivots, and moving in axes at right angles to
one another. Further stability may be obtained by weighting the bottom
of the bowl with lead. There are also liquid compasses, in which the
card is floated on the surface of dilute alcohol, and many modern
ships’ compasses have their movements regulated by a gyrostat.
The large amount of iron and steel used in the construction of modern
vessels has a considerable effect upon the compass needle, and unless
the compass is protected from this influence its readings are liable
to serious errors. The most satisfactory way of giving this protection
is by placing on each side of the compass a large globe of soft iron,
twelve or more inches in diameter.
On account of the fact that the magnetic poles of the Earth do not
coincide with the geographical north and south poles, a compass
needle seldom points exactly north and south, and the angle between
the magnetic meridian and the geographical meridian is called the
_declination_. The discovery that the declination varies in different
parts of the world was made by Columbus in 1492. For purposes of
navigation it is obviously very important that the declination at all
points of the Earth’s surface should be known, and special magnetic
maps are prepared in which all places having the same declination are
joined by a line.
It is an interesting fact that the Earth’s magnetism is subject to
variation. The declination and the dip slowly change through long
periods of years, and there are also slight annual and even daily
variations.
At one time magnets were credited with extraordinary effects upon
the human body. Small doses of lodestone, ground to powder and mixed
with water, were supposed to prolong life, and Paracelsus, a famous
alchemist and physician, born in Switzerland in 1493, believed in the
potency of lodestone ointment for wounds made with steel weapons. Baron
Reichenbach, 1788–1860, believed that he had discovered the existence
of a peculiar physical force closely connected with magnetism, and he
gave this force the name _Od_. It was supposed to exist everywhere,
and, like magnetism, to have two poles, positive and negative; the left
side of the body being od-positive, and the right side od-negative.
Certain individuals, known as “sensitives,” were said to be specially
open to its influence. These people stated that they saw strange
flickering lights at the poles of magnets, and that they experienced
peculiar sensations when a magnet was passed over them. Some of them
indeed were unable to sleep on the left side, because the north pole of
the Earth, being od-negative, had a bad effect on the od-negative left
side. The pretended revelations of these “sensitives” created a great
stir at the time, but now nobody believes in the existence of _Od_.
Professor Tyndall was once invited to a seance, with the object of
convincing him of the genuineness of spiritualism. He sat beside a
young lady who claimed to have spiritualistic powers, and his record of
his conversation with her is amusing. The Reichenbach craze was in full
swing at the time, and Tyndall asked if the lady could see any of the
weird lights supposed to be visible to “sensitives.”
“_Medium._--Oh yes; but I see the light around all bodies.
_I._--Even in perfect darkness?
_Medium._--Yes; I see luminous atmospheres round all people. The
atmosphere which surrounds Mr. R. C. would fill this room with
light.
_I._--You are aware of the effects ascribed by Baron Reichenbach to
magnets?
_Medium._--Yes; but a magnet makes me terribly ill.
_I._--Am I to understand that, if this room were perfectly dark, you
could tell whether it contained a magnet, without being informed
of the fact?
_Medium._--I should know of its presence on entering the room.
_I._--How?
_Medium._--I should be rendered instantly ill.
_I._--How do you feel to-day?
_Medium._--Particularly well; I have not been so well for months.
_I._--Then, may I ask you whether there is, at the present moment, a
magnet in my possession?
The young lady looked at me, blushed, and stammered, ‘No; I am not
_en rapport_ with you.’
_I sat at her right hand, and a left-hand pocket, within six inches
of her person, contained a magnet._”
Tyndall adds, “Our host here deprecated discussion as it ‘exhausted the
medium.’”
CHAPTER VII
THE PRODUCTION OF MAGNETISM BY ELECTRICITY
[Illustration: FIG. 15.--Diagram to illustrate Magnetic effect of an
Electric Current.]
In the previous chapter attention was drawn to the fact that there
are many close parallels between electric and magnetic phenomena, and
in this chapter it will be shown that magnetism can be produced by
electricity. In the year 1819 Professor Oersted, of the University of
Copenhagen, discovered that a freely swinging magnetized needle, such
as a compass needle, was deflected by a current of electricity flowing
through a wire. In Fig. 15, A, a magnetic needle is shown at rest in
its usual north and south direction, and over it is held a copper wire,
also pointing north and south. A current of electricity is now sent
through the wire, and the needle is at once deflected, Fig. 15, B. The
direction of the current is indicated by an arrow, and the direction
in which the needle has moved is shown by the two small arrows. If the
direction of the current is reversed, the needle will be deflected in
the opposite direction. From this experiment we see that the current
has brought magnetic influences into play, or in other words has
produced magnetism. If iron filings are brought near the wire while the
current is flowing, they are at once attracted and cling to the wire,
but as soon as the current is stopped they drop off. This shows us that
the wire itself becomes a magnet during the passage of the current, and
that it loses its magnetism when the current ceases to flow.
[Illustration: FIG. 16.--Magnetic Field round wire conveying a Current.]
Further, it can be shown that two freely moving parallel wires
conveying currents attract or repel one another according to the
direction of the currents. If both currents are flowing in the same
direction the wires attract one another, but if the currents flow in
opposite directions the wires repel each other. Fig. 16 shows the
direction of the lines of force of a wire conveying a current and
passed through a horizontal piece of cardboard covered with a thin
layer of iron filings; and from this figure it is evident that the
passage of the current produces what we may call magnetic whirls round
the wire.
A spiral of insulated wire through which a current is flowing shows
all the properties of a magnet, and if free to move it comes to rest
pointing north and south. It is attracted or repelled by an ordinary
magnet according to the pole presented to it and the direction of the
current, and two such spirals show mutual attraction and repulsion.
A spiral of this kind is called a _solenoid_, and in addition to the
properties already mentioned it has the peculiar power of drawing
or sucking into its interior a rod of iron. Solenoids have various
practical applications, and in later chapters we shall refer to them
again.
If several turns of cotton-covered wire are wound round an iron rod,
the passing of a current through the wire makes the rod into a magnet
(Plate II. _b_), but the magnetism disappears as soon as the current
ceases to flow. A magnet made by the passage of an electric current
is called an _electro-magnet_, and it has all the properties of the
magnets mentioned in the previous chapter. A bar of steel may be
magnetized in the same way, but unlike the iron rod it retains its
magnetism after the current is interrupted. This provides us with
a means of magnetizing a piece of steel much more strongly than is
possible by rubbing with another magnet. Steel magnets, which retain
their magnetism, are called _permanent_ magnets, as distinguished from
electro-magnets in which soft iron is used, so that their magnetism
lasts only as long as the current flows.
Electro-magnets play an extremely important part in the harnessing
of electricity; in fact they are used in one form or another in
almost every kind of electrical mechanism. In later chapters many of
these uses will be described, and here we will mention only the use
of electro-magnets for lifting purposes. In large engineering works
powerful electro-magnets, suspended from some sort of travelling
crane, are most useful for picking up and carrying about heavy masses
of metal, such as large castings. No time is lost in attaching the
casting to the crane; the magnet picks it up directly the current
is switched on, and lets it go the instant the current is stopped.
In any large steel works the amount of scrap material produced is
astonishingly great, hundreds of tons of turnings and similar scrap
accumulating in a very short time. A huge mound of turnings is
awkward to deal with by ordinary manual labour, but a combination of
electro-magnet and crane solves the difficulty completely, lifting
and loading the scrap into carts or trucks at considerable speed, and
without requiring much attention.
Some time ago a disastrous fire occurred at an engineering works in the
Midlands, the place being almost entirely burnt out. Amongst the débris
was, of course, a large amount of metal, and as this was too valuable
to be wasted, an electro-magnet was set to work on the wreckage. The
larger pieces of metal were picked up in the ordinary way, and then the
remaining rubbish was shovelled against the face of the magnet, which
held on to the metal but dropped everything else, and in this way some
tons of metal were recovered.
The effect produced upon a magnetized needle by a current of
electricity affords a simple means of detecting the existence of such
a current. An ordinary pocket compass can be made to show the presence
of a moderate current, but for the detection of extremely small
currents a much more sensitive apparatus is employed. This is called
a _galvanometer_, and in its simplest form it consists essentially of
a delicately poised magnetic needle placed in the middle of a coil of
several turns of wire. The current thus passes many times round the
needle, and this has the effect of greatly increasing the deflection
of the needle, and hence the sensitiveness of the instrument. Although
such an arrangement is generally called a galvanometer, it is really
a galvanoscope, for it does not measure the current but only shows its
presence.
We have seen that electro-motive force is measured in volts, and that
the definition of a volt is that electro-motive force which will cause
a current of one ampere to flow through a conductor having a resistance
of one ohm. If we make a galvanometer with a long coil of very thin
wire having a high resistance, the amount of current that will flow
through it will be proportionate to the electro-motive force. Such a
galvanometer, fitted with a carefully graduated scale, in this way
will indicate the number of volts, and it is called a _voltmeter_.
If we have a galvanometer with a short coil of very thick wire, the
resistance put in the way of the current is so small that it may be
left out of account, and by means of a graduated scale the number of
amperes may be shown; such an instrument being called an _amperemeter_,
or _ammeter_.
For making exact measurements of electric currents the instruments just
described are not suitable, as they are not sufficiently accurate; but
their working shows the principle upon which currents are measured. The
actual instruments used in electrical engineering and in scientific
work are unfortunately too complicated to be described here.
CHAPTER VIII
THE INDUCTION COIL
The voltaic cell and the accumulator provide us with currents of
electricity of considerable volume, but at low pressure or voltage.
For many purposes, however, we require a comparatively small amount of
current at very high pressure, and in such cases we use an apparatus
called the _induction coil_. Just as an electrified body and a magnet
will induce electrification and magnetism respectively, so a current
of electricity will induce another current; and an induction coil is
simply an arrangement by which a current in one coil of wire is made to
induce a current in another coil.
Suppose we have two coils of wire placed close together, one connected
to a battery of voltaic cells, with some arrangement for starting and
stopping the current suddenly, and the other to a galvanometer. As
soon as we send the current through the first coil, the needle of the
galvanometer moves, showing that there is a current flowing through
the second coil; but the needle quickly comes back to its original
position, showing that this current was only momentary. So long as we
keep the current flowing through the first coil the galvanometer shows
no further movement, but as soon as we stop the current the needle
again shows by its movements that another momentary current has been
produced in the second coil. This experiment shows us that a current
induces another current only at the instant it is started or stopped,
or, as we say, at the instant of making or breaking the circuit.
The coil through which we send the battery current is called the
“primary coil,” and the one in which a current is induced is called the
“secondary coil.” The two momentary currents in the secondary coil do
not both flow in the same direction. The current induced on making the
circuit flows in a direction opposite to that of the current in the
primary coil; and the current induced on breaking the circuit flows
in the same direction as that in the primary coil. If the two coils
are exactly alike, the induced current will have the same voltage as
the primary current; but if the secondary coil has twice as many turns
of wire as the primary coil, the induced current will have twice the
voltage of the primary current. In this way, by multiplying the turns
of wire in the secondary coil, we can go on increasing the voltage of
the induced current, and this is the principle upon which the induction
coil works.
We may now describe the construction of such a coil. The primary coil
is made of a few turns of thick copper wire carefully insulated, and
inside it is placed a core consisting of a bundle of separate wires of
soft iron. Upon this coil, but carefully insulated from it, is wound
the secondary coil, consisting of a great number of turns of very
fine wire. In large induction coils the secondary coil has thousands
of times as many turns as the primary, and the wire forming it may be
more than a hundred miles in length. The ends of the secondary coil are
brought to terminals so that they can be connected up to any apparatus
as desired.
[Illustration: FIG. 17.--Diagram showing working of Contact-Breaker for
Induction Coil.]
In order that the induced currents shall follow each other in quick
succession, some means of rapidly making and breaking the circuit is
required, and this is provided by an automatic contact breaker. It
consists of a small piece of soft iron, A, Fig. 17, fixed to a spring,
B, having a platinum tip at C. The adjustable screw, D, also has a
platinum tip, E. Normally the two platinum tips are just touching one
another, and matters are arranged so that their contact completes
the circuit. When the apparatus is connected to a suitable battery a
current flows through the primary coil, and the iron core, F, becomes
an electro-magnet, which draws A towards it. The platinum tips are thus
no longer in contact and the circuit is broken. Immediately this occurs
the iron core loses its magnetism and ceases to attract A, which is
then moved back again by the spring B, so that the platinum tips touch,
the circuit is once more completed, and the process begins over again.
All this takes place with the utmost rapidity, and the speed at which
the contact-breaker works is so great as to produce a musical note.
There are many other types of contact-breakers, but in every case the
purpose is the same, namely, to make and break the primary circuit as
rapidly as possible.
The efficiency of the coil is greatly increased by a condenser which
is inserted in the primary circuit. It consists of alternate layers of
tinfoil and paraffined paper, and its action is like that of a Leyden
jar. A switch is provided to turn the battery current on or off, and
there is also a reversing switch or commutator, by means of which the
direction of the current may be reversed. The whole arrangement is
mounted on a suitable wooden base, and its general appearance is shown
in Fig. 18.
[Illustration:
_By permission of_] [_Harry W. Cox, Ltd._
FIG. 18.--Typical Induction Coil.]
By means of a large induction coil we can obtain a voltage hundreds
or even thousands of times greater than that of the original battery
current, but on account of the great resistance of a very long, thin
wire, the amperage is much smaller. The induction coil produces a
rapid succession of sparks, similar to those obtained from a Wimshurst
machine. A coil has been constructed capable of giving sparks 42½
inches in length, and having a secondary coil with 340,000 turns of
wire, the total length of the wire being 280 miles. Induction coils are
largely employed for scientific purposes, and they are used in wireless
telegraphy and in the production of X-rays.
The principle of the induction coil can be applied also to the lowering
of the voltage of a current. If we make the secondary coil with less,
instead of more turns of wire than the primary coil, the induced
current will be of lower voltage than the primary current, but its
amperage will be correspondingly higher. This fact is taken advantage
of in cases where it is desirable to transform a high voltage current
from the public mains down to a lower voltage current of greater
amperage.
CHAPTER IX
THE DYNAMO AND THE ELECTRIC MOTOR
Most of my readers will have seen the small working models of electric
tramcars which can be bought at any electrical supply stores. These
usually require a current of about one ampere at three or four volts.
If we connect such a car to the battery recommended for it, and keep
it running continuously, we find that the battery soon begins to
show signs of exhaustion. Now if we imagine our little car increased
to the size of an electric street car, and further imagine, say, a
hundred such cars carrying heavy loads day after day from morning to
night, we shall realize that a battery of cells capable of supplying
the current necessary to run these cars would be so colossal as to be
utterly impracticable. We therefore must look beyond the voltaic cell
for a source of current for such a purpose, and this source we find in
a machine called the “dynamo,” from the Greek word _dynamis_, meaning
force.
Oersted’s discovery of the production of magnetism by electricity
naturally suggested the possibility of producing electricity from
magnetism. In the year 1831 one of the most brilliant of our British
scientists, Michael Faraday, discovered that a current of electricity
could be induced in a coil of wire either by moving the coil towards
or away from a magnet, or by moving a magnet towards or away from the
coil. This may be shown in a simple way by connecting the ends of a
coil of insulated wire to a galvanometer, and moving a bar magnet in
and out of the coil; when the galvanometer shows that a current is
induced in the coil on the insertion of the magnet, and again on its
withdrawal. We have seen that a magnet is surrounded by a field of
magnetic force, and Faraday found that the current was induced when the
lines of force were cut across.
Utilizing this discovery Faraday constructed the first dynamo, which
consisted of a copper plate or disc rotated between the poles of a
powerful horse-shoe magnet, so as to cut the lines of force. The
current flowed either from the shaft to the rim, or _vice versa_,
according to the direction of rotation; and it was conducted away by
means of two wires with spring contacts, one pressing against the
shaft, and the other against the circumference of the disc. This
machine was miserably inefficient, but it was the very first dynamo,
and from it have been slowly evolved the mighty dynamos used to-day
in electric power stations throughout the world. There is a little
story told of Faraday which is worth repeating even if it is not true.
Speaking of his discovery that a magnet could be made to produce
an electric current, a lady once said to him, “This is all very
interesting, but what is the use of it?” “Madam,” replied Faraday,
“what is the use of a baby?” In Faraday’s “baby” dynamo, as in all
others, some kind of power must be used to produce the necessary
motion, so that all dynamos are really machines for converting
mechanical energy into electrical energy.
The copper disc in this first dynamo did not prove satisfactory, and
Faraday soon substituted for it rotating coils of wire. In 1832 a
dynamo was constructed in which a length of insulated wire was wound
upon two bobbins having soft iron cores, and a powerful horse-shoe
magnet was fixed to a rotating spindle in such a position that its
poles faced the cores of the bobbins. This machine gave a fair
current, but it was found that the magnet gradually lost its magnetism
on account of the vibration caused by its rotation. The next step
was to make the magnet a fixture, and to rotate the bobbins of wire.
This was a great improvement, and the power of machines built on this
principle was much increased by having a number of rotating coils
and several magnets. One such machine had 64 separate coils rotating
between the poles of 40 large magnets. Finally, permanent magnets were
superseded by electro-magnets, which gave a much more powerful field of
force.
[Illustration: FIG. 19.--Diagram showing principle of Dynamo producing
Alternating Current.]
Having seen something of the underlying principle and of the history
of the dynamo, we must turn our attention to its actual working. Fig.
19 is a rough representation of a dynamo in its simplest form. The
two poles of the magnet are shown marked north and south, and between
them revolves the coil of wire A¹ A², mounted on a spindle SS. This
revolving coil is called the armature. To each of the insulated rings
RR is fixed one end of the coil, and BB are two brushes of copper or
carbon, one pressing on each ring. From these brushes the current is
led away into the main circuit, and in this case we may suppose that
the current is used to light a lamp.
In speaking of the induction coil we saw that the currents induced on
making and on breaking the circuit flowed in opposite directions, and
similarly, Faraday found that the currents induced in a coil of wire on
inserting and on withdrawing his magnet flowed in opposite directions.
In the present case the magnet is stationary and the coil moves, but
the effect is just the same. Now if we suppose the armature to be
revolving in a clockwise direction, then A¹ is descending and entering
the magnetic field in front of the north pole, consequently a current
is induced in the coil, and of course in the main circuit also, in one
direction. Continuing its course, A¹ passes away from this portion
of the magnetic field, and thus a current is induced in the opposite
direction. In this way we get a current which reverses its direction
every half-revolution, and such a current is called an alternating
current. If, as in our diagram, there are only two magnetic poles, the
current flows backwards and forwards once every revolution, but by
using a number of magnets, arranged so that the coil passes in turn the
poles of each, it can be made to flow backwards and forwards several
times. One complete flow backwards and forwards is called a period, and
the number of periods per second is called the periodicity or frequency
of the current. A dynamo with one coil or set of coils gives what is
called “single-phase” current, that is, a current having one wave which
keeps flowing backwards and forwards. If there are two distinct sets
of coils we get a two-phase current, in which there are two separate
waves, one rising as the other falls. Similarly, by using more sets of
coils, we may obtain three-phase or polyphase currents.
[Illustration: FIG. 20.--Diagram showing principle of Dynamo producing
Continuous Current.]
Alternating current is unsuitable for certain purposes, such as
electroplating; and by making a small alteration in our dynamo we get
a continuous or direct current, which does not reverse its direction.
Fig. 20 shows the new arrangement. Instead of the two rings in Fig.
19, we have now a single ring divided into two parts, each half being
connected to one end of the revolving coil. Each brush, therefore,
remains on one portion of the ring for half a revolution, and then
passes over on to the other portion. During one half-revolution we
will suppose the current to be flowing from brush B¹ in the direction
of the lamp. Then during the next half-revolution the current flows in
the opposite direction; but brush B¹ has passed on to the other half of
the ring, and so the current is still leaving by it. In this way the
current must always flow in the same direction in the main circuit,
leaving by brush B¹ and returning by brush B². This arrangement for
making the alternating current into a continuous current is called a
_commutator_.
[Illustration: PLATE IV.
_By permission of_ _Lancashire Dynamo & Motor Co. Ltd._
A TYPICAL DYNAMO AND ITS PARTS.]
In actual practice a dynamo has a set of electro-magnets, and the
armature consists of many coils of wire mounted on a core of iron,
which has the effect of concentrating the lines of force. The armature
generally revolves in small dynamos, but in large ones it is usually a
fixture, while the electro-magnets revolve. Plate IV. shows a typical
dynamo and its parts.
As we saw in an earlier chapter, an electro-magnet has magnetic powers
only while a current is being passed through its winding, and so some
means of supplying current to the electro-magnets in a dynamo must
be provided. It is a remarkable fact that it is almost impossible to
obtain a piece of iron which has not some traces of magnetism, and
so when a dynamo is first set up there is often sufficient magnetism
in the iron of the electro-magnets to produce a very weak field. The
rapid cutting of the feeble lines of force of this field sets up a
weak current, which, acting upon the electro-magnets, gradually brings
them up to full strength. Once the dynamo is generating current it
keeps on feeding its magnets by sending either the whole or a part of
its current through them. After it has once been set going the dynamo
is always able to start again, because the magnet cores retain enough
magnetism to set up a weak field. If there is not enough magnetism in
the cores to start a dynamo for the first time, a current from some
outside source is sent round the magnets.
The foregoing remarks apply to continuous current dynamos only.
Alternating current can be used for exciting electro-magnets, but in
this case the magnetic field produced is alternating also, so that each
pole of the magnet has north and south magnetism alternately. This will
not do for dynamo field magnets, and therefore an alternating current
dynamo cannot feed its own magnets. The electro-magnets in such dynamos
are supplied with current from a separate continuous current dynamo,
which may be of quite small size.
It is a very interesting fact that electric current can be generated
by a dynamo in which the earth itself is used to provide the magnetic
field, no permanent or electro-magnets being used at all. A simple form
of dynamo of this kind consists of a rectangular loop of copper wire
rotating about an axis pointing east and west, so that the loop cuts
the lines of force of the Earth’s magnetic field.
The dynamo provides us with a constant supply of electric current, but
this current is no use unless we can make it do work for us. If we
reverse the usual order of things in regard to a dynamo, and supply
the machine with current instead of mechanical power, we find that
the armature begins to revolve rapidly, and the machine is no longer
a dynamo, but has become an electric motor. This shows us that an
electric motor is simply a dynamo reversed. Let us suppose that we wish
to use the dynamo in Fig. 20 as a motor. In order to supply the current
we will take away the lamp and substitute a second continuous-current
dynamo. We know from Chapter VII. that when a current is sent through
a coil of wire the coil becomes a magnet with a north and a south
pole. The coil in our dynamo becomes a magnet as soon as the current
is switched on, and the attraction between its poles and the opposite
poles of the magnet causes it to make half a revolution. At this point
the commutator reverses the current, and consequently the polarity of
the coil, so that there is now repulsion where previously there was
attraction, and the coil makes another half-revolution. So the process
goes on until the armature attains a very high speed. In general
construction there is practically no difference between a dynamo and a
motor, but there are differences in detail which adapt each to its own
particular work. By making certain alterations in their construction
electric motors can be run with alternating current.
The fact that a dynamo could be reversed and run as a motor was known
probably as early as 1838, but the great value of this reversibility
does not seem to have been realized until 1873. At an industrial
exhibition held at Vienna in that year, it so happened that a workman
or machinery attendant connected two cables to a dynamo which was
standing idle, and he was much surprised to find that it at once began
to revolve at a great speed. It was then seen that the cables led to
another dynamo which was running, and that the current from this source
had made the first dynamo into a motor. There are many versions of
this story, but the important point in all is that this was the first
occasion on which general attention was drawn to the possibilities of
the electric motor.
The practical advantages afforded by the electric motor are many and
great. Once we have installed a sufficiently powerful dynamo and a
steam or other engine to drive it, we can place motors just where
they are required, either close to the dynamo or miles away, driving
them simply by means of a connecting cable. In factories, motors can
be placed close to the machines they are required to drive, anywhere
in the building, thus doing away with all complicated and dangerous
systems of shafting and belts. In many cases where it would be either
utterly impossible or at least extremely inconvenient to use any form
of steam, gas, or oil engine, electric motors can be employed without
the slightest difficulty. In order to realize this, one only has to
think of the positions in which electrically-driven ventilating fans
are placed, or of the unpleasantly familiar electric drill of the
dentist. An electric motor is small and compact, gives off no fumes and
practically no heat, makes very little noise, is capable of running for
very long periods at high speed and with the utmost steadiness, and
requires extremely little attention.
CHAPTER X
ELECTRIC POWER STATIONS
It is apparently a very simple matter to fit up a power station with
a number of very large dynamos driven by powerful engines, and to
distribute the current produced by these dynamos to all parts of a town
or district by means of cables, but as a matter of fact it is a fairly
complicated engineering problem. First of all the source of power for
driving the dynamos has to be considered. In private and other small
power plants, gas, petrol or oil engines are generally used, but for
large stations the choice lies between steam and water power. In this
country steam power is used almost exclusively. Formerly the ordinary
reciprocating steam engines were always employed, and though these are
still in very extensive use, they are being superseded in many cases
by steam turbines. The turbine is capable of running at higher speeds
than the reciprocating engine, and at the greatest speeds it runs with
a great deal less noise, and with practically no vibration at all.
More than this, turbines take up much less room, and require less oil
and attendance. The turbines are coupled directly to the dynamos, so
that the two machines appear almost as one. In the power station shown
on Plate V. a number of alternating current dynamos coupled to steam
turbines are seen.
A large power station consumes enormous quantities of coal, and
for convenience of supply it is situated on the bank of a river or
canal, or, if neither of these is available, as close to the railway
as possible. The unloading of the coal barges or trucks is done
mechanically, the coal passing into a large receiving hopper. From here
it is taken to another hopper close to the furnaces by means of coal
elevators and conveyors, which consist of a number of buckets fixed
at short intervals on an endless travelling chain. From the furnace
hopper the coal is fed into the furnaces by mechanical stokers, and the
resulting ash and clinker falls into a pit below the furnaces, from
which it is carted away.
The heat produced in the furnaces is used to generate steam, and
from the boilers the steam passes to the engines along a steam pipe.
After doing its work in the engines, the steam generally passes to a
condenser, in which it is cooled to water, freed from oil and grease,
and returned to the boilers to be transformed once more into steam.
As this water from the condenser is quite warm, less heat is required
to raise steam from it than would be the case if the boiler supply
were kept up with cold water. The power generated by the engines is
used to drive the dynamos, and stout copper cables convey the current
from these to what are called “bus” bars. There are two of these,
one receiving the positive cable from the dynamos, and the other the
negative cable, and the bars run from end to end of a large main
switchboard. From this switchboard the current is distributed by other
cables known as feeders.
The nature of the current generated at a power station is determined to
a great extent by the size of the district to be supplied. Generally
speaking, where the current is not to be transmitted beyond a radius
of about two miles from the station, continuous current is generated;
while alternating current is employed for the supply of larger areas.
In some cases both kinds of current are generated at one station.
[Illustration: PLATE V.
_By permission of_ _C. A. Parsons & Co._
LOTS ROAD ELECTRIC POWER STATION, CHELSEA.]
If continuous current is to be used, it is generated usually at a
pressure of from 400 to 500 volts, the average being about 440 volts;
and the supply is generally on what is known as the three-wire system.
Three separate wires are employed. The two outer wires are connected
respectively to the positive and the negative bus bars running along
the main switchboard, these bars receiving positive or negative current
directly from the dynamos. The outer wires therefore carry current at
the full voltage of the system. Between them is a third and smaller
wire, connected to a third bar, much smaller than the outer bars, and
known as the mid-wire bar. This bar is not connected to the dynamos,
but to earth, by means of a large plate of copper sunk into the ground.
Connexion between the mid-wire bar and the outer bars is made by two
machines called “balancers,” one connecting the mid-wire bar and the
positive bus bar, and the other the mid-wire bar and the negative bus
bar. If the pressure between the outer bars is 440 volts, then the
pressure between the mid-wire bar and either of the outer bars will be
220 volts, that is just half.
The balancers serve the purpose of balancing the voltage on each side,
and they are machines capable of acting either as motors or dynamos. In
order to comply with Board of Trade regulations, electric appliances of
all kinds intended for ordinary domestic purposes, including lamps, and
heating and cooking apparatus, are supplied with current at a pressure
not exceeding 250 volts. In a system such as we are describing, all
these appliances are connected between the mid-wire and one or other of
the outer wires, thus receiving current at 220 volts. In practice it is
impossible to arrange matters so that the lamps and other appliances
connected with the positive side of the system shall always take the
same amount of current as those connected with the negative side, and
there is always liable to be a much greater load on one side or the
other. If, for instance, a heavy load is thrown on the negative side,
the voltage on that side will drop. The balancer on the positive side
then acts as an electric motor, drives the balancer on the negative
side as a dynamo, and thus provides the current required to raise the
voltage on the negative side until the balance is restored. The working
of the balancers, which need not be described in further detail, is
practically automatic. Electric motors, for driving electric trams or
machinery of any kind, are connected between the outer wires, so that
they receive the full 440 volts of the system.
In any electric supply system the demand for current does not remain
constant, but fluctuates more or less. For instance, in a system
including an electric tramway, if a car breaks down and remains a
fixture for a short time, all cars behind it are held up, and a long
line of cars is quickly formed. When the breakdown is repaired, all
the cars start practically at the same instant, and consequently a
sudden and tremendous demand for current is made. In a very large
tramway system in a fairly level city, the fluctuations in the demand
for current, apart from accidents, are not very serious, for they tend
to average themselves; but in a small system, and particularly if the
district is hilly, the fluctuations are very great, and the current
demand may vary as much as from 400 to 2000 amperes. Again, in a
system supplying power and light, the current demand rises rapidly as
the daylight fails on winter afternoons, because, while workshop and
other motors are still in full swing, thousands of electric lamps are
switched on more or less at the same time. The power station must be
able to deal with any exceptional demands which are likely to occur,
and consequently more current must be available than is actually
required under average conditions. Instead of having generating
machinery large enough to meet all unusual demands, the generators at
a station using continuous current may be only of sufficient size to
supply a little more than the average demand, any current beyond this
being supplied by a battery of storage cells. The battery is charged
during periods when the demand for current is small, and when a heavy
load comes on, the current from the battery relieves the generators of
the sudden strain. To be of any service for such a purpose the storage
battery of course must be very large. Plate VI. shows a large battery
of no cells, and some idea of the size of the individual cells may be
obtained from the fact that each weighs about 3900 lb.
Alternating current is produced at almost all power stations supplying
large districts. It is generated at high pressure, from 2000 volts
upwards, the highest pressure employed in this country being about
11,000 volts. Such pressures are of course very much too high for
electric lamps or motors, and the object of generating current of
this kind is to secure the greatest economy in transmission through
the long cables. Electric energy is measured in watts, the watts
being obtained by multiplying together the pressure or voltage of the
current, and its rate of flow or amperage. From this it will be seen
that, providing the product of voltage and amperage remains the same,
it makes no difference, so far as electric energy is concerned, whether
the current be of high voltage and low amperage, or of low voltage and
high amperage. Now in transmitting a current through a long cable,
there is a certain amount of loss due to the heating of the conductor.
This heating is caused by the current flow, not by the pressure; and
the heavier the current, the greater the heating, and the greater the
loss. This being so, it is clear that by decreasing the current flow,
and correspondingly increasing the pressure, the loss in transmission
will be reduced; and this is why alternating current is generated at
high pressure when it is to be transmitted to a distance.
The kind of alternating current generated is usually that known as
three-phase current. Formerly single-phase current was in general
use, but it has been superseded by three-phase current because the
latter is more economical to generate and to distribute, and also more
satisfactory for electric motors. The actual voltage of the current
sent out from the station varies according to the distance to which the
current is to be conveyed. In the United States and in other countries
where current has to be conveyed to places a hundred or even more miles
from the station, pressures as high as 120,000 volts are in use. It
is possible to produce alternating current at such pressures directly
from the dynamos, but in practice this is never done, on account of the
great liability to breakdown of the insulation. Instead, the current
is generated at from 2000 to 10,000 or 11,000 volts, and raised to the
required pressure, before leaving the station, by means of a step-up
transformer. We have seen that an induction coil raises, or steps up,
the voltage of the current supplied to it. A step-up transformer works
on the same principle as the induction coil, and in passing through
it the current is raised in voltage, but correspondingly lowered in
amperage. Of course, if the pressure of the current generated by the
dynamos is already sufficiently high to meet the local requirements,
the transformer is not used.
[Illustration: PLATE VI.
_By permission of_ _Chloride Electrical Storage Co. Ltd._
POWER STATION BATTERY OF ACCUMULATORS.]
For town supply the current from the power station is led along
underground cables to a number of sub-stations, situated in different
parts of the town, and generally underground. At each sub-station the
current passes through a step-down transformer, which also acts on the
principle of the induction coil, but in the reverse way, so that the
voltage is lowered instead of being raised. From the transformer
the current emerges at the pressure required for use, but it is still
alternating current; and if it is desired to have a continuous-current
supply this alternating current must be converted. One of the simplest
arrangements for this purpose consists of an electric motor and a
dynamo, the two being coupled together. The motor is constructed to
run on the alternating current from the transformer, and it drives the
dynamo, which is arranged to generate continuous current. There is also
a machine called a “rotary converter,” which is largely used instead
of the motor generator. This machine does the work of both motor and
dynamo, but its action is too complicated to be described here. From
the sub-stations the current, whether converted or not, is distributed
as required by a network of underground cables.
In many parts of the world, especially in America, water power is
utilized to a considerable extent instead of steam for the generation
of electric current. The immense volume of water passing over the Falls
of Niagara develops energy equal to about seven million horse-power,
and a small amount of this energy, roughly about three-quarters
of a million horse-power, has been harnessed and made to produce
electric current for light and power. The water passes down a number
of penstocks, which are tubes or tunnels about 7 feet in diameter,
lined with brick and concrete; and at the bottom of these tubes are
placed powerful water turbines. The falling water presses upon the
vanes of the turbines, setting them revolving at great speed, and the
power produced in this way is used to drive a series of very large
alternating current dynamos. The current is conveyed at a pressure
of about 60,000 volts to various towns within a radius of 200 or 300
miles, and it is anticipated that before very long the supply will
be extended to towns still more distant. Many other American rivers
have been harnessed in a similar way, though not to the same extent;
and Switzerland and Norway are utilizing their water power on a
rapidly increasing scale. In England, owing to the abundance of coal,
little has been done in this direction. Scotland is well favoured in
the matter of water power, and it is estimated that the total power
available is considerably more than enough to run the whole of the
railways of that country. Very little of this power has been utilized
however, and the only large hydro-electric installation is the one at
Kinlochleven, in Argyllshire. It is a mistake to suppose that water
power means power for nothing, but taking things all round the cost of
water power is considerably lower than that of steam.
CHAPTER XI
ELECTRICITY IN LOCOMOTION
The electric tramcar has become such a necessary feature of our
everyday life that it is very difficult to realize how short a time
it has been with us. To most of us a horse-drawn tramcar looks like a
relic of prehistoric times, and yet it is not so many years since the
horse tram was in full possession of our streets. Strikes of tramway
employees are fortunately rare events, but a few have occurred during
the past two or three years in Leeds and in other towns, and they have
brought home to us our great dependence upon the electric tram. During
the Leeds strike the streets presented a most curious appearance, and
the city seemed to have made a jump backward to fifty years ago. Every
available article on wheels was pressed into service to bring business
men into the city from the outlying districts, and many worthy citizens
were seen trying to look dignified and unconcerned as they jogged
along in conveyances which might have come out of the Ark. On such an
occasion as this, if we imagine the electric light supply stopped also,
we can form some little idea of our indebtedness to those who have
harnessed electricity and made it the greatest power of the twentieth
century.
There are three distinct electric tramway systems; the trolley or
overhead system, the surface contact system, and the conduit system.
The trolley system has almost driven the other two from the field, and
it is used almost exclusively throughout Great Britain and Ireland.
On the Continent and in the United States the conduit system still
survives, but probably it will not be long before the trolley system is
universally employed.
The superiority of the trolley system lies in the fact that it is
cheaper to construct and to maintain than the other two, and also in
its much greater reliability under all working conditions. The overhead
wire is not one continuous cable, but is divided into sections of about
half a mile in length, each section being supplied with current from a
separate main. At each point where the current is fed to the trolley
wire a sort of metal box may be seen at the side of the street. These
boxes are called “feeder pillars,” and each contains a switch by means
of which the current can be cut off from that particular section, for
repairing or other purposes. Above the car is fixed an arm provided
with a trolley wheel which runs along the wire, and this wheel takes
the current from the wire. From the wheel the current passes down
the trolley arm to the controller, which is operated by the driver,
and from there to the motors beneath the car. Leaving the motors it
passes to the wheels and then to the rails, from which it is led off
at intervals by cables and so returned to the generating station. The
current carried by the rails is at a pressure of only a few volts, so
that there is not the slightest danger of shock from them. There are
generally two electric motors beneath the car, and the horse-power of
each varies from about fifteen to twenty-five.
The controller consists mainly of a number of graduated resistances. To
start the car the driver moves a handle forward notch by notch, thus
gradually cutting out the resistance, and so the motors receive more
and more current until they are running at full speed. The movement of
the controller handle also alters the connexion of the motors. When
the car is started the motors are connected in series, so that the full
current passes through each, while the pressure is divided between
them; but when the car is well on the move the controller connects the
motors in parallel, so that each receives the full pressure of the
current.
The conduit and surface contact systems are much the same as the
trolley system except in the method of supplying the current to the
cars. In the conduit system two conductors conveying the current are
placed in an underground channel or conduit of concrete strengthened
by iron yokes. The top of the conduit is almost closed in so as to
leave only a narrow slot, through which passes the current collector of
the car. This current collector, or “plough” as it is called, carries
two slippers which make contact with the conductors, and thus take
current from them. In this system the current returns along one of
the conductors, so that no current passes along the track rails. This
is the most expensive of the three systems, both in construction and
maintenance.
The surface contact or stud system is like the conduit system in having
conductors placed in a sort of underground trough, but in this case
contact with the conductors is made by means of metal studs fixed at
intervals in the middle of the track. The studs are really the tops of
underground boxes each containing a switch, which, when drawn up to a
certain position, connects the stud to the conductors. These switches
are arranged to be moved by magnets fixed beneath the car, and thus
when the car passes over a stud the magnets work the switch and connect
the stud to the conductors, so that the stud is then “alive.” The
current is taken from the studs by means of sliding brushes or skates
which are carried by the car. The studs are thus alive only when the
car is passing over them, and at all other times they are dead, and
not in any way dangerous.
The weight and speed of electric cars make it important to have a
thoroughly reliable system of brakes. First of all there are ordinary
mechanical brakes, which press against the wheels. Then there are
electro-magnetic slipper brakes which press on the rails instead of on
the wheels of the car. These brakes are operated by electro-magnets
of great power, the current necessary to excite the magnets being
taken from the motors. Finally there is a most interesting and
ingenious method of regenerative control. Before a car can be stopped
after it has attained considerable speed a certain amount of energy
has to be got rid of in some way. With the ordinary mechanical or
electro-magnetic brakes this energy is wasted, but in the regenerative
method it is turned into electric current, which is sent back into the
circuit. If an electric motor is supplied with mechanical power instead
of electric current it becomes a dynamo, and generates current instead
of using it. In the regenerative system, when a car is “coasting” down
a hill it drives the wheels, and the wheels drive the motors, so that
the latter become dynamos and generate current which is sent back to
the power station. In this way some of the abnormal amount of current
taken by a car in climbing a hill is returned when the car descends the
hill. The regenerative system limits the speed of the car, so that it
cannot possibly get beyond control.
[Illustration: PLATE VII.
_By permission of_ _Siemens Brothers Dynamo Works Ltd._
ELECTRIC COLLIERY RAILWAY.]
A large tramway system spreads outwards from the centre of a city to
the suburbs, and usually terminates at various points on the outskirts
of these suburbs. It often happens that there are villages lying some
distance beyond these terminal points, and it is very desirable that
there should be some means of transport between these villages and
the city. An extension of the existing tramway is not practicable
in many cases, because the traffic would not be sufficient to pay for
the heavy outlay, and also because the road may not be of sufficient
width to admit of cars running on a fixed track. The difficulty may
be overcome satisfactorily by the use of trackless trolley cars. With
these cars the costly business of laying a rail track is altogether
avoided, only a system of overhead wires being necessary. As there is
no rail to take the return current, a second overhead wire is required.
The car is fitted with two trolley arms, and the current is taken from
one wire by the first arm, sent through the controller and the motors,
and returned by the second arm to the other wire, and so back to the
generating station. The trolley poles are so arranged that they allow
the car to be steered round obstructions or slow traffic, and the car
wheels are usually fitted with solid rubber tyres. Trackless cars are
not capable of dealing with a large traffic, but they are specially
suitable where an infrequent service, say a half-hourly one, is enough
to meet requirements.
We come now to electric railways. These may be divided into two
classes, those with separate locomotives and those without. The
separate locomotive method is largely used for haulage purposes in
collieries and large works of various kinds. In Plate VII. is seen an
electric locomotive hauling a train of coal waggons in a colliery near
the Tyne, and it will be seen that the overhead system is used, the
trolley arm and wheel being replaced by sliding bows. In a colliery
railway it is generally impossible to select the most favourable track
from the railway constructor’s point of view, as the line must be
arranged to serve certain points. This often means taking the line
sometimes through low tunnels or bridges where the overhead wire must
be low, and sometimes over public roads where the wire must be high;
and the sliding bow is better able than the trolley arm and wheel to
adapt itself to these variations. In the colliery where this locomotive
is used the height of the overhead wire ranges from 10 feet 6 inches
through tunnels or bridges, to 21 feet where the public road is
crossed. The locomotive weighs 33½ tons, and has four electric motors
each developing 50 horse-power with the current employed. It will be
noticed that the locomotive has two sets of buffers. This is because
it has to deal with both main line waggons and the smaller colliery
waggons, the upper set of buffers being for the former, and the lower
and narrower set for the latter. Plate VIII. shows a 50-ton locomotive
on the British Columbia Electric Railway, and a powerful locomotive in
use in South America. In each case it will be seen that the trolley
wheel is used.
In this country electric railways for passenger traffic are mostly
worked on what is known as the multiple-unit system, in which no
separate locomotives are used, the motors and driving mechanism being
placed on the cars themselves. There are also other cars without this
equipment, so that a train consists of a single motor-car with or
without trailer, or of two motor-cars with trailer between, or in fact
of any other combination. When a train contains two or more motor-cars
all the controllers, which are very similar to those on electric
tramcars, are electrically connected so as to be worked together from
one master controller. This system allows the length of the train to
be adjusted to the number of passengers, so that no power is wasted
in running empty cars during periods of small traffic. In suburban
railways, where the stopping-places are many and close together, the
efficiency of the service depends to a large extent upon the time
occupied in bringing the trains from rest to full speed. In this
respect the electric train has a great advantage over the ordinary
train hauled by a steam locomotive, for it can pick up speed at three
or more times the rate of the latter, thus enabling greater average
speeds and a more frequent service to be maintained.
Electric trains are supplied with current from a central generating
station, just as in the case of electric tramcars, but on passenger
lines the overhead wire is in most cases replaced by a third rail. This
live rail is placed upon insulators just outside the track rail, and
the current is collected from it by sliding metal slippers which are
carried by the cars. The return current may pass along the track rails
as in the case of trolley tramcars, or be conveyed by another insulated
conducting rail running along the middle of the track.
The electric railways already described are run on continuous current,
but there are also railways run on alternating current. A section of
the London, Brighton, and South Coast Railway is electrically operated
by alternating current, the kind of current used being that known as
single-phase. The overhead system is used, and the current is led to
the wire at a pressure of about 6000 volts. This current is collected
by sliding bows and conveyed to transformers carried on the trains,
from which it emerges at a pressure of about 300 volts, and is then
sent through the motors. The overhead wires are not fixed directly to
the supports as in the case of overhead tramway wires, but instead two
steel cables are carried by the supports, and the live wires are hung
from these. The effect of this arrangement is to make the sliding bows
run steadily and evenly along the wires without jumping or jolting. If
ever electricity takes the place of steam for long distance railway
traffic, this system, or some modification of it, probably will be
employed.
Mention must be made also of the Kearney high speed electric
mono-railway. In this system the cars, which are electrically driven,
are fitted above and below with grooved wheels. The lower wheels run on
a single central rail fixed to sleepers resting on the ground, and the
upper wheels run on an overhead guide rail. It is claimed that speeds
of 150 miles an hour are attainable with safety and economy in working.
This system is yet only just out of the experimental stage, but its
working appears to be exceedingly satisfactory.
A self-contained electric locomotive has been constructed by the North
British Locomotive Company. It is fitted with a steam turbine which
drives a dynamo generating continuous current, and the current is used
to drive four electric motors. This locomotive has undergone extensive
trials, but its practical value as compared with the ordinary type of
electric locomotive supplied with current from an outside source is not
yet definitely established.
At first sight it appears as though the electric storage cell or
accumulator ought to provide an almost perfect means of supplying
power for self-propelled electric vehicles of all kinds. In practice,
however, it has been found that against the advantages of the
accumulator there are to be set certain great drawbacks, which have not
yet been overcome. Many attempts have been made to apply accumulator
traction to electric tramway systems, but they have all failed, and
the idea has been abandoned. There are many reasons for the failure
of these attempts. The weight of a battery of accumulators large
enough to run a car with a load of passengers is tremendous, and this
is of course so much dead weight to be hauled along, and it becomes
a very serious matter when steep hills have to be negotiated. When a
car is started on a steep up-gradient a sudden and heavy demand for
current is made, and this puts upon the accumulators a strain which
they are not able to bear without injury. Another great drawback is
the comparatively short time for which accumulators can give a heavy
current, for this necessitates the frequent return of the cars to the
central station in order to have the batteries re-charged. Finally,
accumulators are sensitive things, and the continuous heavy vibration
of a tramcar is ruinous to them.
The application of accumulators to automobiles is much more feasible,
and within certain limits the electric motor-car may be considered
a practical success. The electric automobile is superior to the
petrol-driven car in its delightfully easy and silent running, and its
freedom from all objectionable smells. On the other hand high speeds
cannot be attained, and there is the trouble of having the accumulators
re-charged, but for city work this is not a serious matter. Two sets
of accumulators are used, so that one can be left at the garage to be
charged while the other is in use, the replacing of the exhausted set
by the freshly charged one being a matter of only a few minutes. The
petrol-driven car is undoubtedly superior in every way for touring
purposes. Petrol can now be obtained practically anywhere, whereas
accumulator charging stations are comparatively few and far between,
especially in country districts; and there is no comparison as regards
convenience between the filling of a petrol tank and the charging of a
set of accumulators, for one process takes a few minutes and the other
a few hours.
Accumulator-driven locomotives are not in general use, but for certain
special purposes they have proved very satisfactory. A large locomotive
of this kind was used for removing excavated material and for taking
in the iron segments, sleepers, rails, and other materials in the
construction of the Great Northern, Piccadilly, and Brompton Tube
Railway. This locomotive is 50 feet 6 inches long, and it carries
a battery of eighty large “chloride” cells, the total weight of
locomotive and battery being about 64 tons. It is capable of hauling a
load of 60 tons at a rate of from 7 to 9 miles an hour on the level.
Amongst the latest developments of accumulator traction is a complete
train to take the place of a steam locomotive hauling a single
coach on the United Railways of Cuba. According to the _Scientific
American_ the train consists of three cars, each having a battery of
216 cells, supplying current at 200 volts to the motors. Each car has
accommodation for forty-two passengers, and the three are arranged
to work on the multiple-unit system from one master controller. The
batteries will run from 60 to 100 miles for each charging of seven
hours.
CHAPTER XII
ELECTRIC LIGHTING
In the first year of the nineteenth century one of the greatest of
England’s scientists, Sir Humphry Davy, became lecturer on chemistry
to the Royal Institution, where his brilliant lectures attracted large
and enthusiastic audiences. He was an indefatigable experimenter, and
in order to help on his work the Institution placed at his disposal
a very large voltaic battery consisting of 2000 cells. In 1802 he
found that if two rods of carbon, one connected to each terminal of
his great battery, were first made to touch one another and then
gradually separated, a brilliant arch of light was formed between them.
The intense brilliance of this electric arch, or _arc_ as it came to
be called, naturally suggested the possibility of utilizing Davy’s
discovery for lighting purposes, but the maintaining of the necessary
current proved a serious obstacle. The first cost of a battery of the
required size was considerable, but this was a small matter compared
with the expense of keeping the cells in good working order. Several
very ingenious and more or less efficient arc lamps fed by battery
current were produced by various inventors, but for the above reason
they were of little use except for experimental purposes, and the
commercial success of the arc lamp was an impossibility until the
dynamo came to be a really reliable source of current. Since that time
innumerable shapes and forms of arc lamps have been devised, while the
use of such lamps has increased by leaps and bounds. To-day, wherever
artificial illumination on a large scale is required, there the arc
lamp is to be found.
When the carbon rods are brought into contact and then slightly
separated, a spark passes between them. Particles of carbon are torn
off by the spark and volatilized, and these incandescent particles
form a sort of bridge which is a sufficiently good conductor for the
current to pass across it from one rod to the other. When the carbons
are placed horizontally, the glowing mass is carried upwards by the
ascending currents of heated air, and it assumes the arch-like form
from which it gets its name. If the carbons are vertical the curve is
not produced, a more or less straight line being formed instead. The
electric arc may be formed between any conducting substances, but for
practical lighting purposes carbon is found to be most suitable.
Either continuous or alternating currents may be used to form the arc.
With continuous current, if the carbon rods are fully exposed to the
air, they gradually consume away, and minute particles of carbon are
carried across from the positive rod to the negative rod, so that the
former wastes at about twice the rate of the latter. The end of the
positive rod becomes hollowed out so as to resemble a little crater,
and the end of the negative rod becomes more or less pointed. The fact
that with continuous current the positive rod consumes away twice
as fast as the negative rod, may be taken advantage of to decrease
the cost of new carbons, by replacing the wasted positive rod with a
new one, and using the unconsumed portion of the old positive rod as
a new negative rod.[1] If alternating current is used, each rod in
turn becomes the positive rod, so that no crater is formed, and both
the carbons have the same shape and are consumed at the same rate. A
humming noise is liable to be produced by the alternating current arc,
but by careful construction of the lamp this noise is reduced to the
minimum.
[1] In actual practice the positive carbon is made double the
thickness of the negative, so that the two consume at about
the same rate.
If the carbons are enclosed in a suitable globe the rate of wasting is
very much less. The oxygen inside the globe becomes rapidly consumed,
and although the globe is not air-tight, the heated gases produced
inside it check the entrance of further supplies of fresh air as long
as the lamp is kept burning. When the light is extinguished, and the
lamp cools down, fresh air enters again freely.
Arc lamp carbons may be either solid or cored. The solid form is made
entirely of very hard carbon, while the cored form consists of a narrow
tube of carbon filled up with soft graphite. Cored carbons usually
burn more steadily than the solid form. In what are known as flame arc
lamps the carbons are impregnated with certain metallic salts, such as
calcium. These lamps give more light for the same amount of current.
The arc is long and flame-like, and usually of a striking yellow
colour, but it is not so steady as the ordinary arc.
[Illustration: FIG. 21.--Diagram showing simple method of carbon
regulation for Arc Lamps.]
As the carbon rods waste away, the length of the arc increases, and
if this increase goes beyond a certain limit the arc breaks and the
current ceases. If the arc is to be kept going for any length of time
some arrangement for pushing the rods closer together must be provided,
in order to counteract the waste. In arc lamps this pushing together,
or “feeding” as it is called, is done automatically, as is also the
first bringing together and separating of the rods to start or strike
the arc. Fig. 21 shows a simple arrangement for this purpose. A is the
positive carbon, and B the negative. C is the holder for the positive
carbon, and this is connected to the rod D, which is made of soft
iron. This rod is wound with two separate coils of wire as shown,
coil E having a low resistance, and coil F a high one. These two coils
are solenoids, and D is the core, (Chapter VII.). When the lamp is not
in use, the weight of the holder keeps the positive carbon in contact
with the negative carbon. When switched on, the current flows along the
cable to the point H. Here it has two paths open to it, one through
coil E to the positive carbon, and the other through coil F and back
to the source of supply. But coil E has a much lower resistance than
coil F, and so most of the current chooses the easier path through E,
only a small amount of current taking the path through the other coil.
Both coils are now magnetized, and E tends to draw the rod D upwards,
while F tends to pull it downwards. Coil E, however, has much greater
power than coil F, because a much larger amount of current is passing
through it; and so it overcomes the feeble pull of F, and draws up the
rod. The raising of D lifts the positive carbon away from the negative
carbon, and the arc is struck. The carbons now begin to waste away,
and very slowly the distance between them increases. The path of the
current passing through coil E is from carbon A to carbon B by way of
the arc, and as the length of the gap between A and B increases, the
resistance of this path also increases. The way through coil E thus
becomes less easy, and as time goes on more and more current takes the
alternative path through coil F. This results in a decrease in the
magnetism of E, and an increase in that of F, and at a certain point F
becomes the more powerful of the two, and pulls down the rod. In this
way the positive carbon is lowered and brought nearer to the negative
carbon. Directly the diminishing distance between A and B reaches
a certain limit, coil E once more asserts its superiority, and by
overcoming the pull of F it stops the further approach of the carbons.
So, by the opposing forces of the two coils, the carbons are maintained
between safe limits, in spite of their wasting away.
[Illustration: PLATE IX.]
[Illustration:
_By permission of_ _Union Electric Co. Ltd._
NIGHT PHOTOGRAPHS, TAKEN BY THE LIGHT OF THE ARC LAMPS.]
The arc lamp is largely used for the illumination of wide streets,
public squares, railway stations, and the exteriors of theatres,
music-halls, picture houses, and large shops. The intense brilliancy
of the light produced may be judged from the accompanying photographs
(Plate IX.), which were taken entirely by the light of the arc lamps.
Still more powerful arc lamps are constructed for use in lighthouses.
The illuminating power of some of these lamps is equal to that of
hundreds of thousands of candles, and the light, concentrated by large
reflectors, is visible at distances varying from thirty to one hundred
miles.
Arc lamps are also largely used for lighting interiors, such as large
showrooms, factories or workshops. For this kind of lighting the
dazzling glare of the outdoor lamp would be very objectionable and
harmful to the eyes, so methods of indirect lighting are employed to
give a soft and pleasant light. Most of the light in the arc lamp comes
from the positive carbon, and for ordinary outdoor lighting this carbon
is placed above the negative carbon. In lamps for interior lighting
the arrangement is frequently reversed, so that the positive carbon is
below. Most of the light is thus directed upwards, and if the ceiling
is fairly low and of a white colour the rays are reflected by it, and
a soft and evenly diffused lighting is the result. Some light comes
also from the negative carbon, and those downward rays are reflected to
the ceiling by a reflector placed beneath the lamp. Where the ceiling
is very high or of an unsuitable colour, a sort of artificial ceiling
in the shape of a large white reflector is placed above the lamp to
produce the same effect. Sometimes the lamp is arranged so that part of
the light is reflected to the ceiling, and part transmitted directly
through a semi-transparent reflector below the lamp. The composition of
the light of the arc lamp is very similar to that of sunlight, and by
the use of such lamps the well-known difficulty of judging and matching
colours by artificial light is greatly reduced. This fact is of great
value in drapery establishments, and the arc lamp has proved a great
success for lighting rooms used for night painting classes.
The powerful searchlights used by warships are arc lamps provided with
special arrangements for projecting the light in any direction. A
reflector behind the arc concentrates the light and sends it out as a
bundle of parallel rays, and the illuminating power is such that a good
searchlight has a working range of nearly two miles in clear weather.
According to the size of the projector, the illumination varies from
about 3000 to 30,000 or 40,000 candle-power. For some purposes, such
as the illuminating of narrow stretches of water, a wider beam is
required, and this is obtained by a diverging lens placed in front
of the arc. In passing through this lens the light is dispersed or
spread out to a greater or less extent according to the nature of the
lens. Searchlights are used in navigating the Suez Canal by night,
for lighting up the buoys along the sides of the canal. The ordinary
form of searchlight does this quite well, but at the same time it
illuminates equally an approaching vessel, so that the pilot on this
vessel is dazzled by the blinding glare. To avoid this dangerous state
of things a split reflector is used, which produces two separate beams
with a dark space between them. In this way the sides of the canal are
illuminated, but the light is not thrown upon oncoming vessels, so that
the pilots can see clearly.
Glass reflectors are much more efficient than metallic ones, but they
have the disadvantage of being easily put out of action by gunfire.
This defect is remedied by protecting the glass reflector by a screen
of wire netting. This is secured at the back of the reflector, and
even if the glass is shattered to a considerable extent, as by a rifle
bullet, the netting holds it together, and keeps it quite serviceable.
Reflectors protected in this way are not put out of action by even two
or three shots fired through them. Searchlight arcs and reflectors
are enclosed in metal cylinders, which can be moved in any direction,
vertically or horizontally.
In the arc lamps already described, a large proportion of the light
comes from the incandescent carbon electrodes. About the year 1901 an
American electrician, Mr. P. C. Hewitt, brought out an arc lamp in
which the electrodes took no part in producing the light, the whole
of which came from a glowing stream of mercury vapour. This lamp,
under the name of the Cooper-Hewitt mercury vapour lamp, has certain
advantages over other electric illuminants, and it has come into
extensive use.
[Illustration: FIG. 22.--Sketch of Mercury Vapour Lamp.]
It consists of a long glass tube, exhausted of air, and containing a
small quantity of mercury. Platinum wires to take the current from
the source of supply are sealed in at each end. The tube is attached
to a light tubular framework of metal suspended from the ceiling, and
this frame is arranged so that it can be tilted slightly downwards by
pulling a chain. As shown in Fig. 22, the normal position of the lamp
is not quite horizontal, but tilted slightly downwards towards the end
of the tube having the bulb containing the mercury. The platinum wire
at this end dips into the mercury, so making a metallic contact with
it. The lamp is lighted by switching on the current and pulling down
the chain. The altered angle makes the mercury flow along the tube
towards the other platinum electrode, and as soon as it touches this a
conducting path for the current is formed from end to end of the tube.
The lamp is now allowed to fall back to its original angle, so that the
mercury returns to its bulb. There is now no metallic connexion between
the electrodes, but the current continues to pass through the tube as
a vacuum discharge. Some of the mercury is immediately vaporized and
rendered brilliantly incandescent, and so the light is produced. The
trouble of pulling down the chain is avoided in the automatic mercury
vapour lamp, which is tilted by an electro-magnet. This magnet is
automatically cut out of circuit as soon as the tilting is completed
and the arc struck.
The average length of the tube in the ordinary form of mercury vapour
lamp is about 30 inches, and a light of from 500 to 3000 candle-power
is produced, according to the current used. Another form, known as the
“Silica” lamp, is enclosed in a globe like that of an ordinary electric
arc lamp. The tube is only about 5 or 6 inches in length, and it is
made of quartz instead of glass, the arrangements for automatically
tilting the tube being similar to those in the ordinary form of lamp.
The light of the mercury vapour lamp is different from that of all
other lamps. Its peculiarity is that it contains practically no red
rays, most of the light being yellow, with a certain proportion of
green and blue. The result is a light of a peacock-blue colour. The
absence of red rays alters colour-values greatly, scarlet objects
appearing black; and on this account it is impossible to match colours
by this light. In many respects, however, the deficiency in red rays is
a great positive advantage. Every one who has worked by mercury vapour
light must have noticed that it enables very fine details to be seen
with remarkable distinctness. This property is due to an interesting
fact. Daylight and ordinary artificial light is a compound or mixture
of rays of different colours. It is a well-known optical fact that a
simple lens is unable to bring all these rays to the same focus; so
that if we sharply focus an image by red light, it is out of focus or
blurred by blue light. This defect of the lens is called “chromatic
aberration.” The eye too suffers from chromatic aberration, so that
it cannot focus sharply all the different rays at the same time. The
violet rays are brought to a focus considerably in front of the red
rays, and the green and the yellow rays come in between the two. The
eye therefore automatically and unconsciously effects a compromise,
and focuses for the greenish-yellow rays. The mercury vapour light
consists very largely of these rays, and consequently it enables the
image to be focused with greater sharpness; or, in other words, it
increases the acuteness of vision. Experiments carried out by Dr.
Louis Bell and Dr. C. H. Williams demonstrated this increase in visual
sharpness very conclusively. Type, all of exactly the same size, was
examined by mercury vapour light, and by the light from an electric
incandescent lamp with tungsten filament. The feeling of sharper
definition produced by the mercury vapour light was so strong that
many observers were certain that the type was larger, and they were
convinced that it was exactly the same only after careful personal
examination.
Mercury vapour light apparently imposes less strain upon the eyes than
ordinary artificial light, and this desirable feature is the result of
the absence of the red rays, which, besides having little effect in
producing vision, are tiring to the eyes on account of their heating
action. The light is very highly actinic, and for this reason it is
largely used for studio and other interior photographic work. In cases
where true daylight colour effects are necessary, a special fluorescent
reflector is used with the lamp. By transforming the frequency of the
light waves, this reflector supplies the missing red and orange rays,
the result being a light giving normal colour effects.
Another interesting vapour lamp may be mentioned briefly. This has a
highly exhausted glass tube containing neon, a rare gas discovered
by Sir William Ramsay. The light of this lamp contains no blue rays,
and it is of a striking red colour. Neon lamps are used chiefly for
advertising purposes, and they are most effective for illuminated
designs and announcements, the peculiar and distinctive colour of the
light attracting the eye at once.
An electric current meets with some resistance in passing through any
substance, and if the substance is a bad conductor the resistance is
very great. As the current forces its way through the resistance, heat
is produced, and a very thin wire, which offers a high resistance, may
be raised to a white heat by an electric current, and it then glows
with a brilliant light. This fact forms the basis of the electric
incandescent or glow lamp.
In the year 1878, Thomas A. Edison set himself the task of producing
a perfect electric incandescent lamp, which should be capable of
superseding gas for household and other interior lighting. The first
and the greatest difficulty was that of finding a substance which
could be formed into a fine filament, and which could be kept in a
state of incandescence without melting or burning away. Platinum was
first chosen, on account of its very high melting-point, and the fact
that it was not acted upon by the gases of the air. Edison’s earliest
lamps consisted of a piece of very thin platinum wire in the shape of a
spiral, and enclosed in a glass bulb from which the air was exhausted.
The ends of the spiral were connected to outside wires sealed into
the bulb. It was found, however, that keeping platinum continuously
at a high temperature caused it to disintegrate slowly, so that the
lamps had only a short life. Fine threads or filaments of carbon were
then tried, and found to be much more durable, besides being a great
deal cheaper. The carbon filament lamp quickly became a commercial
success, and up to quite recent years it was the only form of electric
incandescent lamp in general use.
In 1903 a German scientist, Dr. Auer von Welsbach, of incandescent gas
mantle fame, produced an electric lamp in which the filament was made
of the metal osmium, and this was followed by a lamp using the metal
tantalum for the filament, the invention of Siemens and Halske. For a
while the tantalum lamp was very successful, but more recently it has
been superseded in popularity by lamps having a filament of the metal
tungsten. The success of these lamps has caused the carbon lamp to
decline in favour. The metal filaments become incandescent much more
easily than the carbon filament, and for the same candle-power the
metal filament lamp consumes much less current than the carbon lamp.
The construction of tungsten lamps is very interesting. Tungsten is a
very brittle metal, and at first the lamps were fitted with a number
of separate filaments. These were made by mixing tungsten powder with
a sort of paste, and then squirting the mixture through very small
apertures, so that it formed hair-like threads. Early in 1911 lamps
having a filament consisting of a single continuous piece of drawn
tungsten wire were produced. It had been known for some time that
although tungsten was so brittle at ordinary temperatures, it became
quite soft and flexible when heated to incandescence in the lamp, and
that it lost this quality again as soon as it cooled down. A process
was discovered by which the metal could be made permanently ductile, by
mechanical treatment while in the heated state. In this process pure
tungsten powder is pressed into rods and then made coherent by heating.
While still hot it is hammered, and finally drawn out into fine wires
through diamond dies. The wire is no thicker than a fine hair, and it
varies in size from about 0·012 mm. to about 0·375 mm., according to
the amount of current it is intended to take. It is mounted by winding
it continuously zigzag shape round a glass carrier, which has at the
top and the bottom a number of metal supports arranged in the form
of a star, and insulated by a central rod of glass. One star is made
of strong, stiff material, and the other consists of fine wires of
some refractory metal, molybdenum being used in the Osram lamps. These
supports act as springs, and keep the wire securely in its original
shape, no matter in what position the lamp is used. The whole is placed
in a glass bulb, which is exhausted of air and sealed up.
For some purposes lamps with specially small bulbs are required, and in
these the tungsten wire is made in the shape of fine spirals, instead
of in straight pieces, so that it takes up much less room. In the
“Axial” lamp the spiral is mounted in such a position that most of the
light is sent out in one particular direction.
The latest development in electric incandescent lamps is the
“half-watt” lamp. The watt is the standard of electrical energy, and
it is the rate of work represented by a current of one ampere at a
pressure of 1 volt. With continuous currents the watts are found very
simply by multiplying together the volts and the amperes. For instance,
a dynamo giving a current of 20 amperes at a pressure of 50 volts would
be called a 1000-watt dynamo. With alternating currents the calculation
is more complicated, but the final result is the same. The ordinary
form of tungsten lamp gives about one candle-power for every watt, and
is known as a one-watt lamp. As its name suggests, the half-watt lamp
requires only half this amount of energy to give the same candle-power,
so that it is very much more economical in current. In this lamp the
tungsten filament is wound in a spiral, but instead of being placed in
the usual exhausted bulb, it is sealed into a bulb containing nitrogen
gas. The increased efficiency is obtained by running the filament at a
temperature from 400° to 600° C. higher than that at which the filament
in the ordinary lamp is used.
In spite of the great advances in artificial lighting made during
recent years, no one has yet succeeded in producing light without
heat. This heat is not wanted, and it represents so much waste energy.
It has often been said that the glow-worm is the most expert of all
illuminating engineers, for it has the power of producing at will a
light which is absolutely without heat. Perhaps the nearest approach
to light without heat is the so-called “cold light” invented by M.
Dussaud, a French scientist. His device consists of a revolving ring
of exactly similar tungsten lamps. Each of these lamps has current
passed through it in turn, and the duration of the current in each is
so short, being only a fraction of a second, that the lamp has not
sufficient time to develop any appreciable amount of heat. The light
from the ring of lamps is brought to a focus, and passed through a
lens to wherever it is required. Electric incandescent lamps are made
in a variety of sizes, each one being intended for a certain definite
voltage. If a lamp designed for, say, 8 volts, is used on a circuit
of 32 volts, its candle-power is greatly increased, while the amount
of current consumed is not increased in proportion. In this way the
lamp becomes a more efficient source of light, but the “over-running,”
as it is called, has a destructive effect on the filament, so that
the life of the lamp is greatly shortened. In the Dussaud system
however the time during which each lamp has current passing through
it is so short, followed by a period of rest, that the destructive
effect of over-running is reduced to the minimum; so that by using
very high voltages an extremely brilliant light is safely obtained
with a comparatively small consumption of current. It might be thought
that the constant interchange of lamps would result in an unsteady
effect, but the substitution of one lamp for another is carried out
so rapidly that the eye gets the impression of perfect steadiness.
The Dussaud system is of little use for ordinary lighting purposes,
but for lighthouse illumination, photographic studio work, and the
projection of lantern slides and cinematograph films, it appears to be
of considerable value.
Electric light has many advantages over all other illuminants. It gives
off very little heat, and does not use up the oxygen in the air of a
room as gas does; while by means of flexible wires the lamps can be put
practically anywhere, so that the light may be had just where it is
wanted. Another great advantage is that the light may be switched on
without any trouble about matches, and there is none of the danger from
fire which always exists with a flame.
The current for electric lamps is generally taken from the public
mains, but in isolated country houses a dynamo has to be installed
on the premises. This is usually driven by a small engine running on
petrol or paraffin. In order to avoid having to run the engine and
dynamo continually, the current is not taken directly from the dynamo,
but from a battery of accumulators. During the day the dynamo is used
to charge the accumulators, and these supply the current at night
without requiring any attention.
Electric lighting from primary cells is out of the question if a
good light is wanted continuously for long periods, for the process
is far too costly and troublesome. If a light of small candle-power
is required for periods of from a few minutes to about an hour, with
fairly long intervals of rest, primary cells may be made a success.
Large dry cells are useful for this purpose, but probably the most
satisfactory cell is the sack Leclanché. This is similar in working to
the ordinary Leclanché cell used for bells, but the carbon mixture is
placed in a canvas bag or sack, instead of in a porous pot, and the
zinc rod is replaced by a sheet of zinc surrounding the sack. These
cells give about 1½ volt each, so that four, connected in series,
are required to light a 6-volt lamp. The lamps must take only a very
small current, or the cells will fail quickly. Small metal filament
lamps taking from a third to half an ampere are made specially for this
purpose, and these always should be used. A battery of sack Leclanché
cells with a miniature lamp of this kind forms a convenient outfit
for use as a night-light, or for lighting a dark cupboard, passage
or staircase. Lamps with ruby glass, or with a ruby cap to slip over
the bulb, may be obtained for photographic purposes. If the outfit is
wanted for use as a reading-lamp it is better to have two separate
batteries, and to use them alternately for short periods. With this
arrangement each battery has a short spell of work followed by a rest,
and the light may be kept on for longer periods without overworking the
cells.
CHAPTER XIII
ELECTRIC HEATING
The light of the electric incandescent lamp is produced by the heating
to incandescence of a thin filament of metal or carbon, and the heat
itself is produced by the electric current forcing its way through
the great resistance opposed to it by the filament. In such lamps the
amount of heat produced is too small to be of much practical use, but
by applying the same principle on a larger scale we get an effective
electric heater.
The most familiar and the most attractive of all electric heaters is
the luminous radiator. This consists of two or more large incandescent
lamps, having filaments of carbon. The lamps are made in the form of
long cylinders, the glass being frosted, and they are set, generally
in a vertical position, in an ornamental case or frame of metal. This
case is open at the front, and has a metal reflector behind. The
carbon filaments are raised to an orange-red heat by the passage of
the current, and they then radiate heat rays which warm the bulbs and
any other objects in their path. The air in contact with these heated
bodies is warmed, and gradually fills the room. This form of heater,
with its bright glowing lamps, gives a room a very cheerful appearance.
In the non-luminous heaters, or “convectors” as they are called, the
heating elements consist of strips of metal or wires having a very
high resistance. These are placed in a frame and made red-hot by the
current. Cold air enters at the bottom of the frame, becomes warm by
passing over the heating elements, and rises out at top and into the
room. More cold air enters the frame and is heated in the same way, and
in a very short time the whole of the air of the room becomes warmed.
The full power of the heater is used in the preliminary warming of the
room, but afterwards the temperature may be kept up with a much smaller
consumption of current, and special regulating switches are provided
to give different degrees of heat. Although these heaters are more
powerful than the luminous radiators, they are not cheerful looking;
but in some forms the appearance is improved by an incandescent lamp
with a ruby glass bulb, which shines through the perforated front of
the frame.
The Bastian, or red glow heater, has thin wires wound in a spiral and
enclosed in tubes made of quartz. These tubes are transparent both
to light and heat, and so the pleasant glow of the red-hot wire is
visible. A different type of heater, the hot oil radiator, is very
suitable for large rooms. This has a wire of high resistance immersed
in oil, which becomes hot and maintains a steady temperature.
Electric cooking appliances, like the heaters just described, depend
upon the heating of resistance wires or strips of metal. The familiar
electric kettle has a double bottom, and in the cavity thus formed is
placed the resistance material, protected by strips of mica, a mineral
substance very largely used in electrical appliances of all kinds
on account of its splendid insulating qualities. Electric irons are
constructed in much the same way as kettles, and sometimes they are
used with stands which cut off the current automatically when the iron
is laid down upon them, so that waste and overheating are prevented.
There are also a great many varieties of electric ovens, grillers,
hot-plates, water-heaters, glue-pots, and foot and bed warmers. These
of course differ greatly in construction, but as they all work on the
same principle there is no need to describe them.
Electric hot-plates are used in an interesting way in Glasgow, to
enable the police on night duty to have a hot supper. The plates are
fitted to street telephone signal boxes situated at points where a
number of beats join. By switching on current from the public mains the
policemen are able to warm their food and tea, and a supper interval of
twenty minutes is allowed. Even policemen are sometimes absent-minded,
and to avoid the waste of current and overheating of the plate that
would result if a “bobby” forgot to switch off, an arrangement is
provided which automatically switches off the current when the plate is
not in use.
[Illustration: FIG. 23.--Diagram to illustrate principle of Electric
Furnace.]
We must turn now to electric heating on a much larger scale, in the
electric furnaces used for industrial purposes. The dazzling brilliance
of the light from the electric arc lamp is due to the intense heat
of the stream of vaporized carbon particles between the carbon rods,
the temperature of this stream being roughly about 5400° F. This
great heat is made use of in various industries in the electric arc
furnace. Fig. 23 is a diagram of a simple furnace of this kind. A
is a vertical carbon rod which can be raised or lowered, and B is
a bed of carbon, forming the bottom of the furnace, and acting as
a second rod. A is lowered until it touches B, the current, either
continuous or alternating, is switched on, and A is then raised. The
arc is thus struck between A and B, and the material contained in the
furnace is subjected to intense heat. When the proper stage is reached
the contents of the furnace are drawn off at C, and fresh material
is fed in from above, so that if desired the process may be kept
going continuously. Besides the electric arc furnace there are also
resistance furnaces, in which the heat is produced by the resistance of
a conductor to a current passing through it. This conductor may be the
actual substance to be heated, or some other resisting material placed
close to it.
It will be of interest to mention now one or two of the uses of
electric furnaces. The well-known substance calcium carbide, so much
used for producing acetylene gas for lighting purposes, is a compound
of calcium and carbon; it is made by raising a mixture of lime and coke
to an intense heat in an electric furnace. The manufacture of calcium
carbide is carried on on a very large scale at Niagara, with electric
power obtained from the Falls, and at Odda in Norway, where the power
is supplied by the river Tysse. Carborundum, a substance almost as hard
as the diamond, is largely used for grinding and polishing purposes.
It is manufactured by sending a strong current through a furnace
containing a core of coke surrounded by a mixture of sand, sawdust, and
carbon. The core becomes incandescent, and the heating is continued
until the carbon combines with the sand, the process taking about
a day. Graphite, a kind of carbon, occurs naturally in the form of
plumbago, which is used for making black lead pencils. It is obtained
by mining, but many of the mines are already worked out, and others
will be exhausted before long. By means of the electric furnace,
graphite can now be made artificially, by heating anthracite coal,
and at Niagara a quantity running into thousands of tons is produced
every year. Electric furnaces are now largely employed, particularly
in France, in the production of the various alloys of iron which are
used in making special kinds of steel; and they are used also to a
considerable extent in the manufacture of quartz glass.
For many years past a great deal of time and money has been spent in
the attempt to make artificial diamonds. Quite apart from its use in
articles of jewellery, the diamond has many very important industrial
applications, its value lying in its extreme hardness, which is not
equalled by any other substance. The very high price of diamonds
however is at present a serious obstacle to their general use. If they
could be made artificially on a commercial scale they would become
much cheaper, and this would be of the greatest importance to many
industries, in which various more or less unsatisfactory substitutes
are now used on account of their much smaller cost. Recent experiments
seem to show that electricity will solve the problem of diamond making.
Small diamonds, one-tenth of an inch long, have been made in Paris
by means of the electric arc furnace. The furnace contains calcium
carbide, surrounded by a mixture of carbon and lime, and the arc,
maintained by a very powerful current, is kept in operation for several
hours. A black substance, something like coke, is formed round the
negative carbon, and in this are found tiny diamonds. The diamonds
continue to increase slowly in size during the time that the arc is
at work, and it is estimated that they grow at the rate of about
one-hundredth of an inch per hour. So far only small diamonds have
been made, but there seems to be no reason why large ones should not be
produced, by continuing the process for three or four days.
A chapter on electric heating would not be complete without some
mention of electric welding. Welding is the process of uniting two
pieces of metal by means of a combination of heat and pressure, so
that a strong and permanent joint is produced. The chief difficulty
in welding is that of securing and keeping up the proper temperature,
and some metals are much more troublesome than others in this respect.
Platinum, iron, and steel are fairly easy to weld, but most of the
other metals, and alloys of different metals, require very exact
regulation of temperature. It is almost impossible to obtain this
exact regulation by ordinary methods of heating, but the electric
current makes it a comparatively easy matter. The principle of ordinary
electric welding is very simple. The ends of the two pieces of metal
are placed together, and a powerful current is passed through them.
This current meets with a high resistance at the point of contact
of the two pieces, and so heat is produced. When the proper welding
temperature is reached, and the metal is in a sort of pasty condition,
the two pieces are pressed strongly together, and the current is
switched off. The pieces are now firmly united together. The process
may be carried out by hand, the welding smith switching the current
on and off, and applying pressure at the right moment by means of
hydraulic power. There are also automatic welders, which perform the
same operations without requiring any manual control. Alternating
current is used, of low voltage but very high amperage.
Steel castings are sometimes found to have small defects, such as
cracks or blow-holes. These are not discarded as useless, but are made
quite sound by welding additional metal into the defective places by
means of the electric arc. The arc is formed between the casting and a
carbon rod, and the tremendous heat reduces the surface of the metal to
a molten condition. Small pieces or rods of metal are then welded in
where required.
CHAPTER XIV
ELECTRIC BELLS AND ALARMS
The most familiar of all electrically worked appliances is probably
the electric bell, which in some form or other is in use practically
all over the world. Electric bells are operated by means of a current
of electricity sent through the coils of an electro-magnet, and one of
the very simplest forms is that known as the single-stroke bell. In
this bell an armature or piece of soft iron is placed across, but at a
little distance from, the poles of an electro-magnet, and to this piece
of iron is fixed a lever terminating in a sort of knob which lies close
to a bell or gong. When a current is sent round the electro-magnet the
armature is attracted, so that the lever moves forward and strikes a
sharp blow upon the gong. Before the gong can be sounded a second time
the current must be interrupted in order to make the magnet release the
armature, so that the lever may fall back to its original position.
Thus the bell gives only one ring each time the circuit is closed.
Bells of this kind may be used for signalling in exactly the same way
as the Morse sounder, and sometimes they are made with two gongs of
different tones, which are arranged so as to be sounded alternately.
[Illustration: FIG. 24.--Mechanism of Electric Bell.]
[Illustration: FIG. 25.--Diagram showing principle of Bell-push.]
For most purposes however another form called the trembler bell is much
more convenient. Fig. 24 is a rough diagram of the usual arrangement
of the essential parts of a trembler bell. When the circuit is closed
by pressing the bell-push, a current flows from the battery to the
electro-magnet EE, by way of terminal T. The electro-magnet then
attracts the soft iron armature A, thus causing the hammer H to strike
the gong. But immediately the armature is pulled away from the terminal
T¹ the circuit is broken and the magnet loses its attraction for the
armature, which is moved back again into contact with T¹ by the spring
S. The circuit is thus again closed, the armature is again attracted,
and the hammer strikes the gong a second time. This process goes on
over and over again at a great speed as long as the bell-push is kept
pressed down, resulting in an extremely rapid succession of strokes
upon the gong. It will be noticed that the working of this bell is very
similar to that of the automatic contact-breaker used for induction
coils (Chapter VIII.). For household purposes this form of bell has
completely driven out the once popular wire-pulled bell. Bell-pushes
are made in a number of shapes and forms, and Fig. 25 will make clear
the working principle of the familiar form which greets us from almost
every doorway with the invitation, “Press.” In private offices and
elsewhere the rather aggressive sound of an ordinary trembler bell is
apt to become a nuisance, and in such cases a modified form which gives
a quiet buzzing sound is often employed.
It is frequently necessary to have an electric bell which, when once
started, will continue ringing until it is stopped. Such bells are
used for fire and burglar alarms and for many other similar purposes,
and they are called continuous-ringing bells as distinguished from
the ordinary trembler bells. In one common form of continuous-ringing
bell two separate batteries are used, one to start the bell and the
other to keep it ringing. When a momentary current from the first
battery is sent over the bell lines the armature is attracted by the
electro-magnet, and its movement allows a lever to fall upon a metal
contact piece. This closes the circuit of the second battery, which
keeps the bell ringing until the lever is replaced by pulling a cord
or pressing a knob. Continuous-ringing bells are often fitted to alarm
clocks. The alarm is set in the usual way, and at the appointed hour
the bell begins to ring, and goes on ringing until its owner, able to
stand the noise no longer, gets out of bed to stop it.
There is another form of electric bell which has been devised to do
away with the annoyance of bells suddenly ceasing to work on account
of the failure of the battery. In this form the battery is entirely
dispensed with, and the current for ringing the bell is taken from a
very small dynamo fitted with a permanent steel horse-shoe magnet. The
armature is connected to a little handle, and current is generated by
twisting the handle rapidly to and fro between the thumb and finger. A
special form of bell is required for this arrangement, which is not in
general use.
In the days of wire-pulled bells it was necessary to have quite a
battery of bells of different tones for different rooms, but a single
electric bell can be rung from bell-pushes placed in any part of
a house or hotel. An indicator is used to show which push has been
pressed, and, this like the bell itself, depends upon the attraction
of an armature by an electro-magnet. Before reaching the bell the wire
from each bell-push passes round a separate small electro-magnet, which
is thus magnetized by the current at the same time that the bell is
rung. In the simplest form of indicator the attraction of the magnet
causes a little flag to swing backwards and forwards over its number.
Another form is the drop indicator, in which the movement of the
armature when attracted by the magnet allows a little flag to drop,
thus exposing the number of the room from which the bell was rung. The
dropped flag has to be replaced, either by means of a knob fixed to a
rod which pushes the flag up again, or by pressing a push which sends
the current through another little electro-magnet so arranged as to
re-set the flag.
The electric current is used to operate an almost endless variety
of automatic alarms for special purposes. Houses may be thoroughly
protected from undesired nocturnal visitors by means of a carefully
arranged system of burglar alarms. Doors and windows are fitted with
spring contacts so that the slightest opening of them closes a battery
circuit and causes an alarm to sound, and even if the burglar succeeds
in getting inside without moving a door or window, say by cutting out
a pane of glass, his troubles are not by any means at an end. Other
contacts are concealed under the doormats, and under the carpets in
passages and stairways, so that the burglar is practically certain to
tread on one or other of them and so rouse the house. A window may be
further guarded by a blind contact. The blind is left down, and is
secured at the bottom to a hook, and the slightest pressure upon it,
such as would be given by a burglar trying to get through the window,
sets off the alarm. Safes also may be protected in similar ways, and
a camera and flashlight apparatus may be provided, so that when the
burglar closes the circuit by tampering with the safe he takes his own
photograph.
The modern professional burglar is a bit of a scientist in his way, and
he is wily enough to find and cut the wires leading to the contacts, so
that he can open a door or window at his leisure without setting off
the alarm. In order to circumvent this little game, burglar alarms are
often arranged on the closed-circuit principle, so that the alarm is
sounded by the breaking of the circuit. A burglar who deftly cut the
wires of an alarm worked on this principle would not be particularly
pleased with the results of his handiwork. The bells of burglar alarms
may be arranged to ring in a bedroom or in the street, and in the
United States, where burglar and in fact all electric alarms are in
more general use than in England, large houses are sometimes connected
to a police station, so that the alarm is given there by bell or
otherwise.
[Illustration: PLATE X.
_By permission of_ _Vickers Limited._
WHERE ELECTRICAL MACHINERY IS MADE.]
When an outbreak of fire is discovered it is of the utmost importance
that the nearest fire-station should be notified instantly, for
fire spreads with such rapidity that a delay of even a few minutes
in getting the fire-engines to the spot may result in the total
destruction of a building which otherwise might have been saved. In
almost all large towns some system of public fire alarms is now in use.
The signal boxes are placed in conspicuous positions in the streets,
and sometimes also in very large buildings. The alarm is generally
given by the starting of a clockwork mechanism which automatically
makes and breaks a circuit a certain number of times. When this occurs
an alarm bell rings at the fire-station, and the number of strokes on
the bell, which depends upon the number of times the alarm mechanism
makes and breaks the circuit, tells the attendant from which box
the alarm has been given. One well-known form of call box has a glass
front, and the breaking of the glass automatically closes the circuit.
In other forms turning a handle or pulling a knob serves the same
purpose.
It is often required to maintain a room at one particular temperature,
and electricity may be employed to give an alarm whenever the
temperature rises above or falls below a certain point. One arrangement
for this purpose consists of an ordinary thermometer having the top
of the mercury tube fitted with an air-tight stopper, through which
a wire is passed down into the tube as far as the mark indicating
the temperature at which the alarm is desired to sound. Another wire
is connected with the mercury in the bulb, and the free ends of both
wires are taken to a suitable battery, a continuous-ringing bell
being inserted in the circuit at some convenient point. If a rise in
temperature takes place the mercury expands and moves up the tube, and
at the critical temperature it touches the wire, thus completing the
circuit and sounding the alarm. This arrangement only announces a rise
in temperature, but by making the thermometer tube in the shape of a
letter U an alarm may be given also when the temperature falls below
a certain degree. A device known as a “thermostat” is also used for
the same purpose. This consists of two thin strips of unlike metals,
such as brass and steel, riveted together and suspended between two
contact pieces. The two metals expand and contract at different rates,
so that an increase in temperature makes the compound strip bend in one
direction, and a decrease in temperature makes it bend in the opposite
direction. When the temperature rises or falls beyond a certain limit
the strip bends so far as to touch one or other of the contact pieces,
and the alarm is then given. Either of the preceding arrangements can
be used also as an automatic fire alarm, or if desired matters may
be arranged so that the closing of the circuit, instead of ringing
a bell, turns on or off a lamp, or adjusts a stove, and in this way
automatically keeps the room at a constant temperature.
Electric alarms operated by ball floats are used to some extent for
announcing the rise or fall beyond a pre-arranged limit of water or
other liquids, and there is a very ingenious electrical device by which
the level of the water in a tank or reservoir can be ascertained at any
time by indicators placed in convenient positions any distance away.
In factories and other large buildings a watchman is frequently
employed to make a certain number of rounds every night. Being human,
a night-watchman would much rather sit and snooze over his fire than
tramp round a dark and silent factory on a cold winter night; and
in order to make sure that he pays regular visits to every point
electricity is called in to keep an eye on him. A good eight-day
clock is fitted with a second dial which is rotated by the clockwork
mechanism, and a sheet of paper, which can be renewed when required, is
placed over this dial. On the paper are marked divisions representing
hours and minutes, and other divisions representing the various
places the watchman is required to visit. A press-button is fixed at
each point to be visited, and connected by wires with the clock and
with a battery. As the watchman reaches each point on his rounds he
presses the button, which is usually locked up so that no one else
can interfere with it, and the current passes round an electro-magnet
inside the clock case. The magnet then attracts an armature which
operates a sort of fine-pointed hammer, and a perforation is made in
the paper, thus recording the exact time at which the watchman visited
that particular place.
The current for ordinary electric bells is generally supplied by
Leclanché cells, which require little attention, and keep in good
working order for a very long time. As we saw in Chapter IV., these
bells soon polarize if used continuously, but as in bell work they
are required to give current for short periods only, with fairly long
intervals of rest, no trouble is caused on this account. These cells
cannot be used for burglar or other alarms worked on the closed-circuit
principle, and in such cases some form of Daniell cell is usually
employed.
CHAPTER XV
ELECTRIC CLOCKS
Amongst the many little worries of domestic life is the keeping
in order of the various clocks. It ought to be a very simple
matter to remember to wind up a clock, but curiously enough almost
everybody forgets to do so now and then. We gaze meditatively at the
solemn-looking machine ticking away on the mantelpiece, wondering
whether we wound it up last week or not; and we wish the wretched thing
would go without winding, instead of causing us all this mental effort.
There is usually a way of getting rid of little troubles of this kind,
and in this case the remedy is to be found in an electrically-driven
clock. The peculiar feature about clocks driven by electricity is that
they reverse the order of things in key-wound clocks, the pendulum
being made to drive the clockwork instead of the clockwork driving
the pendulum. No driving spring is required, and the motive power is
supplied by a small electro-magnet.
The actual mechanism varies considerably in different makes of clock.
In one of the simplest arrangements there is a pendulum with an
armature of soft iron fixed to the extremity of its bob. Below the
pendulum is an electro-magnet, and this is supplied with current from a
small battery of dry cells. A short piece of metal, called a “pallet,”
is attached to the rod of the pendulum by means of a pivot; and as the
pendulum swings it trails this pallet backwards and forwards along
a horizontal spring. In this spring are cut two small notches, one
on each side of the centre of the swing. As long as the pendulum is
swinging sufficiently vigorously, the pallet slides over these notches;
but when the swing has diminished to a certain point the pallet catches
in one or other of the notches. This has the effect of pressing down
the spring so that it touches a contact piece just below, and the
battery circuit is then completed. The electro-magnet now comes into
action and attracts the armature, thus giving the pendulum a pull which
sets it swinging vigorously again. The spring is then freed from the
pressure of the pallet, and it rises to its original position, so that
the circuit is broken. This puts out of action the electro-magnet, and
the latter does no further work until the pendulum requires another
pull. The movement of the pendulum drives the wheelwork, which is
similar to that of an ordinary clock, and the wheelwork moves the hands
in the usual way. A clock of this kind will run without attention
for several months, and then the battery requires to be renewed. As
time-keepers, electrically-driven clocks are quite as good as, and
often very much better than key-wound clocks.
Everybody must have noticed that the numerous public clocks in a large
town do not often agree exactly with one another, the differences
sometimes being quite large; while even in one building, such as a
large hotel, the different clocks vary more or less. This state of
things is very unsatisfactory, for it is difficult to know which of the
clocks is exactly right. Although large clocks are made with the utmost
care by skilled workmen, they cannot possibly be made to maintain
anything like the accuracy of a high-class chronometer, such as is
used by navigators; and the only way to keep a number of such clocks
in perfect agreement is to control their movements from one central or
master clock. This can be done quite satisfactorily by electricity.
The master-clock and the various sub-clocks are connected electrically,
so that a current can be sent from the master-clock to all the others.
Each sub-clock is fitted with an electro-magnet placed behind the
figure XII at the top of the dial. At the instant when the master-clock
reaches the hour, the circuit is closed automatically, and the current
energizes these magnets. The minute hands of all the sub-clocks are
gripped by the action of the magnets, and pulled exactly to the hour;
the pulling being backward or forward according to whether the clocks
are fast or slow. In this way all the clocks in the system are in exact
agreement at each hour. The same result may be attained by adjusting
all the sub-clocks so that they gain a little, say a few seconds in the
hour. In this case the circuit is closed about half a minute before
the hour. As each sub-clock reaches the hour, its electro-magnet comes
into action, and holds the hands so that they cannot proceed. When the
master-clock arrives at the hour the circuit is broken, the magnets
release their captives, and all the clocks move forward together.
It is possible to control sub-clocks so that their pendulums actually
beat exactly with the pendulum of the master-clock; but only a small
number of clocks can be controlled in this way, and they must be
of the best quality. The method is similar to that used for hourly
corrections, the main difference being that the circuit is closed by
the pendulum of the master-clock at each end of its swing, so that the
pendulums of the sub-clocks are accelerated or held back as may be
required.
In the correcting systems already described the sub-clocks are
complete in themselves, so that they work quite independently, except
at the instant of correction. For hotels, schools, and other large
buildings requiring clocks at a number of different points, a simpler
arrangement is adopted. Only one complete clock is used, this being
the master-clock, which may be wound either electrically or by key.
The sub-clocks are dummies, having only a dial with its hands, and an
electro-magnetic arrangement behind the dial for moving the hands. The
sub-clocks are electrically connected with the master-clock, and the
mechanism of this clock is arranged to close the circuit automatically
every half-minute. Each time this occurs the magnet of each sub-clock
moves forward the hands half a minute, and in this way the dummy clocks
are made to travel on together by half-minute steps, exactly in unison
with the master-clock.
CHAPTER XVI
THE TELEGRAPH
We come now to one of the most important inventions of the nineteenth
century, the electric telegraph. From very early times men have felt
the necessity for some means of rapidly communicating between two
distant points. The first really practical method of signalling was
that of lighting beacon fires on the tops of hills, to spread some
important tidings, such as the approach of an enemy. From this simple
beginning arose more complicated systems of signalling by semaphore,
flags, or flashing lights. All these methods proved incapable of
dealing with the rapidly growing requirements of commerce, for they
were far too slow in action, and in foggy weather they were of no
use at all. We are so accustomed to walking into a telegraph office,
filling up a form, and paying our sixpence or more, that it is very
difficult for us to realize the immense importance of the electric
telegraph; and probably the best way of doing this is to try to imagine
the state of things which would result if the world’s telegraphic
instruments were put out of action for a week or two.
The earliest attempts at the construction of an electric telegraph
date back to a time long before the discovery of the electric current.
As early as 1727 it was known that an electric discharge could be
transmitted to a considerable distance through a conducting substance
such as a moistened thread or a wire, and this fact suggested the
possibility of a method of electric signalling. In 1753 a writer in
_Scott’s Magazine_ brought forward an ingenious scheme based upon
the attraction between an electrified body and any light substance.
His telegraph was worked by an electric machine, and it consisted
of twenty-six separate parallel wires, every wire having a metal
ball suspended from it at each end. Close to each ball was placed a
small piece of paper upon which was written a letter of the alphabet.
When any wire was charged, the paper letters at each end of it were
attracted towards the metal balls, and in this way words and sentences
were spelled out. Many other systems more or less on the same lines
were suggested during the next fifty years, but although some of them
had considerable success in an experimental way, they were all far too
unreliable to have any commercial success.
With the invention of the voltaic cell, inventors’ ideas took a new
direction. In 1812 a telegraph based upon the power of an electric
current to decompose water was devised by a German named Sömmering.
He used a number of separate wires, each connected to a gold pin
projecting from below into a glass vessel filled with acidulated water.
There were thirty-five wires in all, for letters and numbers, and
when a current was sent along any wire bubbles of gas formed at the
pin at the end of it, and so the letters or numbers were indicated.
This telegraph, like its predecessors, never came into practical
use. Oersted’s discovery in 1829 of the production of magnetism by
electricity laid the foundation of the first really practical electric
telegraphs, but little progress was made until the appearance of the
Daniell cell, in 1836. The earlier forms of voltaic cells polarized so
rapidly that it was impossible to obtain a constant current from them,
but the non-polarizing Daniell cell at once removed all difficulty in
this respect. In the year 1837 three separate practical telegraphs
were invented: by Morse in the United States, by Wheatstone and Cooke
in England, and by Steinheil in Munich.
[Illustration: FIG. 26.--Dial of Five-Needle Telegraph.]
The first telegraph of Wheatstone and Cooke consisted of five magnetic
needles pivoted on a vertical dial. The letters of the alphabet were
marked on the dial, and the needles were deflected by currents made to
pass through wires by the depression of keys, so that two needles would
point towards the required letter. Fig. 26 is a sketch of the dial of
this apparatus. This telegraph was tried successfully on the London
and North-Western Railway, over a wire a mile and a half in length.
Wheatstone and Cooke afterwards invented a single-needle telegraph in
which the letters were indicated by movements of the needle to the
right or to the left, according to the direction of a current sent
through a coil of wire. Wheatstone subsequently produced an apparatus
which printed the letters on paper.
In the United States, Morse had thought out a scheme of telegraphy
in 1832, but it was not until 1837 that he got his apparatus into
working order. He was an artist by profession, and for a long time he
was unable to develop his ideas for lack of money. After many efforts
he succeeded in obtaining a State grant of £6000 for the construction
of a telegraph line between Baltimore and Washington, and the first
message over this line was sent in 1844, the line being thrown open
to the public in the following year. Amongst the features of this
telegraph were a receiving instrument which automatically recorded the
messages on a moving paper ribbon, by means of a pencil actuated by
an electro-magnet; and an apparatus called a relay, which enabled the
recording instrument to be worked when the current was enfeebled by the
resistance of a very long wire. Morse also devised a telegraphic code
which is practically the same as that in use to-day.
The great discovery of the German Steinheil was that a second wire for
the return of the current was not necessary, and that the earth could
be used for this part of the circuit.
In reading the early history of great inventions one is continually
struck with the indifference or even hostility shown by the general
public. In England the electric telegraph was practically ignored
until the capture of a murderer by means of it literally forced the
public to see its value. The murder was committed near Slough, and the
murderer succeeded in taking train for London. Fortunately the Great
Western Railway had a telegraph line between Slough and London, and a
description telegraphed to Paddington enabled the police to arrest the
murderer on his arrival. In the United States too there was just the
same indifference. The rate for messages on the line between Baltimore
and Washington was one cent for four words, and the total amount taken
during the first four days was one cent!
One of the simplest forms of telegraph is the single-needle
instrument. This consists of a magnetic needle fixed to a spindle at
the back of an upright board through which the spindle is passed. On
the same spindle, but in front of the board, is fixed a dial needle,
which, of course, moves along with the magnetic needle. A coil of wire
is passed round the magnetic needle, and connected to a commutator for
reversing the direction of the current. By turning a handle to the left
a current is made to flow through the coil, and the magnetic needle
moves to one side; but if the handle is turned to the right the current
flows through the coil in the opposite direction, and the needle moves
to the other side. Instead of a handle, two keys may be used, the
movement of the needle varying according to which key is pressed. A
good operator can transmit at the rate of about twenty words a minute
with this instrument. The Morse code, which consists of combinations
of dots and dashes, is used, a movement of the dial needle to the left
meaning a dot, and one to the right a dash. The code as used in the
single-needle instrument is shown in Fig. 27.
[Illustration: FIG. 27.--Code for Single-Needle Telegraph.]
Needle instruments are largely used in railway signal cabins, but for
general telegraphic work an instrument called the Morse sounder is
employed. This consists of an electro-magnet which, when a current is
passed through it, attracts a small piece of iron fixed to one end of
a pivoted lever. The other end of this lever moves between two stops.
At the transmitting station the operator closes a battery circuit by
pressing a key, when the electro-magnet of the sounder at the receiving
station attracts the iron, and the lever flies from one stop to the
other with a sharp click, returning again as soon as the circuit is
broken. A dot is signalled when the lever falls back immediately after
the click, and a dash when it makes a short stay before returning. Fig.
28 shows the code of signals for the Morse telegraph.
[Illustration: FIG. 28.--The Morse Code.]
In passing through a very long wire an electric current becomes greatly
reduced in strength owing to the resistance of the wire. If two
telegraph stations are a great distance apart the energy of the current
thus may be unequal to the task of making the electro-magnet move the
lever of the sounder so as to produce a click, but this difficulty is
overcome by the use of an ingenious arrangement called a “relay.” It
consists of a very small electro-magnet which attracts a light bar,
the movement of the bar being made to close the circuit of another
battery at the receiving station. The feeble current works the relay,
and the current in the local circuit operates the sounder.
The word “telegraph,” which is derived from the Greek _tele_, far
off, and _grapho_, I write, strictly signifies writing at a distance.
The needle instrument and the sounder do not write in any way, but by
modifying the construction of the sounder it can be made to record the
messages it receives. A small wheel is fitted to the free end of the
lever of the sounder, and an ink-well is placed so that the wheel dips
into it when the lever is in the normal position. When the circuit is
closed the lever moves just as in the ordinary sounder, but instead
of clicking against a stop it presses the inked wheel against a paper
ribbon which is kept slowly moving forward by clockwork. In this way
the wheel continues to mark a line along the paper as long as the
circuit remains closed, and according to the time the transmitting key
is kept down a short mark or dot, or a long mark or dash, is produced.
The clockwork which moves the paper ribbon is started automatically by
the current, and it continues working until the message is finished.
[Illustration: FIG. 29.--A Morse Message.
(_a_) Perforated Tape.
(_b_) Printed Tape.
TRANSLATION.
_Series of alternate dots and dashes indicating commencement of
message._
Sec (_section_) A. D. T. (_Daily Telegraph_) Fm (_from_) Berri,
Antivari.
_Then follow the letters_ G. Q., _signifying fresh line_.
They hd (_had_) bn (_been_) seen advancing in t (_the_) distance and
wr (_were_) recognised by thr (_their_) usual uniform wh (_which_)
consists o (_of_) a white fez.
_Finally double dots indicating full stop._]
A good Morse operator can maintain a speed of about thirty words a
minute, but this is far too slow for certain kinds of telegraphic
work, such as the transmission of press news, and for such work the
Wheatstone automatic transmitter is used. First of all the messages
are punched on a paper ribbon. This is done by passing the ribbon
from right to left by clockwork through a punching machine which is
provided with three keys, one for dots, one for dashes, and the other
for spaces. If the left-hand key is pressed, two holes opposite to one
another are made, representing a dot; and if the right-hand key is
pressed, two diagonal holes are punched, representing a dash. In Fig.
29, which shows a piece of ribbon punched in this way, a third line of
holes will be noticed between the outside holes representing the dots
and dashes. These holes are for the purpose of guiding the paper ribbon
steadily along through the transmitting machine. The punched ribbon
is then drawn by clockwork through a Wheatstone transmitter. In this
machine two oscillating needles, connected with one pole of a battery,
are placed below the moving ribbon. Each time a hole passes, these
needles make contact with a piece of metal connected with the other
pole of the battery, thus making and breaking the circuit with much
greater rapidity than is possible with the Morse key. At the receiving
station the messages are recorded by a form of Morse inker, coming out
in dots and dashes as though sent by hand. Below the punched ribbon
in Fig. 29 is shown the corresponding arrangement of dots and dashes.
The same punched ribbon may be used repeatedly when the message has
to be sent on a number of different lines. The Wheatstone automatic
machine is capable of transmitting at the rate of from 250 to 400
words a minute. Fig. 29 is a fragment of a _Daily Telegraph_ Balkan
War special, as transmitted to the _Yorkshire Post_ over the latter’s
private wire from London to Leeds. In the translation it will be seen
that many common words are abbreviated.
One weak point of telegraphy with Wheatstone instruments is that the
messages are received in Morse code, and have to be translated. During
recent years telegraphs have been invented which actually produce their
messages in ordinary written or printed characters. A very ingenious
instrument is the Hughes printing telegraph, which turns out messages
in typewritten form. Its mechanism is too complicated to be described
here, but in general it consists of a transmitter having a keyboard
something like that of a typewriter, by means of which currents of
electricity are made to press a sheet of paper at the right instant
against a revolving type-wheel bearing the various characters. This
telegraph has been modified and brought to considerable perfection,
and in one form or another it is used in European countries and in the
United States.
In the Pollak-Virag system of telegraphy the action of light upon
sensitized photographic paper is utilized. An operator punches special
groupings of holes on a paper ribbon about 1 inch wide, by means of
a perforating machine resembling a typewriter, and the ribbon is
then passed through a machine which transmits by brush contacts. The
receiver consists of a very small mirror connected to two vibrating
diaphragms, which control its movements according to the currents
received, one diaphragm moving the mirror in a vertical direction, and
the other in a horizontal direction. The mirror reflects a ray of light
on to photographic bromide paper in the form of a moving band about 3
inches in width, and the combined action of the two diaphragms makes
the mirror move so that the ray of light traces out the messages in
ordinary alphabetical characters. As it moves forward after being acted
upon by the light, the paper is automatically developed and fixed, and
then passed through drying rollers. Although the writing is rather
imperfect in formation it is quite legible enough for most messages,
but trouble occasionally occurs with messages containing figures, owing
to confusion arising from the similarity of the figures, 3, 5, and 8.
The whole process is carried out with such rapidity that 40,000 or even
more words can be transmitted easily in an hour.
One of the most remarkable of present-day telegraphs is the Creed
high-speed automatic printing telegraph. This has been devised to do
away with hand working as far as possible, and to substitute quicker
and more accurate automatic methods. In this system a perforated paper
tape is produced by a keyboard perforator at the sending station. This
tape is just ordinary Wheatstone tape, its perforations representing in
the Morse code the message to be transmitted; and the main advantage
of the Creed perforator over the three-key punching machine already
described lies in the ease and speed with which it can be worked. The
keyboard contains a separate key for each letter or signal of the Morse
code, and the pressing of any key brings into operation certain punches
which make the perforations corresponding to that particular letter.
The perforator can be worked by any one who understands how to use an
ordinary typewriter, and a speed of about 60 words a minute can be
maintained by a fairly skilful operator. If desired a number of tapes
may be perforated at the same time.
The tape prepared in this way is passed through a Wheatstone
transmitter, and long or short currents, according to the arrangement
of the perforations, are sent out along the telegraph line. At the
receiving station these signals operate a receiving perforator. This
machine produces another perforated tape, which is an exact copy of
the tape at the sending station, and it turns out this duplicate tape
at the rate of from 150 to 200 words a minute. There are two forms of
this receiving perforator, one worked entirely by electricity, and
the other by a combination of electricity and compressed air, both
forms serving the same purpose. The duplicate tape is then passed
through an automatic printer, which reproduces the message in large
Roman characters on a paper tape. The printer works at a speed of from
80 to about 100 words a minute, and the printed tape is pasted on a
telegraphic form by a semi-automatic process, and the message is then
ready for delivery. Plate XI. shows a specimen of the tape from the
receiving perforator, and the corresponding translation as turned out
by the printer. This message formed part of a leading article in the
_Daily Mail_. Some idea of the wonderful capabilities of the Creed
system may be gained from the fact that by means of it practically the
whole contents of the _Daily Mail_ are telegraphed every night from
London to Manchester and Paris, for publication next morning.
One of the most remarkable features about present-day telegraphy is
the ease with which two or more messages can be sent simultaneously
over one line. Duplex telegraphy, or the simultaneous transmission of
two separate messages in opposite directions over one wire, is now
practised on almost every line of any importance. At first sight duplex
telegraphy seems to be an impossibility, for if we have two stations,
one at each end of a single wire, and each station fitted with a
transmitter and a receiver, it appears as if each transmitter would
affect not only the receiver at the opposite end of the wire, but also
the receiver at its own end, thus causing hopeless confusion when both
transmitters were in use at the same time. This actually would be the
case with ordinary telegraphic methods, but by the use of a special
arrangement all confusion in working is avoided.
[Illustration: PLATE XI.
_By permission of_ _Creed, Bille & Co. Ltd._
SPECIMEN OF THE WORK OF THE CREED HIGH-SPEED PRINTING TELEGRAPH.]
We have seen that a magnetic needle is deflected by a current passing
through a coil of wire placed round it, and that the direction in which
the needle is deflected depends upon the direction of the current in
the coil. Now suppose we place round the needle two coils of wire,
wound so that the current in one flows in a direction opposite to that
of the current in the other. Then, if we pass two equal currents,
one through each coil, it is evident that they will neutralize one
another, so that the needle will not be deflected at all. In a duplex
system one end of one of these coils is connected to earth, say to a
copper plate buried in the ground, and one end of the other to the
line wire. The two remaining ends are arranged as branches leading
off from a single wire connected with the transmitting key. The whole
arrangement of coils and needle is repeated at the other end of the
line. If now the transmitting key at station A is pressed, the circuit
is closed and a current flows along the single wire, and then divides
into two where the wire branches, half of it taking the path through
one coil and half the path through the other. Equal currents thus flow
through the oppositely wound coils, and the needle at station A is not
deflected. Leaving the coils, one of these equal currents flows away to
earth, while the other passes out along the line wire. On its arrival
at station B the current is able to pass through only one of the coils
round the needle, and consequently the needle is deflected and the
signal given. In this way the transmitting operator at station A is
able to signal to station B without affecting the receiver at his own
end, and similarly the operator at station B can transmit to A without
affecting the B receiver. Thus there can be no confusion whether the
transmitters are worked at different times or simultaneously, for each
transmitter affects only the receiver at the opposite end of the line.
The diagram in Fig. 30 will help to make clearer the general principle.
K and K¹ are the two transmitting keys which close the circuit, and C
and C¹ are the points at which the current divides into two. Instead
of coils and needles, electro-magnets operating sounders may be used,
such magnets having two separate and oppositely wound coils, acting
in exactly the same way as the coils round the needles. The above
description is of course only a rough outline of the method, and
in practice matters are more complicated, owing to the necessity for
carefully adjusted resistances and for condensers. There is also
another and different method of duplexing a line, but we have not space
to describe it. Duplex telegraphy requires two operators at each end of
the line, one to send and the other to receive.
Diplex telegraphy is the simultaneous transmission of two separate
messages in the same direction over one line. Without going into
details it may be said that for this purpose two different transmitting
keys are required, one of which alters the direction, and the other
the strength of the current though the line wire. The receivers are
arranged so that one responds only to a strong current, and the other
only to a current in one particular direction. A line also may be
quadruplexed, so that it is possible to transmit simultaneously two
messages from each end, four operators being required at each station,
two to transmit and two to receive. Systems of multiplex telegraphy
have been devised by which very large numbers of messages can be sent
at once over a single wire, and the Baudot multiplex telegraph has
proved very successful.
[Illustration: FIG. 30.--Diagram to illustrate principle of Duplex
Telegraphy.]
The wires for telegraphic purposes may be conveyed either above or
below the ground. Overground wires are carried on poles by means of
insulators of porcelain or other non-conducting material, protected
by a sort of overhanging screen. The wires are left bare, and they
are generally made of copper, but iron is used in some cases. In
underground lines the wires formerly were insulated by a covering of
gutta-percha, but now paper is generally used. Several wires, each
covered loosely with thoroughly dry paper, are laid together in a
bundle, the whole bundle or cable being enclosed in a strong lead
pipe. The paper coverings are made to fit loosely so that the wires
are surrounded by an insulating layer of dry air. As many as 1200
separate wires are sometimes enclosed in one pipe. In order to keep
telegraph lines in working order frequent tests are necessary, and the
most important British Postal Telegraph lines are tested once a week
between 7.30 and 7.45 a.m. The earth is generally used for the return
circuit in telegraphy, and the ends of the return wires are connected
either to metal plates buried in the ground to a depth at which the
earth is permanently moist, or to iron gas or water pipes. The current
for telegraph working on a small scale is usually supplied by primary
cells, the Daniell cell being a favourite for this purpose. In large
offices the current is generally taken from a battery of storage cells.
During the early days of telegraphy, overhead lines were a source of
considerable danger when thunderstorms were taking place. Lightning
flashes often completely wrecked the instruments, giving severe shocks
to those in the vicinity, and in a few cases operators were killed
at their posts. Danger of this kind is now obviated by the use of
contrivances known as lightning arresters. There are several forms of
these, but only one need be mentioned. The main features of this are
two metal plates separated slightly from one another, so that there
is a small air gap between them. One plate is connected to the line
wire, and the other to earth. Almost all lightning flashes consist
of an oscillatory discharge, that is one which passes a number of
times backwards and forwards between a cloud and the earth. A very
rapidly alternating discharge of this kind finds difficulty in passing
along the line wire, being greatly impeded by the coils of wire in
the various pieces of apparatus; and although the resistance of this
air gap is very high, the lightning discharge will cross the gap
sooner than struggle along the line wire. In this way, when a flash
affects the line, the discharge jumps the gap between the plates of
the arrester and passes away harmlessly to earth, without entering
the telegraph office at all. As was mentioned in Chapter III., the
prevalence of magnetic storms sometimes renders telegraph lines quite
unworkable for a time, but although such disturbances cause great delay
and general inconvenience, they are not likely to be at all dangerous.
It is often possible to maintain telegraphic communication during
magnetic disturbances by using two lines to form a complete metallic
loop, so that there is no earth return.
CHAPTER XVII
SUBMARINE TELEGRAPHY
The story of submarine telegraphy is a wonderful record of dogged
perseverance in the face of tremendous obstacles and disastrous
failures. It would be of no interest to trace the story to its very
beginning, and so we will commence with the laying of the first cable
across the English Channel from Dover to Calais, in 1850. A single
copper wire covered with a layer of gutta-percha half an inch thick
was used, and leaden weights were attached to it at intervals of one
hundred yards, the fixing of each weight necessitating the stoppage of
the cable-laying ship. The line was laid successfully, but it failed
after working for a single day, and it afterwards turned out that a
Boulogne fisherman had hauled up the cable with his trawl. This line
proved that telegraphic communication between England and France was
possible, but the enterprise was assailed with every imaginable kind of
abuse and ridicule. It is said that some people really believed that
the cable was worked in the style of the old-fashioned house bell,
and that the signals were given by pulling the wire! In the next year
another attempt was made by Mr. T. R. Crampton, a prominent railway
engineer, who himself contributed half of the £15,000 required. The
form of cable adopted by him consisted of four copper wires, each
covered with two layers of gutta-percha, and the four enclosed in a
covering formed of ten galvanized iron wires wound spirally round
them. The line proved a permanent success, and this type of cable,
with certain modifications, is still in use. In 1852 three attempts
were made to connect England and Ireland, but the first two failed
owing to the employment of cables too light to withstand the strong
tidal currents, and the third was somehow mismanaged as regards the
paying-out, so that there was not enough cable to reach across. A
heavier cable was tried in the next year, and this was a lasting
success.
The success of these two cables led to the laying of many other
European cables over similar distances, but we must now pass on to a
very much bigger undertaking, the laying of the Atlantic cable. In
1856 the Atlantic Telegraph Company was formed, with the object of
establishing and working telegraphic communication between Ireland and
Newfoundland, the three projectors being Messrs. J. W. Brett, C. T.
Bright, and C. W. Field. The British and the United States Governments
granted a subsidy, in return for which Government messages were to
have priority over all others, and were to be transmitted free. The
objections launched against the scheme were of course many, some of
them making very amusing reading. It is however very strange to find so
eminent a scientist as Professor Airy, then Astronomer Royal, seriously
stating that it was a mathematical impossibility to submerge a cable
safely to such depths, and that even if this could be done, messages
could not be transmitted through such a great length of cable.
It was estimated that a length of about 2500 nautical miles would be
enough to allow for all contingencies, and the construction of the
cable was commenced in February 1857, and completed in June of that
year. It is difficult to realize the gigantic nature of the task of
making a cable of such dimensions. The length of copper wire used in
making the conductor was 20,500 miles, while the outer sheathing took
367,500 miles of iron wire; the total length of wire used being enough
to go round the Earth thirteen times. The cable was finally stowed away
on board two warships, one British and the other American.
The real troubles began with the laying of the cable. After landing the
shore end in Valentia Bay, the paying-out commenced, but scarcely had
five miles been laid when the cable caught in the paying-out machinery
and parted. By tracing it from the shore the lost end was picked up
and spliced, and the paying-out began again. Everything went well for
two or three days, and then, after 380 miles had been laid, the cable
snapped again, owing to some mismanagement of the brakes, and was lost
at a depth of 2000 fathoms. The cable had to be abandoned, and the
ships returned to Plymouth.
In the next year, 1858, another attempt was made, with new and improved
machinery and 3000 miles of cable, and this time it was decided that
the two ships should start paying-out from mid-ocean, proceeding in
opposite directions towards the two shores after splicing their cables.
On the voyage out the expedition encountered one of the most fearful
storms on record, which lasted over a week, and the British man-of-war,
encumbered with the dead weight of the cable, came near to disaster.
Part of the cable shifted, and those on board feared that the whole of
the huge mass would break away and crash through the vessel’s side.
Sixteen days after leaving Plymouth the rendezvous was reached, the
cables were spliced and the ships started. After the British ship had
paid out 40 miles it was discovered that the cable had parted at some
distance from the ship, and the vessels once more sought each other,
and spliced again ready for another effort. This time the cable parted
after each vessel had paid out a little more than 100 miles, and the
ships were forced to abandon the attempt.
The failure of this second expedition naturally caused great
discouragement, and the general feeling was that the whole enterprise
would have to be given up. The chairman of the company recommended
that in order to make the best of a bad job the remainder of the cable
should be sold, and the proceeds divided amongst the shareholders, but
after great efforts on the part of a dauntless few who refused to admit
defeat, it was finally decided to make one more effort. No time was
lost, and on 17th July 1858 the vessels again sailed from Queenstown.
As before, the cables were spliced in mid-ocean, and this time, after
many anxious days, many false alarms, and one or two narrow escapes
from disaster through faulty pieces of cable discovered almost too
late, the cable was landed successfully on both shores of the Atlantic
early in August.
The Atlantic cable was now an accomplished fact, and dismal forebodings
were turned into expressions of extravagant joy. The first messages
passed between Queen Victoria and the President of the United States,
and amongst the more important communications was one which prevented
the sailing from Canada of two British regiments which had been ordered
to India during the Mutiny. In the meantime the Indian Mutiny had
been suppressed, and therefore these regiments were not required. The
dispatch of this message saved a sum of about £50,000. The prospects
of the cable company seemed bright, but after a short time the signals
began to grow weaker and weaker, and finally, after about seven hundred
messages had been transmitted, the cable failed altogether. This was
a great blow to the general public, and we can imagine the bitter
disappointment of the engineers and electricians who had laboured so
hard and so long to bring the cable into being. It was a favourable
opportunity for the croakers, and amongst a certain section of the
public doubts were expressed as to whether any messages had been
transmitted at all.
A great consultation of experts took place with the object of
determining the cause of the failure, and the unanimous opinion was
that the cable had been injured by the use of currents of too great
intensity. Some years elapsed before another attempt could be made, but
the idea was never abandoned, and a great deal of study was given to
the problems involved. Mr. Field, the most energetic of the original
projectors, never relaxed his determination that the cable should be
made a success, and he worked incessantly to achieve his ambition. It
is said that in pursuance of his object he made sixty-four crossings
of the Atlantic, and considering that he suffered greatly from
sea-sickness every time this shows remarkable pluck and endurance.
In 1865, new capital having been raised, preparations were made for
another expedition. It was now decided to use only one vessel for
laying the cable, and the _Great Eastern_ was chosen for the task. This
vessel had been lying idle for close on ten years, owing to her failure
as a cargo boat, but her great size and capacity made her most suitable
for carrying the enormous weight of the whole cable. In July 1865 the
_Great Eastern_ set sail, under the escort of two British warships.
When 84 miles had been paid out, a fault occurred, and after drawing
up about 10½ miles it was found that a piece of iron wire had pierced
the coating of the cable. The trouble was put right, and the paying-out
continued successfully until over 700 miles had been laid, when another
fault appeared. The cable was again drawn in until the fault was
reached, and another piece of iron was found piercing clean through.
It was evident that two such pieces of iron could not have got there
by accident, and there was no doubt that they had been inserted
intentionally by some malicious scoundrel, most likely with the object
of affecting the company’s shares. A start was made once more, and all
went well until about two-thirds of the distance had been covered, when
the cable broke and had to be abandoned after several nearly successful
attempts to recover it.
In spite of the loss, which amounted to £600,000, the energetic
promoters contrived to raise fresh capital, and in 1866 the _Great
Eastern_ started again. This effort was completely successful, and
on 28th July 1866 the cable was landed amidst great rejoicing. The
following extracts from the diary of the engineer Sir Daniell Gooch,
give us some idea of the landing.
“Is it wrong that I should have felt as though my heart would burst
when that end of our long line touched the shore amid the booming of
cannon, the wild, half-mad cheers and shouts of the men?... I am given
a never-dying thought; that I aided in laying the Atlantic cable....
The old cable hands seemed as though they could eat the end; one man
actually put it into his mouth and sucked it. They held it up and
danced round it, cheering at the top of their voices. It was a strange
sight, nay, a sight that filled our eyes with tears.... I did cheer,
but I could better have silently cried.”
This time the cable was destined to have a long and useful life, and
later in the same year the 1865 cable was recovered, spliced to a new
length, and safely brought to land, so that there were now two links
between the Old World and the New. It was estimated that the total
cost of completing the great undertaking, including the cost of the
unsuccessful attempts, was nearly two and a half millions sterling.
Since 1866 cable-laying has proceeded very rapidly, and to-day
telegraphic communication exists between almost all parts of the
civilized world. According to recent statistics, the North Atlantic
Ocean is now crossed by no less than 17 cables, the number of cables
all over the world being 2937, with a total length of 291,137 nautical
miles.
Before describing the actual working of a submarine cable, a few words
on cable-laying may be of interest. Before the cable-ship starts,
another vessel is sent over the proposed course to make soundings.
Galvanized steel pianoforte wire is used for sounding, and it is
wound in lengths of 3 or 4 nautical miles on gun-metal drums. The
drums are worked by an engine, and the average speed of working is
somewhere about 100 fathoms a minute in descending, and 70 fathoms a
minute in picking up. Some idea of the time occupied may be gained
from a sounding in the Atlantic Ocean which registered a depth of 3233
fathoms, or nearly 3½ miles. The sinker took thirty-three minutes fifty
seconds in descending, and forty-five minutes were taken in picking
up. The heavy sinker is not brought up with the line, but is detached
from the sounder by an ingenious contrivance and left at the bottom.
The sounder is fitted with an arrangement to bring up a specimen of the
bottom, and also a sample of water; and the temperature at any depth is
ascertained by self-registering thermometers.
When the soundings are complete the cable-ship takes up her task.
The cable is coiled in tanks on board, and is kept constantly under
water to prevent injury to the gutta-percha insulation by overheating.
As each section is placed in the tank, the ends of it are led to a
test-box, and labelled so that they can be easily recognized. Insulated
wires run from the test-box to instruments in the testing-room, so
that the electrical condition of the whole cable is constantly
under observation. During the whole time the cable is being laid its
insulation is tested continuously, and at intervals of five minutes
signals are sent from the shore end to the ship, so that a fault is
instantly detected. The cable in its tank is eased out by a number of
men, and mechanics are posted at the cable drums and brakes, while
constant streams of water cool the cable and the bearings and surfaces
of the brakes. The tension, as shown by the dynamometer, is at all
times under careful observation. When it becomes necessary to wind
back the cable on account of some fault, cuts are made at intervals of
a quarter or half a mile, tests being made at each cutting until the
fault is localized in-board. As soon as the cable out-board is found
“O.K.,” the ends are spliced up and the paying-out begins again. If the
cable breaks from any cause, a mark-buoy is lowered instantly on the
spot, and the cable is grappled for. This may take a day or two in good
weather, but a delay of weeks may be caused by bad weather, which makes
grappling impossible.
The practical working of a submarine cable differs in many respects
from that of a land telegraph line. The currents used in submarine
telegraphy are extremely small, contrary to the popular impression.
An insulated cable acts like a Leyden jar, in the sense that it
accumulates electricity and does not quickly part with it, as does
a bare overhead wire. In the case of a very long cable, such as one
across the Atlantic, a current continues to flow from it for some time
after the battery is disconnected. A second signal cannot be sent until
the electricity is dissipated and the cable clear, and if a powerful
current were employed the time occupied in this clearing would be
considerable, so that the speed of signalling would be slow. Another
objection to a powerful current is that if any flaw exists in the
insulation of the cable, such a current is apt to increase the flaw,
and finally cause the breakdown of the line.
The feebleness of the currents in submarine telegraphy makes it
impossible to use the ordinary land telegraph receiver, and a more
sensitive instrument known as the “mirror receiver” is used. This
consists of a coil of very fine wire, in the centre of which a tiny
magnetic needle is suspended by a fibre of unspun silk. A magnet placed
close by keeps the needle in one position when no current is flowing.
As the deflections of the needle are extremely small, it is necessary
to magnify them in some way, and this is done by fixing to the needle
a very small mirror, upon which falls a ray of light from a lamp. The
mirror reflects this ray on to a sheet of white paper marked with a
scale, and as the mirror moves along with the needle the point of light
travels over the paper, a very small movement of the needle causing the
light to travel some inches. The receiving operator sits in a darkened
room and watches the light, which moves to the right or to the left
according to the direction of the current. The signals employed are the
same as those for the single-needle instrument, a movement to the left
indicating a dot, and one to the right a dash. In many instruments the
total weight of magnet and mirror is only two or three grains, and the
sensitiveness is such that the current from a voltaic cell consisting
of a lady’s silver thimble with a few drops of acidulated water and a
diminutive rod of zinc, is sufficient to transmit a message across the
Atlantic.
The mirror receiver cannot write down its messages, and for recording
purposes an instrument invented by Lord Kelvin, and called the “siphon
recorder,” is used. In this instrument a coil of wire is suspended
between the poles of an electro-magnet, and to it is connected by
means of a silk fibre a delicate glass tube or siphon. One end of the
siphon dips into an ink-well, and capillary attraction causes the ink
to fill the siphon. The other end of the siphon almost touches a moving
paper ribbon placed beneath it. The ink and the paper are oppositely
electrified, and the attraction between the opposite charges causes
the ink to spurt out of the siphon in very minute drops, which fall
on to the paper. As long as no current is passing the siphon remains
stationary, but when a current flows from the cable through the coil,
the latter moves to one side or the other, according to the direction
of the current, and makes the siphon move also. Consequently, instead
of a straight line along the middle of the paper ribbon, a wavy line
with little peaks on each side of the centre is produced by the minute
drops of ink. This recorder sometimes refuses to work properly in damp
weather, owing to the loss of the opposite charges on ink and paper,
but a later inventor, named Cuttriss, has removed this trouble by
using a siphon kept constantly in vibration by electro-magnetism. The
ordinary single-needle code is used for the siphon recorder.
CHAPTER XVIII
THE TELEPHONE
In our younger days most of us have amused ourselves with a toy
telephone consisting of a long piece of string having each end passed
through the bottom of a little cardboard box, and secured by a knot.
If the string is stretched tightly this arrangement enables whispered
words to be heard at a distance of 20 or 30 yards. Simple as is this
little toy, yet it is probable that many people would be rather
nonplussed if asked suddenly to explain how the sounds travel along
the string from one box to the other. If the toy had some complicated
mechanism most likely every one would want to know how it worked, but
the whole thing is so extremely simple that generally it is dismissed
without a thought.
If we strike a tuning-fork and then hold it close to the ear, we hear
that it produces a sound, and at the same time, from a slight sensation
in the hand, we become aware that the fork is in vibration. As the fork
vibrates it disturbs the tiny particles of air round it and sets them
vibrating, and these vibrations are communicated from one particle to
another until they reach the drum of the ear, when that also begins to
vibrate and we hear a sound. This is only another way of saying that
the disturbances of the air caused by the vibrations of the tuning-fork
are propagated in a series of waves, which we call “sound waves.” Sound
is transmitted better through liquids than through the air, and better
still through solids, and this is why words spoken so softly as to be
inaudible through the air at a distance of, say, 100 feet, can be heard
fairly distinctly at that distance by means of the string telephone.
The sound reaches us along the string in exactly the same way as
through the air, that is, by means of minute impulses passed on from
particle to particle.
A more satisfactory arrangement than the string telephone consists of
two thin plates of metal connected by a wire which is stretched very
tightly. Words spoken close to one plate are heard by a listener at the
other plate up to a considerable distance. Let us try to see exactly
what takes place when this apparatus is used. In the act of speaking,
vibrations are set up in the air, and these in turn set up vibrations
in the metal plate. The vibrations are then communicated to the wire
and to the metal plate at the other end, and finally the vibrations
of this plate produce vibrations in the air between the plate and the
listener, and the sound reaches the ear.
This simple experiment shows the remarkable fact that a plate of
metal is able to reproduce faithfully all the vibrations communicated
to it by the human voice, and from this fact it follows that if we
can communicate the vibrations set up in one plate by the voice, to
another plate at a distance of 100 miles, we shall be able to speak
to a listener at the further plate just as if he were close to us. A
stretched string or wire transmits the vibrations fairly well up to
a certain distance, but beyond this distance the vibrations become
weaker and weaker until no sound at all reaches the air. By the aid of
electricity, however, we can transmit the vibrations to a tremendous
distance, the range being limited only by the imperfections of our
apparatus.
The first attempt at the construction of an electric telephone, that
is an instrument by means of which the vibrations set up by the voice
or by a musical instrument are transmitted by electricity, was made in
1860 by Johann Philipp Reis, a teacher in a school at Friedrichsdorf,
in Germany. His transmitting apparatus consisted of a box having a
hole covered by a tightly stretched membrane, to which was attached
a little strip of platinum. When the membrane was made to vibrate by
sounds produced close to the box, the strip of platinum moved to and
fro against a metal tip, which closed the circuit of a battery. The
receiver was a long needle of soft iron round which was wound a coil
of wire, and the ends of the needle rested on two little bridges of
a sounding box. The vibrations of the membrane opened and closed the
circuit at a great speed, and the rapid magnetization of the needle
produced a tone of the same pitch as the one which set the membrane
vibrating. This apparatus transmitted musical sounds and melodies with
great accuracy, but there is considerable difference of opinion as to
whether it was able to transmit speech. Professor Sylvanus Thompson
distinctly states that Reis’s telephone could and did transmit speech,
but other experts dispute the fact. We probably shall be quite safe
in concluding that this telephone did transmit speech, but very
imperfectly. In any case it is certain that the receiver of this
apparatus is not based on the same principle as the modern telephone
receiver.
Some years later Graham Bell, Professor of Vocal Physiology in the
University of Boston, turned his attention to the electric transmission
of speech, probably being led to do so from his experiments in
teaching the deaf and dumb. His apparatuses shown at an exhibition
in Philadelphia in 1876, consisted of a tube having one end open for
speaking into, and the other closed by a tightly stretched membrane to
which was attached a very light steel bar magnet. The vibrations set
up in the membrane by the voice made the little magnet move to and
fro in front of the poles of an electro-magnet, inserted in a battery
circuit, thus inducing currents of electricity in the coils of the
latter magnet. The currents produced in this way varied in direction
and strength according to the vibratory movements of the membrane,
and being transmitted along a wire they produced similar variations
in current in another electro-magnet in the receiver. The currents
produced in this manner in the receiver set up vibrations in a metal
diaphragm in front of the magnet poles, and so the words spoken into
the transmitter were reproduced.
Since the year 1876 the telephone has developed with remarkable
rapidity, and an attempt to trace its growth would involve a series
of detailed descriptions of closely similar inventions which would
be quite uninteresting to most readers. Now, therefore, that we have
introduced the instruments, and seen something of its principle and
its early forms, it will be most satisfactory to omit the intermediate
stages and to go on to the telephone as used in recent years. The first
telephone to come into general use was the invention of Graham Bell,
and was an improved form of his early instrument just described. A case
or tube of ebonite, which forms the handle of the instrument, contains
a steel bar magnet having a small coil of insulated wire at the end
nearest the mouthpiece of the tube, the ends of the coil passing along
the tube to be connected to the line wires. Close to the coil end of
the magnet, and between it and the mouthpiece, is fixed a diaphragm of
thin sheet-iron. A complete outfit consists of two of these instruments
connected by wires, and it will be noticed that no battery is employed.
The air vibrations set up by the voice make the diaphragm vibrate
also, so that it moves backwards and forwards. These movements are
infinitesimally small, but they are sufficient to affect the lines of
force of the magnet to such an extent that rapidly alternating currents
of varying degrees of strength are set up in the coil and sent along
the line wire. On arriving at the receiver these currents pass through
the coil and produce rapid variations in the strength of the magnet, so
that instead of exerting a uniform attraction upon the iron diaphragm,
the magnet pulls it with constantly varying force, and thus sets it
vibrating. The air in front of the diaphragm now begins to vibrate,
and the listener hears a reproduction of the words spoken into the
transmitter. The way in which the fluctuations of the current make the
second diaphragm vibrate exactly in accordance with the first is very
remarkable, and it is important to notice that the listener does not
hear the actual voice of the speaker, but a perfect reproduction of it;
in fact, the second diaphragm speaks.
The reader probably will be surprised to be told that the transmitter
and the receiver of a magneto-electric telephone are respectively a
dynamo and electric motor of minute proportions. We provide a dynamo
with mechanical motion and it gives us electric current, and by sending
this current through an electric motor we get mechanical motion
back again. In the transmitter of the telephone just described, the
mechanical motion is in the form of vibrations of the metal diaphragm,
which set up currents of electricity in the coil of wire round the
magnet, so that the transmitter is really a tiny dynamo driven by
the voice. The receiver is provided with electric current from the
transmitter, and it converts this into mechanical motion in the
diaphragm, so that the receiver is a little electric motor.
Transmitters of the type just described work well over short distances,
but the currents they produce are too feeble for transmission over
a very long wire, and on this account they have been superseded
by transmitters on the microphone principle. A microphone is an
instrument for making extremely small sounds plainly audible. If
a current is passed through a box containing loose bits of broken
carbon, it meets with great resistance, but if the bits of carbon are
compressed their conducting power is considerably increased. Even
such slight differences in pressure as are produced by vibrating the
box will affect the amount of current passing through the carbon. If
this current is led by wires to an ordinary telephone receiver the
arrangement becomes a simple form of microphone. The vibrations of the
box vary the resistance of the carbon, and the corresponding variations
in the current set up vibrations in the receiver, but in a magnified
form. The smallest sound vibrations alter the resistance of the carbon,
and as these vibrations are magnified in the receiver, the reproduced
sound is magnified also. The footsteps of a fly may be heard quite
distinctly by means of a good microphone, and the ticks of a watch
sound like the strokes of a hammer.
[Illustration: FIG. 31.--Diagram of Microphone Transmitter.]
By means of this power of magnifying vibrations a microphone
transmitter can be used on a line of tremendous length, where an
ordinary Bell transmitter would be utterly useless. The general
features of this transmitter, Fig. 31, are a diaphragm and a block
of carbon separated slightly from one another, the intervening space
being filled with granules of carbon. These are enclosed in a case of
ebonite having a mouthpiece in front and two terminals behind, one
terminal being connected with the carbon block and the other with the
diaphragm. From these terminals wires are led to a battery and to the
receiver, which is of the Bell type. The current has to pass through
the carbon granules, and the movements of the diaphragm when set in
vibration by the voice vary the pressure upon the granules, and in this
way set up variations in the current. Carbon dust also may be used
instead of granular carbon, and then the instrument is called a “dust
transmitter.”
[Illustration: FIG. 32.--Combined Telephone Transmitter and Receiver.]
It is usual to have a transmitter and a receiver on one handle for
the greater convenience of the user. The arrangement is shown in
Fig. 32, and it will be seen that when the user places the receiver
to his ear the transmitting mouthpiece is in position for speaking.
The microphone with its carbon dust is placed at A, just below the
mouthpiece, and the earpiece or receiver B contains a little magnet and
coil with a diaphragm in front, so that it is really a Bell instrument.
A little lever will be noticed at C. This is a switch which brings the
transmitter into circuit on being pressed with the finger.
It is now time to see something of the arrangement and working
of telephone systems. As soon as the telephone became a
commercially practicable instrument the necessity for some means of
inter-communication became evident, and the telephone exchange was
brought into being. The first exchange was started in 1877, in Boston,
but this was a very small affair and it was run on very crude lines.
When one subscriber wished to communicate with another he had to call
up an operator, who received the message and repeated it to the person
for whom it was intended; there was no direct communication between
the various subscribers’ instruments. As the number of users increased
it became necessary to devise some system whereby each subscriber could
call the attention of an operator at the central station, and be put
into direct communication with any other subscriber without delay; and
the exchange system of to-day, which fulfils these requirements almost
to perfection, is the result of gradual improvements in telephone
methods extending over some thirty-five years.
When a subscriber wishes to telephone, he first must call up the
operator at the exchange. Until comparatively recently this was done
by turning a handle placed at the side of the instrument. This handle
operated a little dynamo, and the current produced caused a shutter at
the exchange to drop and reveal a number, just as in the electric bell
indicator, so that the operator knew which instrument was calling. As
soon as the operator answered the call, the shutter replaced itself
automatically. The signal to disconnect was given in the same way, but
the indicator was of a different colour in order to prevent confusion
with a call signal. These handle-operated telephones are still in
common use, but they are being replaced by instruments which do away
with handle-turning on the part of the subscriber, and with dropping
shutters at the exchange. In this latest system all that the subscriber
has to do is to lift his telephone from its rest, when a little
electric lamp lights up at the exchange; and when he has finished his
conversation he merely replaces the telephone, and again a little lamp
glows.
We must now see what happens at the exchange when a call is made. Each
operator has control of a number of pairs of flexible cords terminating
in plugs, the two cords of each pair being electrically connected. The
plugs rest on a shelf in front of the operator, and the cords pass
through the shelf and hang down below it. If a plug is lifted, the cord
comes up through the shelf, and it is drawn back again by a weight
when the plug is not in use. Two lamps are provided for each pair of
cords, one being fixed close to each cord. The two wires leading from
each subscriber’s instrument are connected to a little tube-shaped
switch called a “jack,” and each jack has a lamp of its own. When a
subscriber lifts his telephone from its rest a lamp glows, and the
operator inserts one plug of a pair into the jack thus indicated, and
the lamp goes out automatically. She then switches on her telephone to
the caller and asks for the number of the subscriber to whom he wishes
to speak; and as soon as she gets this she inserts the other plug of
the pair into the jack belonging to this number. By a simple movement
she then rings up the required person by switching on the current to
his telephone bell.
Here comes in the use of the two lamps connected with the cords. As
long as the subscribers’ telephones are on their rests the lamps are
lighted, but as soon as they are lifted off the lamps go out. The
caller’s telephone is of course off its rest, and so the lamp connected
with the first cord is not lit; but until the subscriber rung up lifts
his instrument to answer the call, the lamp of the second cord remains
lit, having first lighted up when the plug was inserted in the jack
of his number. When the second lamp goes out the operator knows that
the call has been responded to, and that the two subscribers are in
communication with each other. Having finished their conversation,
both subscribers replace their instruments on the rests, whereupon
both lamps light up, informing the operator that she may disconnect by
pulling out the plugs.
It is manifestly impossible for one operator to attend to the calls of
all the subscribers in the exchange, and so a number of operators are
employed, each one having to attend to the calls of a certain number
of subscribers. At the same time it is clear that each operator may be
called upon to connect one of her subscribers to any other subscriber
in the whole exchange. In order to make this possible the switchboard
is divided into sections, each having as many jacks as there are lines
in the exchange, so that in this respect all the sections are multiples
of each other, and the whole arrangement is called a “multiple
switchboard,” the repeated jacks being called “multiple jacks.” Then
there are other jacks which it is not necessary to duplicate. We have
seen that when a subscriber calls the exchange a lamp glows, and
the operator inserts a plug into the jack beside the lamp, in order
to answer the call and ascertain what number is required. These are
called “answering jacks,” and the lamp is the line signal. It is usual
to have three operators to each section of the switchboard, and each
operator has charge of so many answering jacks, representing so many
subscribers. At the same time she has access to the whole section, so
that she can connect any of her subscribers to any other line in the
exchange.
When a number is called for, the operator must be able to tell at
once whether the line is free or not. The jack in her section may be
unoccupied, but she must know also whether all the multiple jacks
belonging to that number are free, for an operator at another section
may have connected the line to one of her subscribers. To enable an
operator to ascertain this quickly an electrical test is provided. When
two lines are connected, the whole of the multiple jacks belonging to
each are charged with electricity, and if an operator at any section
touches one of these jacks with a plug, a current through her receiver
makes a click, and on hearing the click she knows that the line is
engaged. The testing takes an extremely short time, and this is why a
caller receives the reply, “Number engaged,” so promptly that he feels
inclined to doubt whether the operator has made any attempt at all to
connect him up to the number.
In order that an operator may have both hands free to manipulate the
plugs, her telephone receiver is fixed over one ear by a fastening
passing over her head, and the transmitter is hung from her shoulders
so as to be close to her mouth.
In telegraphy it is the rule to employ the earth for the return part of
the circuit, but this is not customary in telephony. The telephone is
a much more sensitive instrument than the telegraph, and a telephone
having an earth return is subject to all kinds of strange and weird
noises which greatly interfere with conversation. These noises may
be caused by natural electrical disturbances, or by the proximity of
telegraph and other wires conveying electric currents. On this account
telephone lines are made with a complete metallic circuit. As in
telegraphy, protection from lightning flashes is afforded by lightning
arresters. The current for the working of a telephone exchange is
supplied from a central battery of accumulators, and also from dynamos.
[Illustration: PLATE XII.
_By permission of_ _Craven Brothers Ltd._
LARGE ELECTRIC TRAVELLING CRANE AT A RAILWAY WORKS.]
Although the manual exchange telephone system of to-day works with
remarkable efficiency, it has certain weak points. For instance, if
an operator cares to do so, she can listen to conversations between
subscribers, so that privacy cannot be assured. As a matter of
fact, the operators have little time for this kind of thing, at any
rate during the busy hours of the day, and as a rule they are not
sufficiently interested in other people’s affairs to make any attempt
to listen to their remarks. The male operators who work through the
slack hours of the night are occasionally guilty of listening. Some
time ago the writer had to ring up a friend in the very early
morning, and during the conversation this gentleman asked what time it
was. Before the writer had time to get a word out, a deep bass voice
from the exchange replied, “Half-past two.” Little incidents of this
sort remind one that it is not wise to speak too freely by telephone.
Then again operators are liable to make wrong connexions through faulty
hearing of the number called for, and these are equally annoying to the
caller and to the person rung up in mistake. Many other defects might
be mentioned, but these are sufficient to show that the manual system
is not perfect.
For a long time inventors have been striving to do away with all
such defects by abolishing the exchange operators, and substituting
mechanism to work the exchanges automatically, and during the last
few years the system of the Automatic Electric Company, of Chicago,
has been brought to great perfection. This system is in extensive use
in the United States, and is employed in two or three exchanges in
this country. Unfortunately the mechanism of this system is extremely
complicated, so that it is impossible to describe it fully in a book of
this kind; but some idea of the method of working may be given without
entering into technical details.
Each subscriber’s telephone instrument is fitted with a dial which
turns round on a pivot at its centre. This dial has a series of holes
round its circumference, numbered consecutively from 1 to 9, and 0.
Suppose now a subscriber wishes to speak to a friend whose telephone
number is 2583. He removes the receiver from its hook, places his
finger in the hole marked 2, and turns the dial round in a clockwise
direction until his finger comes in contact with a stop. He then
removes his finger, and the dial automatically returns to its original
position. He then places his finger in the hole marked 5, and again
turns the dial as far as the stop, and when the dial has returned
to the normal position he repeats the process with his finger placed
successively in the holes marked 8 and 3. He now places the receiver
to his ear, and by the time he has done this the automatic mechanism
at the exchange has made the necessary connexions, and has rung the
bell of subscriber number 2583. On completing the conversation each
subscriber returns his receiver to its hook, and the exchange mechanism
returns to its normal position.
The turning of the dial by the finger coils up a spring, and this
spring, acting along with a speed governor, makes the dial return to
its first position at a certain definite speed as soon as the finger
is removed. During this retrograde movement a switch automatically
sends out into the line a certain number of impulses, the number being
determined by the hole in which the finger is placed. In the case
supposed, groups of two, five, eight, and three impulses respectively
would be sent out, each group separated from the next by an interval
during which the subscriber is turning the dial.
Now let us see what takes place at the exchange. The subscriber’s
instrument is connected to a mechanical arrangement known as a “line
switch.” This switch is brought into play by the act of removing
the receiver from its hook, and it then automatically connects the
subscriber’s line to what is called a “first selector” switch. The
group of two impulses sent out by the first turning of the dial raises
this first selector two steps, and it then sweeps along a row of
contacts connected to “trunks” going to the 2000 section. Passing by
occupied trunks, it finds an idle one, and so connects the line to an
idle “second selector.” This selector is operated by the second group
of impulses, five in number, and after being raised five steps it
acts like the first selector, and finds an idle trunk leading to the
2500 section. This places the caller’s line in connexion with still
another switch called a “connector,” and this switch, operated by the
remaining groups of eight and three impulses, finds the required tens
section, and selects the third member of that section. If the number
2583 is disengaged, the connector switch now sends current from the
central battery to this instrument, thus ringing its bell, and it also
supplies speaking current to the two lines during the conversation,
restores the exchange mechanism to its original condition as soon
as the conversation is ended and the subscribers have hung up their
receivers, and registers the call on the calling subscriber’s meter. If
the connector finds the number engaged, it sends out an intermittent
buzzing sound, to inform the caller of the fact. All these operations
take time to describe, even in outline, but in practice they are
carried out with the utmost rapidity, each step in the connecting-up
process taking only a small fraction of a second.
For ordinary local calls the automatic system requires no operators
at all, but for the convenience of users there are usually two clerks
at the exchange, one to give any information required by subscribers,
and the other to record complaints regarding faulty working. For trunk
calls, the subscriber places his finger in the hole marked 0, and
gives the dial one turn. This connects him to an operator at the trunk
switchboard, who makes the required connexion and then calls him up in
the usual way.
It might be thought that the complex mechanism of an automatic exchange
would constantly be getting out of order, but it is found to work with
great smoothness. Each automatic switchboard has a skilled electrician
in attendance, and he is informed instantly of any faulty working
by means of supervisory lamps and other signals. Even without these
signals the attendant would be quickly aware of any breakdown, for his
ear becomes so accustomed to the sounds made by the apparatus during
the connecting-up, that any abnormal sound due to faulty connecting
attracts his attention at once. However detected, the faults are put
right immediately, and it often happens that a defective line is noted
and repaired before the subscriber knows that anything is wrong.
On account of its high speed in making connexions and disconnexions,
its absolute accuracy, and its privacy, the automatic telephone system
has proved most popular wherever it has been given a fair trial. Its
advantages are most obvious in large city exchanges where the traffic
during business hours is tremendously heavy, and it is probable that
before very long the automatic system will have replaced manual methods
for all such exchanges.
The telephone system is more highly developed in the United States than
in this country, and some of the exchanges have been made to do a great
deal more than simply transmit messages. For instance, in Chicago there
is a system by which a subscriber, on connecting himself to a special
circuit, is automatically informed of the correct time, by means of
phonographs, between the hours of 8 a.m. and 10 p.m. New York goes
further than this however, and has a regular system of news circulation
by telephone. According to _Electricity_, the daily programme is
as follows: “8 a.m., exact astronomical time; 8 to 9 a.m., weather
reports, London Stock Exchange news, special news item; 9 to 9.30
a.m., sales, amusements, business events; 9.45 to 10 a.m., personal
news, small notices; 10 to 10.30 a.m., New York Stock Exchange and
market news; 11.30 a.m. to 12 noon, local news, miscellaneous; 12 noon,
exact astronomical time, latest telegrams, military and parliamentary
news; 2 to 2.15 p.m., European cables; 1.15 to 2.30 p.m., Washington
news; 2.30 to 2.45 p.m., fashions, ladies’ news; 2.45 to 3.15 p.m.,
sporting and theatrical news; 3.15 to 3.30 p.m., closing news from Wall
Street; 3.30 to 5 p.m., musical news, recitals, etc.; 5 to 6 p.m.,
feuilleton sketches, literary news; 8 to 10.30 p.m., selected evening
performance--music, opera, recitations.” Considering the elaborate
nature of this scheme one might imagine that the subscription would be
high, but as a matter of fact it is only six shillings per month.
The telephone has proved of great value in mine rescue work, in
providing means of communication between the rescue party and those
in the rear. This end is achieved by means of a portable telephone,
but as the members of a rescue party often wear oxygen helmets, the
ordinary telephone mouthpiece is of no use. To overcome this difficulty
the transmitter is fastened round the throat. The vibrations of the
vocal cords pass through the wall of the throat, and thus operate the
transmitter. The receiver is fixed over one ear by means of suitable
head-gear, and the connecting wire is laid by the advancing rescuers.
A case containing some hundreds of feet of wire is strapped round
the waist, and as the wearer walks forward this wire pays itself out
automatically.
By the time that the telephone came to be a really practical
instrument, capable of communicating over long distances on land, the
Atlantic telegraph cable was in operation, and an attempt was made to
telephone from one continent to the other by means of it, but without
success. In speaking of submarine telegraphy in Chapter XVII. we saw
that the cable acts like a Leyden jar, and it was this fact that made
it impossible to telephone through more than about 20 miles of cable,
so that transatlantic telephony was quite out of the question. It was
evident that little progress could be made in this direction unless
some means could be devised for neutralizing this capacity effect, as
it is called, of the cable, and finally it was discovered that this
could be done by inserting at intervals along the cable a number of
coils of wire. These coils are known as “loading coils,” and a cable
provided with them is called a “loaded cable.” Such cables have been
laid across various narrow seas, such as between England and France,
and England and Ireland, and these have proved very successful for
telephonic communication. The problem of transatlantic telephony
however still remains to be solved. Experiments have been made in
submarine telephony over a bare iron cable, instead of the usual
insulated cable. Conversations have been carried on in this way without
difficulty between Seattle, Washington, U.S.A., and Vashon Island, a
total distance of about 11 miles, and it is possible that uninsulated
cables may play an extremely important part in the development of
submarine telephony.
CHAPTER XIX
SOME TELEGRAPHIC AND TELEPHONIC INVENTIONS
In telegraphy messages not only may be received, but also recorded,
by the Morse printer or one of its modifications, but in ordinary
telephony there is no mechanical method of recording messages. This
means that we can communicate by telephone only when we can call up
somebody to receive the message at the other end, and if no one happens
to be within hearing of the telephone bell we are quite helpless. This
is always annoying, and if the message is urgent the delay may be
serious. Several arrangements for overcoming this difficulty by means
of automatic recording mechanism have been invented, but the only
really successful one is the telegraphone.
This instrument is the invention of Waldemar Poulsen, whose apparatus
for wireless telegraphy we shall speak of in the next chapter. The
telegraphone performs at the same time the work of a telephone and
of a phonograph. In the ordinary type of phonograph the record is
made in the form of depressions or indentations on the surface of a
cylinder of wax; these indentations being produced by a stylus actuated
by vibrations set up in a diaphragm by the act of speaking. In the
telegraphone the same result is obtained entirely by electro-magnetic
action. The wax cylinder of the phonograph is replaced by a steel wire
or ribbon, and the recording stylus by an electro-magnet. The steel
ribbon is arranged to travel along over two cylinders or reels kept in
constant rotation, and a small electro-magnet is fixed midway between
the cylinders so that the ribbon passes close above it. This magnet
is connected to the telephone line, so that its magnetism fluctuates
in accordance with the variations in the current in the line. We have
seen that steel retains magnetism imparted to it. In passing over the
electro-magnet the steel ribbon is magnetized in constantly varying
degrees, corresponding exactly with the variations in the line current
set up by the speaker’s voice, and these magnetic impressions are
retained by the ribbon. When the speaker has finished, the telephone
line is disconnected, the ribbon is carried back to the point at which
it started, and the apparatus is connected to the telephone receiver.
The ribbon now moves forward again, and this time it acts like the
speaker’s voice, the varying intensity of its magnetic record producing
corresponding variations in the strength of the magnet, so causing the
receiver diaphragm to reproduce the sounds in the ordinary way.
The magnetic record made in this manner is fairly permanent, and if
desired it may be reproduced over and over again. In most cases,
however, a permanent record is of no value, and so the magnetic
impressions are obliterated in order that the ribbon may be used to
take a new record. This can be done by passing a permanent magnet along
the ribbon, but it is more convenient to have an automatic obliterating
arrangement. This consists of another electro-magnet fixed close to the
recording magnet, so that the ribbon passes over it before reaching the
latter. The obliterating magnet is connected with a battery, and its
unvarying magnetism destroys all traces of the previous record, and
the ribbon passes forward to the recording magnet ready to receive new
impressions.
For recording telephone messages the telegraphone is attached to the
telephone instrument, and by automatically operated switches it is
set working by a distant speaker. It records all messages received
during the absence of its owner, who, on his return, connects it to
his receiver, and thus hears a faithful reproduction of every word.
By speaking into his instrument before going out, the owner can leave
a message stating the time at which he expects to return, and this
message will be repeated by the telegraphone to anybody ringing up
in the meantime. The most recent forms of telegraphone are capable
of recording speeches over an hour in length, and their reproduction
is as clear as that of any phonograph, indeed in many respects it is
considerably more perfect.
Another electrical apparatus for recording speech may be mentioned.
This rejoices in the uncouth name of the Photographophone, and it is
the invention of Ernst Ruhmer, a German. Its working is based upon
the fact that the intensity of the light of the electric arc may be
varied by sound vibrations, each variation in the latter producing a
corresponding variation in the amount of light. In the photographophone
the light of an arc lamp is passed through a lens which focuses it
upon a moving photographic film. By speaking or singing, the light is
made to vary in brilliance, and proportionate effects are produced in
the silver bromide of the film. On developing the film a permanent
record of the changes in the light intensity is obtained, in the
form of shadings of different degrees of darkness. The film is now
moved forward from end to end in front of a fairly powerful lamp. The
light passes through the film, and falls upon a sort of plate made of
selenium. This is a non-metallic substance which possesses the curious
property of altering its resistance to an electric current according
to the amount of light falling upon it; the greater the amount of
light, the more current will the selenium allow to pass. The selenium
plate is connected with a telephone receiver and with a battery. As
the film travels along, its varying shadings allow an ever-changing
amount of light to pass through and fall upon the selenium, which
varies its resistance accordingly. The resulting variations in the
current make the receiver diaphragm give out a series of sounds, which
are exact reproductions of the original sounds made by the voice. The
reproduction of speech by the photographophone is quite good, but as a
rule it is not so perfect as with the telegraphone.
About ten years ago a German inventor, Professor A. Korn, brought
out the first really practical method of telegraphing drawings or
photographs. This invention is remarkable not only for what it
accomplishes, but perhaps still more for the ingenuity with which
the many peculiar difficulties of the process are overcome. Like the
photographophone, Korn’s photo-telegraphic apparatus utilizes the power
of selenium to alter its resistance with the amount of light reaching
it.
Almost everybody is familiar with the terms “positive” and “negative”
as used in photography. The finished paper print is a positive, with
light and shade in the correct positions; while the glass plate from
which the print is made is a negative, with light and shade reversed.
The lantern slide also is a positive, and it is exactly like the
paper print, except that it has a base of glass instead of paper, so
that it is transparent. Similarly, a positive may be made on a piece
of celluloid, and this, besides being transparent, is flexible. The
first step in transmitting on the Korn system is to make from the
photograph to be telegraphed a positive of this kind, both transparent
and flexible. This is bent round a glass drum or cylinder, and fixed
so that it cannot possibly move. The cylinder is given a twofold
movement. It is rotated by means of an electric motor, and at the same
time it is made to travel slowly along in the direction of its length.
In fact its movement is very similar to that of a screw, which turns
round and moves forward at the same time. A powerful beam of light is
concentrated upon the positive. This beam remains stationary, but owing
to the dual movement of the cylinder it passes over every part of the
positive, following a spiral path. Exactly the same effect would be
produced by keeping the cylinder still and moving the beam spirally
round it, but this arrangement would be more difficult to manipulate.
The forward movement of the cylinder is extremely small, so that the
spiral is as fine as it is possible to get it without having adjacent
lines actually touching. The light passes through the positive into the
cylinder, and is reflected towards a selenium cell; and as the positive
has an almost infinite number of gradations of tone, or degrees
of light and shade, the amount of light reaching the cell varies
constantly all the time. The selenium therefore alters its resistance,
and allows a constantly varying current to pass through it, and so to
the transmission line.
At the receiving end is another cylinder having the same rotating and
forward movement, and round this is fixed a sensitive photographic
film. This film is protected by a screen having a small opening, and
no light can reach it except through this aperture. The incoming
current is made to control a beam of light focused to fall upon
the screen aperture, the amount of light varying according to the
amount of current. In this way the beam of light, like the one at the
transmitting end, traces a spiral from end to end of the film, and
on developing the film a reproduction of the original photograph is
obtained. The telegraphed photograph is thus made up of an enormous
number of lines side by side, but these are so close to one another
that they are scarcely noticed, and the effect is something like that
of a rather coarse-grained ordinary photograph.
It is obvious that the success of this method depends upon the
maintaining of absolute uniformity in the motion of the two cylinders,
and this is managed in a very ingenious way. It will be remembered
that one method of securing uniformity in a number of sub-clocks under
the control of a master-clock is that of adjusting the sub-clocks to
go a little faster than the master-clock. Then, when the sub-clocks
reach the hour, they are held back by electro-magnetic action until the
master-clock arrives at the hour, when all proceed together.
A similar method is employed for the cylinders. They are driven by
electric motors, and the motor at the receiving end is adjusted so
as to run very slightly faster than the motor at the sending end.
The result is that the receiving cylinder completes one revolution
a minute fraction of a second before the transmitting cylinder. It
is then automatically held back until the sending cylinder completes
its revolution, and then both commence the next revolution exactly
together. The pause made by the receiving cylinder is of extremely
short duration, but in order that there shall be no break in the spiral
traced by light upon the film, the pause takes place at the point where
the ends of the film come together. In actual practice certain other
details of adjustment are required to ensure precision in working, but
the main features of the process are as described.
Although the above photo-telegraphic process is very satisfactory in
working, it has been superseded to some extent by another process of
a quite different nature. By copying the original photograph through
a glass screen covered with a multitude of very fine parallel lines,
a half-tone reproduction is made. This is formed of an immense number
of light and dark lines of varying breadth, and it is printed in
non-conducting ink on lead-foil, so that while the dark lines are bare
foil, the light ones are covered with the ink. This half-tone is placed
round a metal cylinder having the same movement as the cylinders in the
previous processes, and a metal point, or “stylus” as it is called,
is made to rest lightly upon the foil picture, so that it travels all
over it, from one end to the other. An electrical circuit is arranged
so that when the stylus touches a piece of the bare foil a current is
sent out along the line wire. This current is therefore intermittent,
being interrupted each time the stylus passes over a part of the
half-tone picture covered with the non-conducting ink, the succeeding
periods of current and no current varying with the breadth of the
conducting and the non-conducting lines. This intermittent current goes
to a similar arrangement of stylus and cylinder at the receiving end,
this cylinder having round it a sheet of paper coated with a chemical
preparation. The coating is white all over to begin with, but it turns
black wherever the current passes through it. The final result is that
the intermittent current builds up a reproduction in black-and-white of
the original photograph. In this process also the cylinders have to be
“synchronized,” or adjusted to run at the same speed. Both this process
and the foregoing one have been used successfully for the transmission
of press photographs, notably by the _Daily Mirror_.
Professor Korn has carried out some interesting and fairly successful
experiments in wireless transmission of photographs, but as yet the
wireless results are considerably inferior to those obtained with
a line conductor. For transmitting black-and-white pictures, line
drawings, or autographs by wireless, a combination of the two methods
just mentioned is employed; the second method being used for sending,
and the first or selenium method for receiving. For true half-tone
pictures the selenium method is used at each end.
CHAPTER XX
WIRELESS TELEGRAPHY AND TELEPHONY--PRINCIPLES AND APPARATUS
Wireless telegraphy is probably the most remarkable and at the
same time the most interesting of all the varied applications
of electricity. The exceptional popular interest in wireless
communication, as compared with most of the other daily tasks which
electricity is called upon to perform, is easy to understand. The
average man does not realize that although we are able to make
electricity come and go at our bidding, we have little certain
knowledge of its nature. He is so accustomed to hearing of the electric
current, and of the work it is made to do, that he sees little to
marvel at so long as there is a connecting wire. Electricity is
produced by batteries or by a dynamo, sent along a wire, and made to
drive the necessary machinery; apparently it is all quite simple. But
take away the connecting wire, and the case is different. In wireless
telegraphy electricity is produced as usual, but instantly it passes
out into the unknown, and, as far as our senses can tell, it is lost
for ever. Yet at some distant point, hundreds or even thousands of
miles away, the electrical influence reappears, emerging from the
unknown with its burden of words and sentences. There is something
uncanny about this, something suggesting telepathy and the occult, and
herein lies the fascination of wireless telegraphy.
The idea of communicating without any connecting wires is an old
one. About the year 1842, Morse, of telegraph fame, succeeded in
transmitting telegraphic signals across rivers and canals without a
connecting wire. His method was to stretch along each bank of the river
a wire equal in length to three times the breadth of the river. One of
these wires was connected with the transmitter and with a battery, and
the other with a receiver, both wires terminating in copper plates sunk
in the water. In this case the water took the place of a connecting
wire, and acted as the conducting medium. A few years later another
investigator, a Scotchman named Lindsay, succeeded in telegraphing
across the river Tay, at a point where it is over a mile and a half
wide, by similar methods. Lindsay appears to have been the first to
suggest the possibility of telegraphing across the Atlantic, and
although at that time, 1845, the idea must have seemed a wild one, he
had the firmest faith in its ultimate accomplishment.
Amongst those who followed Lindsay’s experiments with keen interest
was the late Sir William, then Mr. Preece, but it was not until 1882,
twenty years after Lindsay’s death, that he commenced experiments on
his own account. In March of that year the cable across the Solent
failed, and Preece took the opportunity of trying to signal across
without a connecting wire. He used two overhead wires, each terminating
in large copper plates sunk in the sea, one stretching from Southampton
to Southsea Pier, and the other from Ryde Pier to Sconce Point. The
experiment was successful, audible Morse signals being received on
each side. In this experiment, as in those of Morse and Lindsay, the
water acted as the conducting medium; but a year or two later, Preece
turned his attention to a different method of wireless communication,
by means of induction. This method was based upon the fact that at the
instant of starting and stopping a current in one wire, another current
is induced in a second wire placed parallel to it, even when the two
wires are a considerable distance apart. Many successful experiments
in this induction telegraphy were made, one of the most striking
being that between the Island of Mull and the mainland, in 1895. The
cable between the island and the mainland had broken, and by means
of induction perfect telegraphic communication was maintained during
the time that the cable was being repaired. Although this system of
wireless telegraphy is quite successful for short distances, it becomes
impracticable when the distance is increased, because the length of
each of the two parallel wires must be roughly equal to the distance
between them. These experiments of Preece are of great interest, but
we must leave them because they have little connexion with present-day
wireless telegraphy, in which utterly different methods are used.
All the commercial wireless systems of to-day depend upon the
production and transmission of electric waves. About the year 1837
it was discovered that the discharge of a Leyden jar did not consist
of only one sudden rush of electricity, but of a series of electric
oscillations, which surged backwards and forwards until electric
equilibrium was restored. This discovery was verified by later
experimenters, and it forms the foundation of our knowledge of electric
waves. At this point many readers probably will ask, “What are electric
waves?” It is impossible to answer this question fully, for we still
have a great deal to learn about these waves, and we only can state the
conclusions at which our greatest scientists have arrived after much
thought and many experiments. It is believed that all space is filled
with a medium to which the name “ether” has been given, and that this
ether extends throughout the matter. We do not know what the ether is,
but the important fact is that it can receive and transmit vibrations
in the form of ether waves. There are different kinds of ether waves,
and they produce entirely different effects. Some of them produce the
effect which we call light, and these are called “light waves.” Others
produce the effect known as heat, and they are called “heat waves”; and
still others produce electricity, and these we call “electric waves.”
These waves travel through the ether at the enormous speed of 186,000
miles per second, so that they would cross the Atlantic Ocean in about
1/80 second. The fact that light also travels at this speed suggested
that there might be some connexion between the two sets of waves, and
after much experiment it has been demonstrated that the waves of light
and electricity are identical except in their length.
Later on in this chapter we shall have occasion to refer frequently
to wave-length, and we may take this opportunity of explaining what
is understood by this term. Wave-length is the distance measured from
the crest of one wave to the crest of the next, across the intervening
trough or hollow. From this it will be seen that the greater the
wave-length, the farther apart are the waves; and also that if we have
two sets of waves of different wave-lengths but travelling at the same
speed, then the number of waves arriving at any point in one second
will be greater in the case of the shorter waves, because these are
closer together.
A tuning-fork in vibration disturbs the surrounding air, and sets
up air waves which produce the effect called sound when they strike
against the drums of our ears. In a similar way the discharge of a
Leyden jar disturbs the surrounding ether, and sets up electric ether
waves; but these waves produce no effect upon us in the shape of sight,
sound, or feeling. There is however a very simple piece of apparatus
which acts as a sort of electric eye or ear, and detects the waves for
us. This consists of a glass tube loosely filled with metal filings,
and having a cork at each end. A wire is passed through each cork so
as to project well into the tube, but so that the two ends do not
touch one another, and the outer ends of these wires are connected to
a battery of one or two cells, and to some kind of electrically worked
apparatus, such as an electric bell. So long as the filings lie quite
loosely in the tube they offer a very high resistance, and no current
passes. If now electric waves are set up by the discharge of a Leyden
jar, these waves fall upon the tube and cause the resistance of the
filings to decrease greatly. The filings now form a conducting path
through which the current passes, and so the bell rings. If no further
discharge takes place the electric waves cease, but the filings do not
return to their original highly resistant condition, but retain their
conductivity, and the current continues to pass, and the bell goes on
ringing. To stop the bell it is only necessary to tap the tube gently,
when the filings immediately fall back into their first state, so that
the current cannot pass through them.
Now let us see how the “coherer,” as the filings tube is called,
is used in actual wireless telegraphy. Fig. 33_a_ shows a simple
arrangement for the purpose. A is an induction coil, and B the battery
supplying the current. The coil is fitted with a spark gap, consisting
of two highly polished brass balls CC, one of these balls being
connected to a vertical wire supported by a pole, and the other to
earth. D is a Morse key for starting and stopping the current. When the
key is pressed down, current flows from the battery to the coil, and
in passing through the coil it is raised to a very high voltage, as
described in Chapter VIII. This high tension current is sent into the
aerial wire, which quickly becomes charged up to its utmost limits. But
more current continues to arrive, and so the electricity in the aerial,
unable to bear any longer the enormous pressure, takes the only path
of escape and bursts violently across the air gap separating the brass
balls. Surging oscillations are then produced in the aerial, the ether
is violently disturbed, and electric waves are set in motion. This is
the transmitting part of the apparatus.
[Illustration: _a._ Transmitting.
_b._ Receiving.
FIG. 33.--Diagram of simple Wireless Transmitting and Receiving
Apparatus.]
If a stone is dropped into a pond, little waves are set in motion, and
these spread outwards in ever-widening rings. Electric waves also are
propagated outwards in widening rings, but instead of travelling in
one plane only, like the water waves, they proceed in every plane; and
when they arrive at the receiving aerial they set up in it oscillations
of the same nature as those which produced the waves. Let us suppose
electric waves to reach the aerial wire of Fig. 33_b_. The resistance
of the coherer H is at once lowered so that current from battery N
flows and operates the relay F, which closes the circuit of battery
M. This battery has a twofold task. It operates the sounder E, and
it energizes the electro-magnet of the de-coherer K, as shown by the
dotted lines. This de-coherer is simply an electric bell without the
gong, arranged so that the hammer strikes the coherer tube; and its
purpose is to tap the tube automatically and much more rapidly than
is possible by hand. The sounder therefore gives a click, and the
de-coherer taps the tube, restoring the resistance of the filings. The
circuit of battery N is then broken, and the relay therefore interrupts
the circuit of battery M. If waves continue to arrive, the circuits
are again closed, another click is given, and again the hammer taps
the tube. As long as waves are falling upon the aerial, the alternate
makings and breakings of the circuits follow one another very rapidly
and the sounder goes on working. When the waves cease, the hammer of
the de-coherer has the last word, and the circuits of both batteries
remain broken. To confine the electric waves to their proper sphere two
coils of wire, LL, called choking coils, are inserted as shown.
In this simple apparatus we have all the really essential features of
a wireless installation for short distances. For long distance work
various modifications are necessary, but the principle remains exactly
the same. In land wireless stations the single vertical aerial wire
becomes an elaborate arrangement of wires carried on huge masts and
towers. The distance over which signals can be transmitted and received
depends to a considerable extent upon the height of the aerial, and
consequently land stations have the supporting masts or towers from
one to several hundred feet in height, according to the range over
which it is desired to work. As a rule the same aerial is used both for
transmitting and receiving, but some stations have a separate aerial
for each purpose. A good idea of the appearance of commercial aerials
for long distance working may be obtained from the frontispiece,
which shows the Marconi station at Glace Bay, Nova Scotia, from which
wireless communication is held with the Marconi station at Clifden, in
Galway, Ireland.
In the first wireless stations what is called a “plain aerial”
transmitter was used, and this was almost the same as the transmitting
apparatus in Fig. 33_a_, except, of course, that it was on a larger
scale. This arrangement had many serious drawbacks, including that
of a very limited range, and it has been abandoned in favour of the
“coupled” transmitter, a sketch of which is shown in Fig. 34. In this
transmitter there are two separate circuits, having the same rate of
oscillation. A is an induction coil, supplied with current from the
battery B, and C is a condenser. A condenser is simply an apparatus
for storing up charges of electricity. It may take a variety of forms,
but in every case it must consist of two conducting layers separated
by a non-conducting layer, the latter being called the “dielectric.”
The Leyden jar is a condenser, with conducting layers of tinfoil and
a dielectric of glass, but the condensers used for wireless purposes
generally consist of a number of parallel sheets of metal separated
by glass or mica, or in some cases by air only. The induction coil
charges up the condenser with high tension electricity, until the
pressure becomes so great that the electricity is discharged in the
form of a spark between the brass balls of the spark gap D. The
accumulated electric energy in the condenser then surges violently
backwards and forwards, and by induction corresponding surgings are
produced in the aerial circuit, these latter surgings setting up
electric waves in the ether.
[Illustration: FIG. 34.--Wireless “Coupled” Transmitter.]
For the sake of simplicity we have represented the apparatus as using
an induction coil, but in all stations of any size the coil is replaced
by a step-up transformer, and the current is supplied either from an
electric light power station at some town near by, or from a power
house specially built for the purpose. Alternating current is generally
used, and if the current supplied is continuous, it is converted into
alternating current. This may be done by making the continuous current
drive an electric motor, which in turn drives a dynamo generating
alternating current. In any case, the original current is too low in
voltage to be used directly, but in passing through the transformer
it is raised to the required high pressure. The transmitting key,
which is inserted between the dynamo and the transformer, is specially
constructed to prevent the operator from receiving accidental shocks,
and the spark gap is enclosed in a sort of sound-proof box, to deaden
the miniature thunders of the discharge.
During the time that signals are being transmitted, sparks follow one
another across the spark gap in rapid succession, a thousand sparks per
second being by no means an uncommon rate. The violence of these rapid
discharges raises the brass balls of the gap to a great heat. This has
the effect of making the sparking spasmodic and uncertain, with the
result that the signals at the receiving station are unsatisfactory.
To get over this difficulty Marconi introduced a rotary spark gap.
This is a wheel with projecting knobs or studs, mounted on the shaft
of the dynamo supplying the current, so that it rotates rapidly. Two
stationary knobs are fixed so that the wheel rotates between them, and
the sparks are produced between these fixed knobs and those of the
wheel, a double spark gap thus being formed. Overheating is prevented
by the currents of air set up by the rapid movement of the wheel, and
the sparking is always regular.
[Illustration: PLATE XIII.
_Photo by_ _Daily Mirror_.
(_a_) MARCONI OPERATOR RECEIVING A MESSAGE.]
[Illustration:
_By permission of_ _The Marconi Co. Ltd._
(_b_) MARCONI MAGNETIC DETECTOR.]
In the receiving apparatus already described a filings coherer was used
to detect the ether waves, and, by means of a local battery, to
translate them into audible signals with a sounder, or printed signals
with a Morse inker. This coherer however is unsuitable for commercial
working. It is not sufficiently sensitive, and it can be used only
for comparatively short distances; while its action is so slow that
the maximum speed of signalling is not more than about seventeen or
eighteen words a minute. A number of different detectors of much
greater speed and sensitiveness have been devised. The most reliable of
these, though not the most sensitive, is the Marconi magnetic detector,
Plate XIII._b_. This consists of a moving band made of several soft
iron wires twisted together, and passing close to the poles of two
horse-shoe magnets. As the band passes from the influence of one magnet
to that of the other its magnetism becomes reversed, but the change
takes a certain amount of time to complete owing to the fact that the
iron has some magnetic retaining power, so that it resists slightly the
efforts of one magnet to reverse the effect of the other. The moving
band passes through two small coils of wire, one connected with the
aerial, and the other with a specially sensitive telephone receiver.
When the electric waves from the transmitting station fall upon the
aerial of the receiving station, small, rapidly oscillating currents
pass through the first coil, and these have the effect of making the
band reverse its magnetism instantly. The sudden moving of the lines
of magnetic force induces a current in the second coil, and produces
a click in the telephone. As long as the waves continue, the clicks
follow one another rapidly, and they are broken up into the long and
short signals of the Morse code according to the manipulation of the
Morse key at the sending station. Except for winding up at intervals
the clockwork mechanism which drives the moving band, this detector
requires no attention, and it is always ready for work.
Another form of detector makes use of the peculiar power possessed
by certain crystals to rectify the oscillatory currents received
from the aerial, converting them into uni-directional currents. At
every discharge of the condenser at the sending station a number of
complete waves, forming what is called a “train” of waves, is set in
motion. From each train of waves the crystal detector produces one
uni-directional pulsation of current, and this causes a click in the
telephone receiver. If these single pulsations follow one another
rapidly and regularly, a musical note is heard in the receiver. Various
combinations of crystals, and crystals and metal points, are used, but
all work in the same way. Some combinations work without assistance,
but others require to have a small current passed through them from a
local battery. The crystals are held in small cups of brass or copper,
mounted so that they can be adjusted by means of set-screws. Crystal
detectors are extremely sensitive, but they require very accurate
adjustment, and any vibration quickly throws them out of order.
The “electrolytic” detector rectifies the oscillating currents in a
different manner. One form consists of a thin platinum wire passing
down into a vessel made of lead, and containing a weak solution of
sulphuric acid. The two terminals of a battery are connected to the
wire and the vessel respectively. As long as no oscillations are
received from the aerial the current is unable to flow between the
wire and the vessel, but when the oscillations reach the detector the
current at once passes, and operates the telephone receiver. The action
of this detector is not thoroughly understood, and the way in which the
point of the platinum wire prevents the passing of the current until
the oscillations arrive from the aerial is something of a mystery.
The last detector that need be described is the Fleming valve
receiver. This consists of an electric incandescent lamp, with either
carbon or tungsten filament, into which is sealed a plate of platinum
connected with a terminal outside the lamp. The plate and the filament
do not touch one another, but when the lamp is lighted up a current
can be passed from the plate to the filament, but not from filament to
plate. This receiver acts in a similar way to the crystal detector,
making the oscillating currents into uni-directional currents. It has
proved a great success for transatlantic wireless communication between
the Marconi stations at Clifden and Glace Bay, and is extensively used.
The electric waves set in motion by the transmitting apparatus of a
wireless station spread outwards through the ether in all directions,
and so instead of reaching only the aerial of the particular station
with which it is desired to communicate, they affect the aerials of
all stations within a certain range. So long as only one station is
sending messages this causes no trouble; but when, as is actually the
case, large numbers of stations are hard at work transmitting different
messages at the same time, it is evident that unless something can be
done to prevent it, each of these messages will be received at the
same moment by every station within range, thus producing a hopeless
confusion of signals from which not a single message can be read.
Fortunately this chaos can be avoided by what is called “tuning.”
Wireless tuning consists in adjusting the aerial of the receiving
station so that it has the same natural rate of oscillation as that of
the transmitting station. A simple experiment will make clearer the
meaning of this. If we strike a tuning-fork, so that it sounds its
note, and while it is sounding strongly place near it another fork of
the same pitch and one of a different pitch, we find that the fork of
similar pitch also begins to sound faintly, whereas the third fork
remains silent. The explanation is that the two forks of similar pitch
have the same natural rate of vibration, while the other fork vibrates
at a different rate. When the first fork is struck, it vibrates at a
certain rate, and sets in motion air waves of a certain length. These
waves reach both the other forks, but their effect is different in each
case. On reaching the fork of similar pitch the first wave sets it
vibrating, but not sufficiently to give out a sound. But following this
wave come others, and as the fork has the same rate of vibration as
the fork which produced the waves, each wave arrives just at the right
moment to add its impulse to that of the preceding wave, so that the
effect accumulates and the fork sounds. In the case of the third fork
of different pitch, the first wave sets it also vibrating, but as this
fork cannot vibrate at the same rate as the one producing the waves,
the latter arrive at wrong intervals; and instead of adding together
their impulses they interfere with one another, each upsetting the work
of the one before it, and the fork does not sound. The same thing may
be illustrated with a pendulum. If we give a pendulum a gentle push at
intervals corresponding to its natural rate of swing, the effects of
all these pushes are added together, and the pendulum is made to swing
vigorously. If, on the other hand, we give the pushes at longer or
shorter intervals, they will not correspond with the pendulum’s rate of
swing, so that while some pushes will help the pendulum, others will
hinder it, and the final result will be that the pendulum is brought
almost to a standstill, instead of being made to swing strongly and
regularly. The same principle holds good with wireless aerials. Any
aerial will respond readily to all other aerials having the same rate
of oscillation, because the waves in each case are of the same length;
that is to say, they follow one another at the same intervals. On the
other hand, an aerial will not respond readily to waves from another
aerial having a different rate of oscillation, because these do not
follow each other at intervals to suit it.
If each station could receive signals only from stations having
aerials similar to its own, its usefulness would be very limited,
and so all stations are provided with means of altering the rate of
oscillation of their aerials. The actual tuning apparatus by which this
is accomplished need not be described, as it is complicated, but what
happens in practice is this: The operator, wearing telephone receivers
fixed over his ears by means of a head band, sits at a desk upon which
are placed his various instruments. He adjusts the tuning apparatus
to a position in which signals from stations of widely different
wave-lengths are received fairly well, and keeps a general look out
over passing signals. Presently he hears his own call-signal, and
knows that some station wishes to communicate with him. Immediately he
alters the adjustment of his tuner until his aerial responds freely to
the waves from this station, but not to waves from other stations, and
in this way he is able to cut out signals from other stations and to
listen to the message without interruption.
Unfortunately wireless tuning is yet far from perfect in certain
respects. For instance, if two stations are transmitting at the same
time on the same wave-length, it is clearly impossible for a receiving
operator to cut one out by wave-tuning, and to listen to the other
only. In such a case, however, it generally happens that although the
wave-frequency is the same, the frequency of the wave groups or trains
is different, so that there is a difference in the notes heard in the
telephones; and a skilful operator can distinguish between the two
sufficiently well to read whichever message is intended for him. The
stations which produce a clear, medium-pitched note are the easiest to
receive from, and in many cases it is possible to identify a station
at once by its characteristic note. Tuning is also unable to prevent
signals from a powerful station close at hand from swamping to some
extent signals from another station at a great distance, the nearer
station making the receiving aerial respond to it as it were by brute
force, tuning or no tuning.
Another source of trouble lies in interference by atmospheric
electricity. Thunderstorms, especially in the tropics, interfere
greatly with the reception of signals, the lightning discharges giving
rise to violent, irregular groups of waves which produce loud noises
in the telephones. There are also silent electrical disturbances in
the atmosphere, and these too produce less strong but equally weird
effects. Atmospheric discharges are very irregular, without any real
wave-length, so that an operator cannot cut them out by wave-tuning
pure and simple in the way just described, as they defy him by
affecting equally all adjustments. Fortunately, the irregularity
of the atmospherics produces correspondingly irregular sounds in
the telephones, quite unlike the clear steady note of a wireless
station; and unless the atmospherics are unusually strong this note
pierces through them, so that the signals can be read. The effects of
lightning discharges are too violent to be got rid of satisfactorily,
and practically all that can be done is to reduce the loudness of the
noises in the telephones, so that the operator is not temporarily
deafened. During violent storms in the near neighbourhood of a station
it is usual to connect the aerial directly to earth, so that in the
event of its being struck by a flash the electricity passes harmlessly
away, instead of injuring the instruments, and possibly also the
operators. Marconi stations are always fitted with lightning-arresters.
The methods and apparatus we have described so far are those of
the Marconi system, and although in practice additional complicated
and delicate pieces of apparatus are used, the description given
represents the main features of the system. Although Marconi was not
the discoverer of the principles of wireless telegraphy, he was the
first to produce a practical working system. In 1896 Marconi came from
Italy to England, bringing with him his apparatus, and after a number
of successful demonstrations of its working, he succeeded in convincing
even the most sceptical experts that his system was thoroughly sound.
Commencing with a distance of about 100 yards, Marconi rapidly
increased the range of his experiments, and by the end of 1897 he
succeeded in transmitting signals from Alum Bay, in the Isle of Wight,
to a steamer 18 miles away. In 1899 messages were exchanged between
British warships 85 miles apart, and the crowning achievement was
reached in 1901, when Marconi received readable signals at St. John’s,
Newfoundland, from Poldhu in Cornwall, a distance of about 1800 miles.
In 1907 the Marconi stations at Clifden and Glace Bay were opened
for public service, and by the following year transatlantic wireless
communication was in full swing. The sending of wireless signals across
the Atlantic was a remarkable accomplishment, but it did not represent
by any means the limits of the system, as was shown in 1910. In that
year Marconi sailed for Buenos Ayres, and wireless communication with
Clifden was maintained up to the almost incredible distance of 4000
miles by day, and 6735 miles by night. The Marconi system has had many
formidable rivals, but it still holds the proud position of the most
successful commercial wireless system in the world.
We have not space to give a description of the other commercial
systems, but a few words on some of the chief points in which they
differ from the Marconi system may be of interest. We have seen that
an ordinary spark gap, formed by two metal balls a short distance
apart, becomes overheated by the rapid succession of discharges, with
the result that the sparking is irregular. What actually happens is
that the violent discharge tears off and vaporizes minute particles
of the metal. This intensely heated vapour forms a conducting path
which the current is able to cross, so that an arc is produced just
in the same way as in the arc lamp. This arc is liable to be formed
by each discharge, and it lasts long enough to prevent the sparks
from following one another promptly. In the Marconi system this
trouble is avoided by means of a rotating spark gap, but in the German
“Telefunken” system, so named from Greek _tele_, far off, and German
_Funke_, a spark, a fixed compound spark gap is used for the same
purpose. This consists of a row of metal discs about 1/100 inch apart,
and the spark leaps these tiny gaps one after the other. The discs are
about 3 inches in diameter, and their effect is to conduct away quickly
the heat of the discharge. By this means the formation of an arc is
prevented, and the effect of each discharge is over immediately, the
sparks being said to be “quenched.” The short discharges enable more
energy to be radiated from the aerial into the ether, and very high
rates of sparking are obtained, producing a high-pitched musical note.
The “Lepel” system also uses a quenched spark. The gap consists of two
metal discs clamped together and separated only by a sheet of paper.
The paper has a hole through its centre, and through this hole the
discharge takes place, the discs being kept cool by water in constant
circulation. The discharge is much less noisy than in the Marconi and
Telefunken systems, and the musical note produced is so sensitive that
by varying the adjustments simple tunes can be played, and these can
be heard quite distinctly in the telephone at the receiving stations.
In the three systems already mentioned spark discharges are used to
set up oscillatory currents in the aerial, which in turn set up waves
in the ether. Each discharge sets in motion a certain number of waves,
forming what is known as a train of waves. The discharges follow one
another very rapidly, but still there is a minute interval between
them, and consequently there is a corresponding interval between the
wave-trains. In the “Goldschmidt” system the waves are not sent out in
groups of this kind, but in one long continuous stream. The oscillatory
currents which produce ether waves are really alternating currents
which flow backwards and forwards at an enormous speed. The alternating
current produced at an ordinary power station is of no use for
wireless purposes, because its “frequency,” or rate of flow backwards
and forwards, is far too low. It has been found possible however to
construct special dynamos capable of producing alternating current
of the necessary high frequency, and such dynamos are used in the
Goldschmidt system. The dynamos are connected directly to the aerial,
so that the oscillatory currents in the latter are continuous, and the
ether waves produced are continuous also.
The “Poulsen” system produces continuous waves in an altogether
different manner, by means of the electric arc. The arc is formed
between a fixed copper electrode and a carbon electrode kept in
constant rotation, and it is enclosed in a kind of box filled
with methylated spirit vapour, hydrogen, or coal gas. A powerful
electro-magnet is placed close to the arc, so that the latter is
surrounded by a strong magnetic field. Connected with the terminals of
the arc is a circuit consisting of a condenser and a coil of wire,
and the arc sets up in this circuit oscillatory currents which are
communicated to the aerial. These currents are continuous, and so also
are the resulting waves.
The method of signalling employed in these two continuous-wave
systems is quite different from that used in the Marconi and other
spark systems. It is practically impossible to signal by starting and
stopping the alternating-current dynamos or the arc at long or short
intervals to represent dashes or dots. In one case the sudden changes
from full load to zero would cause the dynamo to vary its speed, and
consequently the wave-length would be irregular; besides which the
dynamo would be injured by the sudden strains. In the other case it
would be extremely difficult to persuade the arc to start promptly
each time. On this account the dynamo and the arc are kept going
continuously while a message is being transmitted, and the signals are
given by altering the wave-length. In other words, the transmitting
aerial is thrown in and out of tune alternately at the required long
or short intervals, and the receiving aerial responds only during the
“in-tune” intervals.
The various receiving detectors previously described are arranged to
work with dis-continuous waves, producing a separate current impulse
from each group or train of waves. In continuous wave systems there are
of course no separate groups, and for this reason these detectors are
of no use, and a different arrangement is required. The oscillatory
currents set up in the aerial are allowed to charge up a condenser,
and this condenser is automatically disconnected from the aerial and
connected to the operator’s telephones at regular intervals of about
1/1000 second. Each time the condenser is connected to the telephones
it is discharged, and a click is produced. These clicks continue only
as long as the waves are striking the aerial, and as the transmitting
operator interrupts the waves at long or short intervals the clicks are
split up into groups of corresponding length.
Continuous waves have certain advantages over dis-continuous waves,
particularly in the matter of sharp tuning, but these advantages
are outweighed to a large extent by weak points in the transmitting
apparatus. The dynamos used to produce the high-frequency currents in
the Goldschmidt system are very expensive to construct and troublesome
to keep in order; while in the Poulsen system the arc is difficult to
keep going for long periods, and it is liable to fluctuations which
greatly affect its working power. Although all the commercial Marconi
installations make use of dis-continuous waves exclusively, Mr. Marconi
is still carrying out experiments with continuous waves.
There are many points in wireless telegraphy yet to be explained
satisfactorily. Our knowledge of the electric ether waves is still
limited, and we do not know for certain how these waves travel from
place to place, or exactly what happens to them on their journeys. For
this reason we are unable to give a satisfactory explanation of the
curious fact that, generally speaking, it is easier to signal over long
distances at night than during the day. Still more peculiar is the fact
that it is easier to signal in a north and south direction than in an
east and west direction. There are also remarkable variations in the
strength of the signals at certain times, particularly about sunset
and sunrise. Every station has a certain normal range which does not
vary much as a rule, but at odd times astonishing “freak” distances
are covered, stations having for a short time ranges far beyond their
usual limits. These and other problems are being attacked by many
investigators, and no doubt before very long they will be solved.
Wireless telegraphy already has reached remarkable perfection, but it
is still a young science, and we may confidently expect developments
which will enable us to send messages with all speed across vast gulfs
of distance at present unconquered.
Wireless telephony, like wireless telegraphy, makes use of electric
waves set up in, and transmitted through the ether. The apparatus is
practically the same in each case, except in one or two important
points. In wireless telegraphy either continuous or dis-continuous
waves may be used, and in the latter case the spark-frequency may be as
low as twenty-five per second. On the other hand, wireless telephony
requires waves which are either continuous, or if dis-continuous,
produced by a spark-frequency of not less than 20,000 per second. In
other words, the frequency of the wave trains must be well above the
limits of audibility. Although dis-continuous waves of a frequency
of from 20,000 to 40,000 or more per second can be used, it has been
found more convenient to use absolutely continuous waves for wireless
telephony, and these may be produced by the Marconi disc generator, by
the Goldschmidt alternator, or by the Poulsen arc, the last named being
largely employed.
In wireless telegraphy the wave trains are split up by a transmitting
key so as to form groups of signals; but in telephony the waves are
not interrupted at all, but are simply varied in intensity by means
of the voice. All telephony, wireless or otherwise, depends upon the
production of variations in the strength of a current of electricity,
these variations being produced by air vibrations set up in speaking.
In ordinary telephony with connecting wires the current variations are
produced by means of a microphone in the transmitter, and in wireless
telephony the same principle is adopted. Here comes in the outstanding
difficulty in wireless transmission of speech. The currents used in
ordinary telephony are small, and the microphone works with them
quite satisfactorily; but in wireless telephony very heavy currents
have to be employed, and so far no microphone has proved capable of
dealing effectively with these currents. Countless devices to assist
the microphone have been tried. It was found that one of the causes of
trouble was the overheating of the carbon granules, which caused them
to stick together, so becoming insensitive. To remedy this the granules
have been cooled in various ways, by water, gas, or oil, but although
the results have been improved, still the microphones worked far from
perfectly. Improved results have been obtained also by connecting
a number of microphones in parallel. The microphone difficulty is
holding back the development of wireless telephony, and at present no
satisfactory solution of the problem is in sight.
The transmitting and receiving aerials are the same as in wireless
telegraphy, and like them are tuned to the same frequency. The
receiving apparatus too is of the ordinary wireless type, with
telephones and electrolytic or other detectors.
Wireless telephony has been used with considerable success in various
German collieries, and at the Dinnington Main Colliery, Yorkshire.
Early last year Marconi succeeded in establishing communication by
wireless telephony between Bournemouth and Chelmsford, which are about
100 miles apart; and about the same time a song sung at Laeken, in
Belgium, was heard clearly at the Eiffel Tower, Paris, a distance of
225 miles. The German Telefunken Company have communicated by wireless
telephony between Berlin and Vienna, 375 miles, and speech has been
transmitted from Rome to Tripoli, a total distance of more than 600
miles. These distances are of course comparatively small, but if the
microphone trouble can be overcome satisfactorily, transatlantic
wireless telephony appears to be quite possible.
CHAPTER XXI
WIRELESS TELEGRAPHY--PRACTICAL APPLICATIONS
A fairly good idea of the principles and apparatus of wireless
telegraphy should have been gained in reading Chapter XX., but so
far little has been said about its practical use. If we leave their
power out of consideration, wireless stations may be divided into two
classes: fixed stations on land, and moving stations, if the expression
may be allowed, on ships. For moving stations wireless telegraphy
has the field all to itself, but for communication between fixed
stations it comes into conflict with ordinary telegraphy by wire or
cable. As regards land messages over comparatively short distances,
say throughout Great Britain, wireless telegraphy has no advantages
over the older methods; and it is extremely unlikely that it ever will
be substituted for the existing cable telegraphy. For long distances
overland wireless has the great advantage of having all its apparatus
concentrated at two points. A long land line passing through wild
country, and exposed to all kinds of weather, requires constant labour
to keep it in good repair, and when a breakdown occurs at any point,
the repairing gang may be miles away, so that delay is caused. On the
other hand, whatever may go wrong at a wireless station, no time is
lost in effecting the necessary repairs, for everything is on the spot.
At present there is no great competition between wireless and ordinary
telegraphy for overland messages of any kind, but the case is different
when we come to communication across seas and oceans. Already the
cable companies have been affected considerably, and there is little
doubt that they will feel the competition much more seriously before
long. The general public, always conservative in such matters, have
not yet grasped the fact that telegrams can be handed in at any
telegraph office in the British Isles, and at most telegraph offices
in the United States and Canada, for wireless transmission across the
Atlantic, via the Marconi stations at Clifden and Glace Bay. The cost
is remarkably small, being eightpence a word for ordinary messages.
It is impossible to state with any accuracy how many land wireless
stations there are in the world, but the list given in the _Year-Book
of Wireless Telegraphy_ for 1915 enumerates about 700 stations. This
list does not include private or experimental stations, and also many
stations used exclusively for naval or military purposes are not given.
The information available about these 700 stations is incomplete in
many cases, but about 500 are controlled by various departments of
the governments of the different states. Of the remainder, about 100
are controlled by the Marconi Company, the rest being in the hands of
various wireless, commercial, or railway companies.
Amongst the most important land stations are the Clifden and Glace
Bay transatlantic stations. They are very similar in plan, and each
has a separate aerial for sending and for receiving. Contrary to the
usual practice, continuous current is used to charge the condensers.
In Chapter IV. we saw how a current of high voltage could be obtained
by connecting a number of cells in series, and at these stations the
necessary high voltage is produced by connecting a number of powerful
dynamos in series, on the same principle. Along with the dynamos
a huge battery of accumulators, consisting of about 6000 cells, is
used as a sort of reservoir of current. These stations have a normal
range of considerably over 3000 miles. Last year a large transmitting
station was completed at Cefndu, near Carnarvon. This station, which
is probably the most powerful in existence, is intended to communicate
directly with New Jersey, United States, as an alternative to the
Clifden-Glace Bay route.
Other powerful stations are Poldhu, in Cornwall, of which we shall
speak later; the French Eiffel Tower station; the German station at
Nauen, near Berlin, which last year succeeded in exchanging messages
with Windhoek, German South-West Africa, a distance of nearly 6000
miles; and the extremely powerful station at Coltano, Italy. France has
three stations in West Africa with a night range of 1600 miles; and
Italy one in Somaliland with a normal range of about the same distance.
The recently opened Chinese stations at Canton, Foochow, and Woosung
have a range of 1300 miles by night, and 650 miles by day. With the
fall of Tsingtau, China, Germany lost a wireless station capable of
signalling over 1350 miles at night. Japan has six stations with a
night range of over 1000 miles. Massawa, on the Red Sea, has a range
of 1600 miles, and New Zealand has two stations with ranges of 1200
miles by day, and 2500 miles by night. Australia has a large number of
stations with a normal range of about 500 miles. In the United States,
which has a very large number of stations, Arlington, Virginia, covers
1000 miles, and Sayville from 600 to 2300 miles. South America has not
many high-power stations, but Cerrito, in Uruguay, has a range of about
1000 miles.
Until a thoroughly practical system of long-distance wireless telephony
is developed, wireless telegraphy will remain the only possible
means of communication between ships and shore, or between one ship
and another, except where the distance is so small that some method
of semaphore signalling can be used. In the days when wireless was
unknown, a navigator was thrown entirely upon his own resources as
soon as his vessel was out of sight of land, for no information of any
kind could reach him. Even with a wireless installation on board, the
captain of a vessel still needs the same skill and watchfulness as of
old, but in the times of uncertainty and danger to which all ships
are liable, he often is able to obtain information which may prevent
disaster. In order to determine accurately his position, a navigator
requires to know the exact Greenwich Mean Time, and he gets this time
from his chronometers. These are wonderfully reliable instruments, but
even they may err at times. To avoid the possibility of mistakes from
this cause, wireless time signals are sent out at regular intervals by
certain high-power stations, and as long as a vessel is within range of
one of these stations the slightest variation in the chronometers may
be detected immediately. Amongst these stations are the Eiffel Tower,
giving time signals at 10 a.m. and at midnight; and Norddeich, Germany,
giving signals at noon and midnight. These time signals have proved
most useful also on land, more particularly for astronomers and for
explorers engaged on surveying work.
In addition to time signals, other valuable information is conveyed
by wireless to ships at sea. A ship encountering ice, or a derelict,
reports its discovery to other ships and to the shore stations, and
in this way vessels coming along the same route are warned of the
danger in time to take the necessary precautions. Weather reports are
issued regularly from various shore stations in most parts of the
world. The completeness of the information given varies a good deal
with different stations, but in many cases it includes a report of
the existing state of the weather at a number of different places,
a forecast of the winds likely to be encountered at sea, say at a
distance of 100 miles from land, and warnings of approaching storms,
with remarks on any special atmospheric conditions at the time of
sending. In Europe weather reports are issued daily from the Admiralty
station at Cleethorpes, the Eiffel Tower, and Norddeich; and in the
United States more than a dozen powerful stations are engaged in this
work.
Another valuable use of wireless is in carrying on the work of
lighthouses and lightships during snowstorms or dense fogs, which
the light cannot penetrate. So far not much has been done in this
direction, but the French Government have decided to establish wireless
lighthouses on the islands outside the port of Brest, and also at
Havre. Automatic transmitting apparatus will be used, sending out
signals every few seconds, and working for periods of about thirty
hours without attention.
The improvement in the conditions of ocean travel wrought by wireless
telegraphy is very remarkable. The days when a vessel, on passing out
of sight of land, entered upon a period of utter isolation, is gone
for ever. Unless it strays far from all recognized trade routes, a
ship fitted with a wireless installation is never isolated; and with
the rapidly increasing number of high-power stations both on land and
sea, it soon will be almost impossible for a vessel to find a stretch
of ocean beyond the reach of wave-borne messages. The North Atlantic
Ocean is specially remarkable for perfection of wireless communication.
For the first 250 miles or so after leaving British shores, liners are
within reach of various coast stations, and beyond this Poldhu takes
up the work and maintains communication up to about mid-Atlantic.
On passing beyond the reach of Poldhu, liners come within range of
other Marconi stations at Cape Cod, Massachusetts, and Cape Race,
Newfoundland, so that absolutely uninterrupted communication is
maintained throughout the voyage. On many liners a small newspaper is
published daily, in which are given brief accounts of the most striking
events of the previous day, together with Stock Exchange quotations and
market prices. This press news is sent out during the night from Poldhu
and Cape Cod. During the whole voyage messages may be transmitted from
ship to shore, or from shore to ship, with no more difficulty, as
far as the public are concerned, than in sending an ordinary inland
telegram.
The transmitting ranges of ship installations vary greatly, the range
of the average ocean liner being about 250 miles. Small ships come as
low as 50 miles, while a few exceptional vessels have night ranges
up to 1200 or even 2500 miles. Although an outward-bound vessel is
almost always within range of some high-power shore station, it is
evident that it soon must reach a point beyond which it is unable to
communicate directly with the shore. This difficulty is overcome by a
system of relaying from ship to ship. The vessel wishing to speak with
the shore hands on its message to some other vessel nearer to land or
with a longer range, and this ship sends forward the message to a shore
station if one is within its reach, and if not to a third vessel, which
completes the transmission.
The necessity for wireless installations on all sea-going vessels has
been brought home to us in startling fashion on several occasions
during the last few years. Time after time we have read thrilling
accounts of ocean disasters in which wireless has come to the rescue in
the most wonderful way. A magnificent liner, with its precious human
freight, steams majestically out of harbour, and ploughs its way out
into the waste of waters. In mid-ocean comes disaster, swift and awful,
and the lives of all on board are in deadly peril. Agonized eyes sweep
the horizon, but no sail is in sight, and succour seems hopeless. But
on the deck of that vessel is a small, unpretentious cabin, and at a
desk in that cabin sits a young fellow with strange-looking instruments
before him. At the first tidings of disaster he presses a key, and out
across the waters speed electric waves bearing the wireless cry for
help, “S.O.S.,” incessantly repeated. Far away, on another liner, is a
similar small cabin, and its occupant is busy with messages of everyday
matters. Suddenly, in the midst of his work, comes the call from the
stricken vessel, and instantly all else is forgotten. No matter what
the message in hand, it must wait, for lives are in danger. Quickly
the call is answered, the position of the doomed ship received, and
the captain is informed. A few orders are hurriedly given, the ship’s
course is changed, and away she steams to the rescue, urged on by the
full power of her engines. In an hour or two she arrives alongside,
boats are lowered, and passengers and crew are snatched from death and
placed in safety. This scene, with variations, has been enacted many
times, and never yet has wireless failed to play its part. It is only
too true that in some instances many lives have been lost, but in these
cases it is necessary to remember that without wireless every soul on
board might have gone down. The total number of lives already saved by
wireless is estimated at about 5000, and of these some 3000 have been
saved in the Atlantic.
Ship aerials are carried from one mast to another, as high up as
possible. The transmitting and receiving apparatus is much the same as
in land stations, so that it need not be described. In addition, most
liners carry a large induction coil and a suitable battery, so that
distress signals can be transmitted even when the ordinary apparatus
is rendered useless by the failure of the current supply. Most of the
wireless systems are represented amongst ship installations, but the
great majority of vessels have either Marconi or Telefunken apparatus.
Every wireless station, whether on ship or on shore, has a separate
call-signal, consisting of three letters. For instance, Clifden is
MFT, Poldhu MPD, Norddeich KAV, s.s. _Lusitania_ MFA, and H.M.S.
_Dreadnought_ BAU. Glace Bay, GB, and the Eiffel Tower, FL, have two
letters only. In order to avoid confusion, different countries have
different combinations of letters assigned to them exclusively, and
these are allotted to the various ship and shore stations. For example,
Great Britain has all combinations beginning with B, G, and M; France
all combinations beginning with F, and also the combinations UAA to
UMZ; while the United States is entitled to use all combinations
beginning with N and W, and the combinations KIA to KZZ. There are also
special signals to indicate nationality, for use by ships, British
being indicated by - - -- -, Japanese by -- - -- -, and so on.
Wireless telegraphy apparently has a useful future in railway work.
In spite of the great perfection of present-day railway signalling,
no railway company is able to avoid occasional accidents. Some of
these accidents are due to circumstances which no precautions can
guard against entirely, such, for instance, as the sudden breakage
of some portion of the mechanism of the train itself. In many cases,
however, the accident is caused by some oversight on the part of the
signalman or the engine-driver. Probably the great majority of such
accidents are not due to real carelessness or inattention to duty,
but to unaccountable freaks of the brain, through which some little
detail, never before forgotten, is overlooked completely until too
late. We all are liable to these curious mental lapses, but happily in
most cases these do not lead to disaster of any kind. The ever-present
possibility of accidents brought about in this way is recognized fully
by railway authorities, and every effort is made to devise mechanism
which will safeguard a train in case of failure of the human element.
The great weakness of the ordinary railway system is that there is no
reliable means of communicating with the driver of a train except by
the fixed signals, so that when a train has passed one set of signals
it is generally beyond the reach of a message until it arrives at
the next set. On the enterprising Lackawanna Railroad, in the United
States, an attempt has been made to remove this defect by means of
wireless telegraphy, and the experiment has been remarkably successful.
Wireless communication between moving passenger trains and certain
stations along the route has been established, and the system is being
rapidly developed.
The wireless equipment of the stations is of the usual type, and does
not call for comment, but the apparatus on the trains is worth mention.
The aerial, which must be low on account of bridges and tunnels,
consists of rectangles of wire fixed at a height of 18 inches above
the roof of each car. These separate aerials are connected together by
a wire running to a small operating room containing a set of Marconi
apparatus, and situated at the end of one of the cars. The earth
connexion is made to the track rails, and the current is taken from
the dynamos used to supply the train with electric light. With this
equipment messages have been transmitted and received while the train
was running at the rate of 70 miles an hour, and distances up to 125
miles have been covered. During a severe storm in the early part of
last year the telegraph and telephone lines along the railroad broke
down, but uninterrupted communication was maintained by wireless,
and the operations of the relief gangs and the snow-ploughs were
directed by this means. For emergency signalling this system is likely
to prove of enormous importance. If signals are set wrongly, through
some misunderstanding, and a train which should have been held up is
passed forward into danger, it can be stopped by a wireless message in
time to prevent an accident. Again, if a train has a breakdown, or if
it sticks fast in a snow-drift, its plight and its exact position can
be signalled to the nearest station, so that help may be sent without
delay. The possibilities of the system in fact are almost unlimited,
and it seems not unlikely that wireless telegraphy will revolutionize
the long-distance railway travelling of the future.
CHAPTER XXII
ELECTROPLATING AND ELECTROTYPING
In our chapter on the accumulator or storage cell we saw that a current
of electricity has the power of decomposing certain liquids; that
is to say, it is able to split them up into their component parts.
This power has given rise to the important art of electroplating and
electrotyping. Electroplating is the process of depositing a coating
of a rarer metal, such as gold, silver, or nickel, upon the surface of
baser or commoner metals; and electrotyping is the copying of casts,
medals, types, and other similar objects. The fact that metals could
be deposited by the decomposition of a solution by a current was known
in the early days of the voltaic cell, but no one seems to have paid
much attention to it. An Italian chemist published in 1805 an account
of his success in coating two silver medals with gold, and some thirty
years later Bessemer transformed lead castings into fairly presentable
ornaments by coating them with copper, but commercial electroplating
may be said to have begun about 1840, when an Englishman named
Elkington took out a patent for the process. Since then the processes
of electroplating and electrotyping have rapidly come more and more
into use, until to-day they are practised on a vast scale, giving
employment to thousands.
Electroplating on a small scale is a very simple affair. A solution
of the metal which it is desired to deposit is placed in a suitable
vessel. Two metal rods are placed across the top of this vessel, and
from one of these is suspended a plate of the same metal as that in
the solution, and from the other is hung the article to receive the
coating. The positive terminal of a voltaic battery is connected to
the rod supporting the plate, and the negative terminal to the rod
carrying the article to be plated. As the current passes through the
solution from the plate to the article the solution is decomposed, and
the article receives a coating of metal. The solution through which the
current passes, and which is decomposed, is called the _electrolyte_,
and the terminal points at which the current enters and leaves the
solution are called _electrodes_. The electrode by which the current
enters the electrolyte is called the _anode_, and the one by which it
leaves is called the _cathode_.
If we wish to deposit a coating of copper on, say, an old spoon which
has been dismissed from household service, a solution of sulphate
of copper must be made up and placed in a glass or stoneware jar.
Two little rods of brass, copper, or any other good conductor are
placed across the jar, one at each side, and by means of hooks of
wire a plate of copper is hung from one rod and the spoon from the
other. The positive terminal of a battery of Daniell cells is then
connected to the anode rod which supports the copper plate, and the
negative terminal to the cathode rod carrying the spoon. The current
now commences its task of splitting up the copper-sulphate solution
into pure copper and sulphuric acid, and depositing this copper upon
the spoon. The latter is very quickly covered with a sort of “blush”
copper, and the coating grows thicker and thicker as long as the
current is kept at work. If there were no copper plate forming the
anode the process would soon come to a standstill, on account of the
copper in the electrolyte becoming used up; but as it is the sulphuric
acid separated out of the electrolyte takes copper from the plate and
combines with it to form a further supply of copper sulphate. In this
way the strength of the solution is kept up, and the copper anode
becomes smaller and smaller as the coating on the spoon increases
in thickness. It is not necessary that the anode should consist of
absolutely pure copper, because any impurities will be precipitated to
the bottom or mixed with the solution, nothing but quite pure copper
being deposited on the spoon. At the same time if the copper anode
is very impure the electrolyte quickly becomes foul, and has to be
purified or replaced by new solution.
[Illustration:
_By permission of_] [_W. Canning & Co._
FIG. 35.--Small Electroplating Outfit.]
To nickel-plate the spoon we should require a nickel plate for the
anode and a nickel solution; to silver-plate it, a silver anode and
solution, and so on. Fig. 35 shows at simple but effective arrangement
for amateur electroplating in a small way.
Electroplating on a commercial scale is of course a much more elaborate
process, but the principle remains exactly the same. Fig. 36 shows
the general arrangement of a plating shop. It is obviously extremely
important that the deposit on a plated article should be durable,
and to ensure that the coating will adhere firmly the article must be
cleaned thoroughly before being plated. Cleanliness in the ordinary
domestic sense is not sufficient, for the article must be chemically
clean. Some idea of the care required in this respect may be gained
from the fact that if the cleaned surface is touched with the hand
before being plated, the coating will strip off the parts that have
been touched. The surfaces are first cleaned mechanically, and then
chemically by immersion in solutions of acids or alkalies, the cleaning
process varying to some extent with different metals. There is also
a very interesting process of cleaning by electricity. The article
is placed in a vat fitted with anode and cathode rods, just as in an
ordinary plating vat, and containing a solution of hydrate of potash
and cyanide of potassium. The anode consists of a carbon plate, and
the article is hung from the cathode rod. Sufficient current is passed
through the solution to cause gas to be given off rapidly at the
cathode, and as this gas rises to the surface it carries with it the
grease and dirt from the article, in the form of a dirty scum. After
a short time the article becomes oxidized and discoloured, and the
current is then reversed, so that the article becomes the anode, and
the carbon plate the cathode. The current now removes the oxide from
the surface of the article, which is left quite bright and chemically
clean.
[Illustration:
_By Permission of_] [_W. Canning & Co._
FIG. 36.--General Arrangement of an Electroplating Shop.]
When thoroughly cleaned the articles are ready to be placed in the
plating vats. These vats are usually made of wood lined with chemically
pure lead, or of iron lined with enamel or cement. Anode and cathode
rods made of brass are placed across the vats, and from these the
anodes of the various metals and the articles to be plated are hung by
hooks of nickel or brass. Any number of rods may be used, according to
the size of the vat, so long as the articles have an anode on each
side. If three rods are used the articles are hung from the centre one,
and the anodes from the outside ones. If a number of small articles
are to be plated together they are often suspended in perforated metal
trays. Small articles are also plated by placing them in a perforated
barrel of wood, or wood and celluloid, which revolves in the solution.
While the articles are being plated the revolving of the barrel makes
them rub one against the other, so that they are brightly burnished.
Dog chains, cycle chain links, button-hooks, and harness fittings are
amongst the articles plated by means of the revolving barrel.
The strength of current required for different kinds of plating varies
considerably, and if the work is to be of the best quality it is very
important that the current should be exactly right for the particular
process in hand. In order to adjust it accurately variable resistances
of German silver wire are provided for each vat, the current having to
pass through the resistance before reaching the solution. The volume
and the pressure of the current are measured by amperemeters and
voltmeters attached to the resistance boards. If the intensity of the
current is too great the articles are liable to be “burnt,” when the
deposit is dark coloured and often useless.
When exceptionally irregular surfaces have to be plated it is sometimes
necessary to employ an anode of special shape, in order to keep as
uniform a distance as possible between the electrodes. If this is not
done, those parts of the surface nearest the anode get more than their
share of the current, and so they receive a thicker deposit than the
parts farther away.
An interesting process is that known as “parcel-plating,” by which
decorative coatings of different coloured metals can be deposited on
one article. For instance, if it is desired to have gold flowers on a
silver brooch, the parts which are not to be gilded are painted over
with a non-conducting varnish. When this varnish is quite dry the
brooch is placed in the gilding vat and the current sent through in the
usual way. The gold is then deposited only on the parts unprotected
by varnish, and after the gilding the varnish is easily removed by
softening it in turpentine and brushing with a bristle brush. More
elaborate combinations of different coloured metals can be made in the
same way.
Sugar basins, cream jugs, ornamental bowls, cigarette cases, and other
articles are often gilded only on the inside. The article is filled
with gold solution and connected to the cathode rod. A piece of gold
wrapped in calico is attached to the anode rod, suspended in the
solution inside the article, and moved about quickly until the deposit
is of the required thickness.
The time occupied in plating is greatly shortened by stirring or
agitating the solutions. This sets up a good circulation of the liquid,
and a continual supply of fresh solution is brought to the cathode.
At the same time the resistance to the current is decreased, and
more current may be used without fear of burning. Fig. 37 shows an
arrangement for this purpose. The solution is agitated by compressed
air, and at the same time the cathode rods are moved backwards and
forwards. Plating solutions are also frequently heated in order to
hasten the rate of deposition.
When the plating process is complete, the articles are removed from
the vat, thoroughly swilled in water, and dried. They are then ready
for finishing by polishing and burnishing, or they may be given a sort
of frosted surface. During the finishing processes the appearance of
the articles changes considerably, the rather dead-looking surface
produced by the plating giving place to the bright lustre of the
particular metal.
[Illustration:
_By Permission of_] [_W. Canning & Co._
FIG. 37.--Method of agitating solution in Plating Vat.]
It sometimes happens that an article which has been plated and polished
shows little defects here and there in the deposit. In such a case it
is not necessary to re-plate the whole article, for the defects can be
made good by a process of “doctoring.” A piece of the same metal as
that forming the deposit is placed between two pieces of wood, and a
wire fastened to one end of it. At the other end several thicknesses of
flannel are wrapped round and securely tied. This strip, which forms a
miniature anode, is connected to the anode rod of the plating vat, and
the article is connected to the cathode rod. The flannel is saturated
with the plating solution, and the strip is rubbed gently over the
defective places until the deposit formed is as thick as that on the
rest of the article. If the work is done carefully the “doctored”
portions cannot be distinguished from the rest of the surface.
Electroplating may be employed to give ships’ plates a coating of
copper to prevent barnacles from sticking to them. The work is done
in sections by building up to the side of the vessel a sort of vat of
which the plate to be coated forms one side. The plate is thus at the
same time the cathode and part of the vat.
So far we have spoken only of electroplating objects made of metal. If
we tried to copperplate a plaster cast by simply suspending it as we
did our spoon, we should get no result at all, because the plaster is
a non-conductor. But if we sprinkle plumbago over the cast so as to
give it a conducting surface, we can plate it quite well. Practically
all materials can be electroplated, but if they are non-conductors they
must be given a conducting surface in the way just described or by some
similar means. Even flowers and insects may be plated, and by giving
them first a coating of copper and then a coating of gold, delicately
beautiful results are obtained.
Electrotyping is practically the same as electroplating, except that
the coating is removed from the support on which it is deposited.
The process is largely used for copying engraved plates for printing
purposes. The plate is first rubbed over with a very weak solution
of beeswax in turpentine, to prevent the deposit from adhering to it,
and it is then placed in a copperplating vat and given a good thick
coating. The coating is then stripped off, and in this way a reversed
copy of the plate is obtained. This copy is then replaced in the vat,
and a coating of copper deposited upon it, and this coating, when
stripped off, forms an exact reproduction of the original, with every
detail faithfully preserved. An engraved plate may be copied also by
making from it a mould of plaster or composition. The surface of this
mould is then rendered conducting by sprinkling over it a quantity of
plumbago, which is well brushed into all the recesses, and a coating
of copper deposited on it. As the mould was a reversed copy of the
original, the coating formed upon it is of course an exact copy of the
plate. If the copy has to be made very quickly a preliminary deposit
of copper is chemically formed on the mould before it is placed in the
vat. This is done by pouring on to the mould a solution of sulphate of
copper, and sprinkling iron filings over the surface. The filings are
then brushed down on to the face of the mould with a fine brush, and a
chemical reaction takes place, resulting in the precipitation of copper
from the solution. After the filings have been washed away, the mould
is placed in the vat, and the deposition of copper takes place very
rapidly.
Engraved copperplates are often nickel or steel-plated to give their
surface greater hardness, so that the printer may obtain a larger
number of sharp impressions from the same plate. Stereotypes also are
coated with nickel for a similar reason.
Before the dynamo came into general use all electroplating and
electrotyping was done with current supplied by voltaic cells, and
though the dynamo is now used exclusively in large plating works,
voltaic cells are still employed for work on a very small scale. A
cell which quickly polarizes is quite useless for plating purposes,
and one giving a constant and ample supply of current is required. The
Daniell cell, which was described in Chapter IV., is used, and so also
is the Bunsen cell, which consists of a porous pot containing strong
nitric acid and a carbon rod, placed in an outer stoneware vessel
containing dilute sulphuric acid and a zinc plate. The drawback to
this cell is that it gives off very unpleasant fumes. The dynamos used
for plating work are specially constructed to give a large amount of
current at very low pressure. Continuous current only can be used, for
alternating current would undo the work as fast as it was done, making
the article alternately a cathode and an anode.
CHAPTER XXIII
INDUSTRIAL ELECTROLYSIS
The metal copper, as obtained from copper ore, contains many impurities
of various kinds. For most purposes these impurities greatly affect the
value of the copper, and before the metal can be of much commercial
use they must be got rid of in some way. In the previous chapter, in
describing how to copperplate an old spoon, we saw that the anode need
not consist of pure copper, because in any case nothing but the pure
metal would be deposited upon the spoon. This fact forms the basis of
the important industry of electrolytic copper refining. The process is
exactly the same as ordinary copperplating, except that the cathode
always consists of absolutely pure copper. This is generally in the
form of a sheet no thicker than thin paper, but sometimes a number of
suspended wires are used instead. A solution of copper sulphate is used
as usual for the electrolyte, and the anode is a thick cast plate of
the impure copper. The result of passing a current through the solution
is that copper is taken from the anode and carried to the cathode, the
impurities falling to the bottom of the vat and accumulating as a sort
of slime. In this way thick slabs of pure copper are obtained, ready to
be melted down and cast into ingots.
The impurities in the raw copper vary according to the ore from which
it is obtained, and sometimes gold and silver are found amongst them.
When the copper is known to contain these metals the deposit at the
bottom of the refining vats is carefully collected, and from it a
considerable quantity of gold and silver is recovered. It is estimated
that about half a million tons of copper are refined every year. An
immense amount of this pure copper is used for electrical purposes, for
making conducting wires and cables, and innumerable parts of electric
appliances and machinery of all kinds; in fact it is calculated that
more than half of the copper produced all over the world is used in
this way.
A similar method is employed to obtain the precious metals in a pure
state, from the substance known as “bullion”; which consists usually of
an intermingling of gold, silver, and copper, with perhaps also lead.
Just as in copper refining, the raw material is used as the anode, and
a strip of pure gold or silver, according to which metal is required,
as the cathode. A silver solution is used if silver is wanted, and a
gold solution if gold is to be deposited.
The metal aluminium has come into general use with surprising rapidity,
and during the last twenty-five or thirty years the amount of this
metal produced annually has increased from two or three tons to many
thousands of tons. Aluminium occurs naturally in large quantities,
in the form of alumina, or oxide of aluminium, but for a long time
experimenters despaired of ever obtaining the pure metal cheaply
on a commercial scale. The oxides of most metals can be reduced,
that is deprived of their oxygen, by heating them with carbon; but
aluminium oxide holds on to its oxygen with extraordinary tenacity,
and absolutely refuses to be parted from it in this way. One process
after another was tried, without success, and cheap aluminium seemed
to be an impossibility until about 1887, when two chemists, Hall, an
American, and Héroult, a Frenchman, discovered a satisfactory solution
of the problem. These chemists, who were then scarcely out of their
student days, worked quite independently of one another, and it is a
remarkable fact that their methods, which are practically alike, were
discovered at almost the same time. The process is an interesting
mixture of electrolysis and electric heating. An iron crucible
containing a mixture of alumina, fluorspar, and cryolite is heated. The
two last-named substances are quickly fused, and the alumina dissolves
in the resulting fluid. When the mixture has reached the fluid state,
electrodes made of carbon are dipped into it, and a current is passed
through; with the result that oxygen is given off at the anode, and
metallic aluminium is produced at the cathode, in molten drops. This
molten metal is heavier than the rest of the fluid, and so it falls
to the bottom. From here it is drawn off at intervals, while fresh
alumina is added as required, so that the process goes on without
interruption. After the first fusing of the mixture no further outside
heat is required, for the heat produced by the passage of the current
is sufficient to keep the materials in a fluid state. Vast quantities
of aluminium are produced in this way at Niagara Falls, and in Scotland
and Switzerland.
Most of us are familiar with the substance known as caustic soda.
The chemical name for this is sodium hydrate, and its preparation by
electrolysis is interesting. Common salt is a chemical compound of the
metal sodium and the greenish coloured, evil smelling gas chlorine,
its proper name being sodium chloride. A solution of this in water is
placed in a vat or cell, and a current is sent through it. The solution
is then split up into chlorine, at the anode, and sodium at the
cathode. Sodium has a remarkably strong liking for water, and as soon
as it is set free from the chlorine it combines with the water of the
solution, and a new solution of sodium hydrate is formed. The water in
this is then got rid of, and solid caustic soda remains.
Amongst the many purposes for which caustic soda is used is the
preparation of oxygen and hydrogen. Water, to which a little sulphuric
acid has been added, is split up by a current into oxygen and hydrogen,
as we saw in Chapter V. This method may be used for the preparation of
these two gases on a commercial scale, but more usually a solution of
caustic soda is used as the electrolyte. If the oxygen and hydrogen
are not to be used at the place where they are produced, they are
forced under tremendous pressure into steel cylinders, and at a
lantern lecture these cylinders may be seen supplying the gas for the
lime-light. Although the cylinders are specially made and tested for
strength, they are covered with a sort of rope netting; so that if by
any chance one happened to burst, the shattered fragments of metal
would be caught by the netting, instead of flying all over the room and
possibly injuring a number of people. A large quantity of hydrogen is
prepared by this process for filling balloons and military airships.
CHAPTER XXIV
THE RÖNTGEN RAYS
In the chapter on electricity in the atmosphere we saw that whereas
air at ordinary pressure is a bad conductor, its conducting power
increases rapidly as the pressure is lowered. Roughly speaking, if we
wish to obtain a spark across a gap of 1 inch in ordinary air, we must
have an electric pressure of about 50,000 volts. The discharge which
takes place under these conditions is very violent, and it is called
a “disruptive” discharge. If however the air pressure is gradually
lowered, the discharge loses its violent character, and the brilliant
spark is replaced by a soft, luminous glow.
The changes in the character of the discharge may be studied by means
of an apparatus known as the “electric egg.” This consists of an
egg-shaped bulb of glass, having its base connected with an air-pump.
Two brass rods project into the bulb, one at each end; the lower rod
being a fixture, while the upper one is arranged to slide in and out,
so that the distance between the balls can be varied. The outer ends of
the rods are connected to an induction coil or to a Wimshurst machine.
If the distance between the balls has to be, say, half an inch, to
produce a spark with the air at normal pressure, then on slightly
reducing the pressure by means of the air-pump it is found that a
spark will pass with the balls an inch or more apart. The brilliance
of an electric spark is due to the resistance of the air, and as the
pressure decreases the resistance becomes smaller, so that the light
produced is much less brilliant. If the exhaustion is carried still
further the discharge becomes redder in colour, and spreads out wider
and wider until it loses all resemblance to a spark, and becomes a
luminous glow of a purple or violet colour. At first this glow seems to
fill the whole bulb, but at still higher vacua it contracts into layers
of definite shape, these layers being alternately light and dark.
Finally, when the pressure becomes equal to about one-millionth of an
atmosphere, a luminous glow surrounds the cathode or negative rod,
beyond this is dark space almost filling the bulb, and the walls of the
bulb between the cathode and the anode glow with phosphorescent light.
This phosphorescence is produced by rays coming from the cathode and
passing through the dark space, and these rays have been given the name
of “cathode rays.”
Many interesting experiments with these rays may be performed with
tubes permanently exhausted to the proper degree. The power of the
rays to produce phosphorescence is shown in a most striking way with
a tube fixed in a horizontal position upon a stand, and containing a
light cross made of aluminium, placed in the path of the rays. This
is hinged at the base, so that it can be stood up on end or thrown
down by jerking the tube. Some of the rays streaming from the cathode
are intercepted by the cross, while others pass by it and reach the
other end of the tube. The result is that a black shadow of the cross
is thrown on the glass, sharply contrasted with those parts of the
tube reached by the rays, and which phosphoresce brilliantly. After a
little while this brilliance decreases, for the glass becomes fatigued,
and loses to a considerable extent its power of phosphorescing. If
now the cross is jerked down, the rays reach the portions of the tube
before protected by the cross, and this glass, being quite fresh,
phosphoresces with full brilliance. The black cross now suddenly
becomes brilliantly illuminated, while the tired glass is dark in
comparison. If the tired glass is allowed to rest for a while it partly
recovers its phosphorescing powers, but it never regains its first
brilliance.
An even more striking experiment may be made with a horizontal tube
containing a tiny wheel with vanes of mica, something like a miniature
water-wheel, mounted on glass rails. When the discharges are sent
through the tube, the cathode rays strike against the vanes and cause
the little wheel to move forward in the direction of the anode. Other
experiments show that the cathode rays have great heating power, and
that they are deflected by a magnet held close to the tube.
For a long time the nature of these cathode rays was in dispute. German
physicists held that they were of the same character as ordinary light,
while English scientists, headed by Sir William Crookes, maintained
that they were streams of extremely minute particles of matter in a
peculiar fourth state. That is to say, the matter was not liquid, or
solid, or gaseous in the ordinary sense, but was _ultra-gaseous_,
and Crookes gave it the name of _radiant matter_. Most of us have
been taught to look upon the atom as the smallest possible division
of matter, but recent researches have made it clear that the atom
itself is divisible. It is believed that an atom is made up of very
much more minute particles called _electrons_, which are moving
about or revolving all the time with incredible rapidity. According
to Sir Oliver Lodge, if we imagine an atom of hydrogen to be as big
as an ordinary church, then the electrons which constitute it will
be represented by about 700 grains of sand, 350 being positively
electrified and 350 negatively electrified. It is not yet definitely
determined whether these electrons are minute particles of matter
charged with electricity, or whether they are actually atoms of
electricity. The majority of scientists now believe that the cathode
rays consist of a stream of negative electrons repelled from the
cathode at a speed of 124 miles per second, or not quite 1/1000 of the
velocity of light.
In November 1895, Professor Röntgen, a German physicist, announced
his discovery of certain invisible rays which were produced at the
same time as the cathode rays, and which could penetrate easily solids
quite opaque to ordinary light. He was experimenting with vacuum tubes,
and he found that certain rays emerged from the tube. These were not
cathode rays, because they were able to pass through the glass, and
were not deflected by a magnet. To these strange rays he gave the name
of the “_X_,” or unknown rays, but they are very frequently referred to
by the name of their discoverer.
It was soon found that the Röntgen rays affected an ordinary
photographic plate wrapped up in black paper so as to exclude all
ordinary light, and that they passed through flesh much more easily
than through bone. This fact makes it possible to obtain what we may
call “shadow-graphs” of the bones through the flesh, and the value of
this to the medical profession was realized at once. The rays also
were found to cause certain chemical compounds to become luminous. A
cardboard screen covered with one of these compounds is quite opaque to
ordinary light, but if it is examined when the Röntgen rays are falling
upon it, it is seen to be brightly illuminated, and if the hand is held
between the screen and the rays the bones become clearly visible.
[Illustration: FIG. 38.--X-Ray Tube, showing paths of Cathode and
X-Rays.]
Röntgen rays are produced when the cathode rays fall upon, and as it
were bombard, an obstacle of some kind. Almost any tube producing
cathode rays will produce also Röntgen rays, but special forms of
tube are used when the main object is to obtain these latter rays.
Fig. 38 shows a typical form of simple X-ray tube. This, like all
other tubes for X-ray work, is exhausted to a rather higher vacuum
than tubes intended for the production of cathode rays only. The
cathode C is made of aluminium, and is shaped like a saucer, its
curvature being arranged so that the cathode rays are focused on to the
anti-cathode A. The focusing as a rule is not done very accurately,
for although sharper radiographs are obtained when the cathode rays
converge exactly to a point on the anti-cathode, the heating effect
at this point is so great that a hole is quickly burned. The target,
or surface of the anti-cathode, is made of some metal having an
extremely high melting-point, such as platinum, iridium, or tungsten.
It has a flat surface inclined at an angle of about 45°, so that the
rays emanating from it proceed in the direction shown by the dotted
lines in the figure. The continuous lines show the direction of the
cathode rays. The anode is made of aluminium, and it is shown at N. It
is not necessary to have a separate anode, and the anti-cathode may
be used as the anode. In the tube shown in Fig. 38 the anode and the
anti-cathode are joined by an insulated wire, so that they both act as
anodes. The tube is made of soda-glass, as the X-rays do not pass at
all readily through lead-glass.
[Illustration:
_By permission of_] [_C. H. F. Muller._
FIG. 39.--Diagram of Mica Vacuum Regulator for X-Ray Tubes.]
The penetrating power of the X-rays varies with the vacuum of the
tube, a low vacuum giving rays of small penetration, and a high vacuum
rays of great penetration. Tubes are called hard or soft according to
the degree of the vacuum, a hard tube having a high vacuum and a soft
tube a low one. It should be remembered that the terms high and low,
as applied to the vacuum of X-ray tubes, are only relative, because
the vacuum must be very high to admit of the production of X-rays at
all. The vacuum becomes higher as the tube is used, and after a while
it becomes so high that the tube is practically useless, for the
penetrating power of the rays is then so great that sharp contrasts
between different substances, such as flesh and bone, cannot be
obtained, and the resulting radiographs are flat and poor. The vacuum
of a hard tube may be lowered temporarily by gently heating the tube,
but this is not a very convenient or satisfactory process, and tubes
are now made with special arrangements for lowering the vacuum when
necessary. There are several vacuum-regulating devices, and Fig. 39
is a diagram of the “Standard” mica regulator used in most of the
well-known “Muller” X-ray tubes. This consists of a small additional
bulb containing an electrode D carrying a series of mica discs. A wire
DF is attached to D by means of a hinged cap. The vacuum is lowered
while the discharges are passing through the tube. The wire DF is moved
towards the cathode terminal B, and kept there for a few seconds.
Sparks pass between F and B, and the current is now passing through the
electrode D in the regulator chamber. This causes the mica to become
heated, so that it gives off a small quantity of gas, which passes
into the main tube and so lowers the vacuum. The wire DF is then moved
well away from B, and after a few hours’ rest the tube, now of normal
hardness, is ready for further use.
We have already referred to the heating of the anti-cathode caused by
the bombardment of the cathode rays. Even if these rays are not focused
very sharply, the anti-cathode of an ordinary tube becomes dangerously
hot if the tube is run continuously for a fairly long period, and for
hospital and other medical work on an extensive scale special tubes
with water-cooled anti-cathodes are used. These tubes have a small
bulb blown in the anti-cathode neck. This bulb is filled with water,
which passes down a tube to the back of the target of the anti-cathode.
By this arrangement the heat generated in the target is absorbed by
the water, so that the temperature of the target can become only very
slightly higher then 212° F., which is the temperature of boiling
water, and quite a safe temperature for the anti-cathode. In some tubes
the rise in temperature is made slower by the use of broken bits of ice
in place of water. Fig. 40 shows a Muller water-cooled tube, and Fig.
41 explains clearly the parts of an X-ray tube and their names.
[Illustration: FIG. 40.--Muller Water-cooled X-Ray Tube.]
[Illustration:
_By permission of_] [_C. H. F. Muller._
FIG. 41.--Diagram showing parts of X-Ray Tube.]
An induction coil is generally used to supply the high-tension
electricity required for the production of the Röntgen rays. For
amateur or experimental purposes a coil giving continuous 4-inch or
even 3-inch sparks will do, but for medical work, in which it is
necessary to take radiographs with very short exposures, coils giving
sparks of 10, 12, or more inches in length are employed. An electrical
influence machine, such as the Wimshurst, may be used instead of an
induction coil. Very powerful machines with several pairs of plates
of large diameter, and driven by an electric motor, are in regular
use for X-ray work in the United States, but in this country they are
used only to a very small extent. A Wimshurst machine is particularly
suitable for amateur work. If a screen is to be used for viewing bones
through the flesh a fairly large machine is required, but for screen
examination of such objects as coins in a box, or spectacles in a
case, and for taking radiographs of these and other similar objects, a
machine giving a fairly rapid succession of sparks as short as 2 inches
can be used. Of course the exposure required for taking radiographs
with a machine as small as this are very long, but as the objects are
inanimate this does not matter very much.
For amateur X-ray work the arrangement of the apparatus is simple.
The tube is held in the required position by means of a wooden clamp
attached to a stand in such a way that it is easily adjustable.
Insulated wires are led from the coil or from the Wimshurst machine
to the tube, the positive wire being connected to the anode, and the
negative wire to the cathode. With a small Wimshurst machine light
brass chains may be used instead of wires, and these have the advantage
of being easier to manipulate. For medical purposes the arrangements
are more complicated, and generally a special room is set apart for
X-ray work.
If the connexions have been made correctly, then on starting the coil
or the machine the tube lights up. The bulb appears to be sharply
divided into two parts, the part in front of the anti-cathode glowing
with a beautiful greenish-yellow light, while the part behind the
anti-cathode is dark, except for lighter patches close to the anode.
The Röntgen rays are now being produced. The illumination is not steady
like that of an electric lamp, but it consists of a series of flickers,
which, with powerful apparatus, follow one another so rapidly as to
give the impression of continuity. If the connexions are wrong, so that
the negative wire goes to the anode instead of to the cathode, the bulb
is not divided in this way, but has patches of light almost all over.
As soon as this appearance is seen the apparatus must be stopped and
the connexions reversed, for the tube is quickly damaged by passing the
discharge through it in the wrong direction.
Having produced the X-rays, we will suppose that it is desired to
examine the bones of the hand. For this purpose a fluorescent screen is
required. This consists of a sheet of white cardboard coated usually
with crystals of barium platino-cyanide. In order to shut out all light
but that produced by the rays, the cardboard is placed at the larger
end of a box or bellows shaped like a pyramid. This pyramid is brought
close to the X-ray tube, with its smaller end held close to the eyes,
and the hand is placed against the outer side of the cardboard sheet.
The outline of the hand is then seen as a light shadow, and the very
much blacker shadow of the bones is clearly visible. For screen work
it is necessary to darken the room almost entirely, on account of the
feebleness of the illumination of the screen.
If a radiograph of the bones of the hand is to be taken, a very
sensitive photographic plate is necessary. An ordinary extra-rapid
plate will do fairly well, but for the best work plates made specially
for the purpose are used. The emulsion of an ordinary photographic
plate is only partially opaque to the X-rays, so that while some of
the rays are stopped by it, others pass straight through. The silver
bromide in the emulsion is affected only by those rays which are
stopped, so that the energy of the rays which pass through the emulsion
is wasted. If a plate is coated with a very thick film, a larger
proportion of the rays can be stopped, and many X-ray plates differ
from photographic plates only in the thickness of the emulsion. A thick
film however is undesirable because it makes the after processes of
developing, fixing, and washing very prolonged. In the “Wratten” X-ray
plate the emulsion is made highly opaque to the rays in a different and
ingenious manner. Salts of certain metals have the power of stopping
the X-rays, and in this plate a metallic salt of this kind is contained
in the emulsion. The film produced in this way stops a far larger
proportion of the rays than any ordinary film, and consequently the
plate is more sensitive to the rays, so that shorter exposures can be
given.
X-ray plates are sold usually wrapped up separately in light-tight
envelopes of black paper, upon which the film side of the plate is
marked. If there is no such wrapping the plate must be placed in a
light-tight envelope, with its film facing that side of the envelope
which has no folds. The ordinary photographic double envelopes, the
inner one of yellow paper and the outer one of black paper, are very
convenient for this purpose. The plate in its envelope is then laid
flat on the table, film side upwards, and the X-ray tube is clamped in
a horizontal position so that the anti-cathode is over and pointing
towards the plate. The hand is laid flat on the envelope, and the coil
or machine is set working. The exposure required varies so much with
the size of the machine or coil, the distance between the tube and the
plate, the condition of the tube, and the nature of the object, that it
is impossible to give any definite times, and these have to be found
by experiment. The hand requires a shorter exposure than any other
part of the body. If we call the correct exposure for the hand 1, then
the exposures for other parts of the body would be approximately 3 for
the foot and the elbow, 6 for the shoulder, 8 for the thorax, 10 for
the spine and the hip, and about 12 for the head. The exposures for
such objects as coins in a box are much less than for the hand. After
exposure, the plate is developed, fixed, and washed just as in ordinary
photography. Plate XIV. shows a Röntgen ray photograph of a number of
fountain pens, British and foreign.
Prolonged exposure to the X-rays gives rise to a painful and serious
disease known as X-ray dermatitis. This danger was not realized by
the early experimenters, and many of them contracted the disease,
with fatal results in one or two cases. Operators now take ample
precautions to protect themselves from the rays. The tubes are screened
by substances opaque to the rays, so that these emerge only where they
are required, and impenetrable gloves or hand-shields, aprons, and
face-masks made of rubber impregnated with lead-salts are worn.
X-ray work is a most fascinating pursuit, and it can be recommended
strongly to amateurs interested in electricity. There is nothing
particularly difficult about it, and complete outfits can be obtained
at extremely low prices, although it is best to get the most powerful
Wimshurst machine or induction coil that can be afforded. As
radiography is most likely to be taken up by photographers, it may be
well to state here that any photographic plates or papers left in
their usual wrappings in the room in which X-rays are being produced
are almost certain to be spoiled, and they should be placed in a
tightly fitting metal box or be taken into the next room. It is not
necessary for the amateur doing only occasional X-ray work with small
apparatus to take any of the precautions mentioned in the previous
paragraph, for there is not the slightest danger in such work.
[Illustration: PLATE XIV.
_By permission of_ _Kodak Ltd._
RÖNTGEN RAY PHOTOGRAPH OF BRITISH AND FOREIGN FOUNTAIN PENS. TAKEN ON
WRATTEN X-RAY PLATE.]
CHAPTER XXV
ELECTRICITY IN MEDICINE
One of the most remarkable things about electricity is the immense
number of different purposes for which it may be used. We have already
seen it driving trams and trains, lighting and heating our houses, and
carrying our messages thousands of miles over land and sea, and now we
come to its use in medical work. In the minds of many people medical
electricity is associated with absolute quackery. Advertisements of
electric belts, rings, and other similar appliances have appeared
regularly for many years in our newspapers and magazines, and constant
exposures of the utter worthlessness of almost all these appliances
have produced the impression that medical electricity is nothing but
a bare-faced fraud, while the disgusting exhibitions of so-called
electric healing which have been given on the music-hall stage have
greatly deepened this impression. This state of things is very
unfortunate, because electricity, in the hands of competent medical
men, is a healing agent of wonderful potency. Still another source of
prejudice against electricity may be found in the fact that electric
healing is popularly associated with more or less violent shocks. On
this account nervously-sensitive people shrink from the idea of any
kind of electrical treatment. As a matter of fact electric shocks have
no healing value, but on the contrary they are frequently harmful, and
a very severe shock to a sensitive person may cause permanent injury.
No shocks whatever are given in electric treatment by medical men,
and indeed in the majority of cases the treatment is unaccompanied by
unpleasant sensations of any kind.
In the previous chapter we spoke of the use of the Röntgen or X-rays in
examining the various bones of the body. By means of the fluorescent
screen it is quite easy to find and examine fractures and dislocations,
and many of the diseases of the bones and joints can be seen and
recognized. Metals are opaque to the X-rays, and so the screen shows
plainly such objects as needles or bullets embedded in the flesh.
Sometimes people, especially young children, swallow coins and other
small metal articles, and here again the X-rays will show the exact
position of the intruder. A particularly valuable application of the
rays is in the discovering and locating of tiny fragments of metal in
the eye, for very often it is quite impossible to do this by ordinary
observation. Most of these fragments are of steel or iron, and they are
most easily removed by means of an electro-magnet. If the fragment is
very small a powerful magnet is used, one capable of supporting 500 or
600 lb.; but if it is fairly large a weaker magnet, supporting perhaps
30 lb., must be employed, because the forceful and rapid dragging out
of a large body might seriously damage the eye.
If the chest is examined by the Röntgen rays the lungs are seen
as light spaces between the clearly marked ribs, and any spot of
congestion appears as a darker patch. In this way the early stages
of consumption may be revealed, and in pneumonia and other similar
complaints valuable information regarding the condition of the lungs
can be obtained. It is possible also to follow to a considerable extent
the processes of digestion. X-rays easily pass through ordinary food,
but if bismuth oxychloride, which is quite harmless, is mixed with
the food, the mixture becomes opaque to the rays, and so its course
may be followed on the screen. The normal movements of the food are
well known, and an abnormal halt is probably caused by an obstruction
of some kind, and thus the X-rays enable the physician to locate the
obstruction, and to form an opinion of its nature.
In our chapter on wireless telegraphy we saw that the discharge of a
Leyden jar takes the form of a number of rapid oscillations backwards
and forwards. These oscillations take place at a rate of more than half
a million per second, but by the use of an apparatus called a “high
frequency transformer” the rate is increased to more than a million per
second. Electricity in this state of rapid oscillation is known as high
frequency electricity, and high frequency currents are very valuable
for some kinds of medical work. The application of these currents is
quite painless, and but for the strange-looking apparatus the patient
probably would not know that anything unusual was taking place. To
some extent the effect maybe said to be not unlike that of a powerful
tonic. Insomnia and other troubles due to disordered nerves are quickly
relieved, and even such obstinate complaints as neuritis and crippling
rheumatism have been cured. The treatment is also of great value in
certain forms of heart trouble. By increasing the strength of the high
frequency currents the tissues actually may be destroyed, and this
power is utilized for exterminating malignant growths, such as lupus or
cancer.
The heat produced by a current of electricity is made use of in
cauterizing. The burner is a loop of platinum wire, shaped according
to the purpose for which it is intended, and it is used at a dull red
heat. Very tiny electric incandescent lamps, fitted in long holders
of special shape, are largely used for examining the throat and the
various cavities of the body.
In the Finsen light treatment electric light is used for a very
different purpose. The spectrum of white light consists of the colours
red, orange, yellow, green, blue, indigo, and violet. Just beyond the
violet end of the spectrum are the ultra-violet rays. Ultra-violet
light consists of waves of light which are so short as to be quite
invisible to the eye, and Dr. N. R. Finsen, a Danish physician, made
the discovery that this light is capable of destroying bacterial germs.
In the application of ultra-violet rays to medical work, artificial
light is used in preference to sunlight; for though the latter contains
ultra-violet light, a great deal of it is absorbed in passing through
the atmosphere. Besides this, the sun sends out an immense amount of
radiant heat, and this has to be filtered out before the light can be
used. The usual source of light is the electric arc, and the arc is
much richer in ultra-violet rays if it is formed between electrodes of
iron, instead of the usual carbon rods. The light, which, in addition
to the ultra-violet rays, includes the blue, indigo, and violet parts
of the spectrum, is passed along a tube something like that of a
telescope, and is focused by means of a double lens, consisting of two
separate plates of quartz. Glass cannot be used for the lens, because
it is opaque to the extreme ultra-violet rays. A constant stream of
water is passed between the two plates forming the lens, and this
filters out the heat rays, which are not wanted. In some forms of
Finsen lamp an electric spark is used as the source of light, in place
of the arc.
The most important application of the Finsen light is in the cure
of the terribly disfiguring disease called lupus. This is a form of
tuberculosis of the skin, and it is produced by the same deadly microbe
which, when it attacks the lungs, causes consumption. In all but
extreme cases the Finsen light effects a remarkable cure. A number of
applications are necessary, each of half an hour or more; and after a
time the disease begins to disappear, leaving soft, normal skin. The
exact action of the light rays is a disputed point. Finsen himself
believed that the ultra-violet rays attacked and exterminated the
microbe, but a later theory is that the rays stimulate the tissues to
such an extent that they are enabled to cure themselves. As early as
the year 1899 Finsen had employed his light treatment in 350 cases of
lupus, and out of this number only five cases were unsuccessful.
The ultra-violet rays are said to have a very beneficial effect upon
the teeth. Experiments carried out in Paris, using a mercury vapour
lamp as the source of light, show that discoloured teeth are whitened
and given a pearly lustre by these rays, at the same time being
sterilized so that they do not easily decay. The Röntgen rays are used
for the treatment of lupus, and more particularly for deeper growths,
such as tumours and cancers, for which the Finsen rays are useless,
owing to their lack of penetrating power. The action of these two kinds
of rays appears to be similar, but the X-rays are much the more active
of the two.
Electricity is often applied to the body through water, in the form
of the hydro-electric bath, and such baths are used in the treatment
of different kinds of paralysis. Electric currents are used too for
conveying drugs into the tissues of the body. This is done when it
is desired to concentrate the drug at some particular point, and it
has been found that chemicals can be forced into the tissues for a
considerable distance.
Dr. Nagelschmidt, a great authority on medical electricity, has
suggested the use of electricity for weight reducing. In the ordinary
way superfluous flesh is got rid of by a starvation diet coupled with
exercise, but in many cases excessively stout people are troubled
with heart disorders and asthma, so that it is almost impossible for
them to undergo the necessary muscular exertion. By the application
of electric currents, however, the beneficial effects of the gentle
exercise may be produced without any exertion on the part of the
patient, and an hour’s treatment is said to result in a decrease in
weight of from 200 to 800 grammes, or roughly 7 to 27 ounces.
CHAPTER XXVI
OZONE
The great difference between the atmospheric conditions before and
after a thunderstorm must have been noticed by everybody. Before the
storm the air feels lifeless. It does not satisfy us as we draw it
into our lungs, and however deeply we breathe, we feel that something
is lacking. After the storm the air is delightful to inhale, and it
refreshes us with every breath. This remarkable transformation is
brought about to a very large extent by ozone produced by the lightning
discharges.
As far back as 1785 it was noticed that oxygen became changed in
some way when an electric spark was passed through it, and that it
acquired a peculiar odour. No particular attention was paid to the
matter however until about 1840, when Schönbein, a famous German
chemist, and the discoverer of gun-cotton and collodion, became
interested in it. He gave this strange smelling substance the name of
“ozone,” and he published the results of his experiments with it in
a treatise entitled, “On the Generation of Ozone.” Schönbein showed
that ozone could be produced by various methods, chemical as well as
electrical. For instance, if a piece of phosphorus is suspended in
a jar of air containing also a little water, in such a manner that
it is partly in the water and partly out of it, the air acquires
the characteristic smell of ozone, and it is found to have gained
increased chemical energy, so that it is a more powerful oxidizing
agent. For a long time the exact chemical nature of ozone could not be
determined, mainly because it was impossible to obtain the substance
in quantities sufficiently large for extensive experimental research,
but also on account of its extremely energetic properties, which made
it very troublesome to examine. These difficulties were so great that
investigators were in doubt as to whether ozone was an element or a
compound of two or more elements; but finally it was proved that it was
simply oxygen in a condensed or concentrated state.
Apparently ozone is formed by the contraction of oxygen, so that from
three volumes of oxygen two volumes of ozone are produced. In other
words, ozone has one and a half times the density of oxygen. Ozone has
far greater oxidizing power than oxygen itself; in fact it is probably
the most powerful of all oxidizing agents, and herein lies its great
value. It acts as nature’s disinfectant or sterilizer, and plays a very
important part in keeping the air pure, by destroying injurious organic
matter. Bacteria apparently have a most decided objection to dying; at
any rate they take an extraordinary amount of killing. Ozone is more
than a match for them however, and under its influence they have a
short life and probably not a merry one.
Ozone exists naturally in the atmosphere in the open country, and more
especially at the seaside. It is produced by lightning discharges, by
silent electrical discharges in the atmosphere, by the evaporation of
water, particularly salt water, by the action of sunlight, and also by
the action of certain vegetable products upon the air. The quantity
of ozone in the air is always small, and even pure country or sea air
contains only one volume of ozone in about 700,000 volumes of air. No
ozone can be detected in the air of large towns, or over unhealthy
swamps or marshes. The exhilarating effects of country and sea air,
and the depressing effects of town air, are due to a very large extent
to the presence or absence of ozone.
A great proportion of our common ailments are caused directly or
indirectly by a sort of slow poisoning, produced by the impure air in
which we live and work. It is popularly supposed that the tainting of
the air of rooms in which large numbers of people are crowded together
is due to an excessive amount of carbonic acid gas. This is a mistake,
for besides being tasteless and odourless, carbonic acid gas is
practically harmless, except in quantities far greater than ever exist
even in the worst ventilated rooms. The real source of the tainted
air is the great amount of animal matter thrown off as waste products
from the skin and lungs, and this tainting is further intensified
by the absence of motion in the air. Even in an over-crowded room
the conditions are made much more bearable if the air is kept in
motion, and in a close room ladies obtain relief by the use of their
fans. What we require, therefore, in order to maintain an agreeable
atmosphere under all conditions, is some means of keeping the air in
gentle motion, and at the same time destroying as much as possible of
the animal matter contained in it. Perhaps the most interesting and
at the same time the most scientific method of doing this is by ozone
ventilation.
In the well-known “Ozonair” system of ventilation, ozone is generated
by high-tension current. Low-tension current is taken from the public
mains or from accumulators, and raised to a very high voltage by
passing it through a step-up transformer. The secondary terminals
of the transformer are connected to a special form of condenser,
consisting of layers of fine metal gauze separated by an insulating
substance called “micanite.” The high tension between the gauze layers
produces a silent electrical discharge or glow. A small fan worked
by an electric motor draws the air over the condenser plates, and so
a certain proportion of the oxygen is ozonized, and is driven out of
the other side of the apparatus into the room. The amount of ozone
generated and the amount of air drawn over the condenser are regulated
carefully, so that the ozonized air contains rather less than one
volume of ozone in one million volumes of air, experiment having shown
that this is the most suitable strength for breathing. Ozone diluted to
this degree has a slight odour which is very refreshing, and besides
diminishing the number of organic germs in the air, it neutralizes
unpleasant smells, such as arise from cooking or stale tobacco smoke.
Ozone ventilation is now employed successfully in many hotels,
steamships, theatres and other places of entertainment, municipal and
public buildings, and factories.
[Illustration:
_By permission of_] [_Ozonair, Ltd._
FIG. 42.--Diagram of Ozonizing Plant, Central London Tube Electric
Railway.]
One of the most interesting examples of ozone ventilation is that of
the Central London tube electric railway. The installation consists of
a separate ozonizing plant at every station, except Shepherd’s Bush,
which is close to the open end of the tunnel. Fig. 42 is a diagram of
the general arrangement of one of these plants, and it shows how the
air is purified, ozonized, and sent into the tunnel. The generating
plant is seen at the top left-hand corner of the figure. Air is drawn
in as shown by the arrows, and by passing through the filter screen
F it is freed from dirt and smuts, and from most of the injurious
gases which always are present in town air. The filter screen is kept
moist by a continual flow of water from jets above it, the waste
water falling into the trough W. The ozone generator is shown at O.
Continuous current at about 500 volts, from the power station, is
passed through a rotary converter, which turns it into alternating
current at 380 volts. This current goes to the transformer T, from
which it emerges at a pressure of 5000 volts, and is supplied to the
ozone generator. From the generator the strongly ozonized air is
taken by way of the ozone pipe P, to the mixing chamber of the large
ventilating fan M, where it is mixed with the main air current and then
blown down the main air trunk. From this trunk it is distributed to
various conduits, and delivered at the air outlets marked A. Altogether
the various plants pump more than eighty million cubic feet of ozonized
air into the tunnels every working day.
In many industries pure air is very essential, especially during
certain processes. This is the case in brewing, in cold storage, and in
the manufacture and canning of food products; and in these industries
ozone is employed as an air purifier, with excellent results. Other
industries cannot be carried on without the production of very
unpleasant fumes and smells, which are a nuisance to the workers and
often also to the people living round about; and here again ozone is
used to destroy and remove the offending odours. It is employed also in
the purification of sewage and polluted water; in bleaching delicate
fabrics; in drying and seasoning timber; in maturing tobacco, wines and
spirits, and in many other processes too numerous to mention.
CHAPTER XXVII
ELECTRIC IGNITION
The petrol motor, which to-day is busily engaged all over the world
in driving thousands upon thousands of self-propelled vehicles or
automobiles, belongs to the important class of internal-combustion
engines. Combustion means the operation of burning, and an
internal-combustion engine is one in which the motive power is produced
by the combustion of a highly explosive mixture of gases. In the
ordinary petrol motor this mixture consists of petrol and air, and it
is made by means of a device called a “carburetter.” By suction, a
quantity of petrol is forced through a jet with a very fine nozzle, so
that it is reduced to an extremely fine spray. A certain proportion of
air is allowed to enter, and the mixture passes into the cylinder. Here
it is compressed by the rising piston so that it becomes more and more
heated, and at the right point it is ignited. Combustion takes place
with such rapidity that it takes the form of an explosion, and the
energy produced in this way drives forward the piston, which turns the
crank-shaft and so communicates motion to the driving-wheels.
The part played by electricity in this process is confined to the
ignition of the compressed charge of petrol and air. This may be done
in two ways; by means of an accumulator and a small induction coil, or
by means of a dynamo driven by the engine. At one time the first method
was employed exclusively, but to-day it is used as a rule only for
starting the car engine, the second or magneto method being used when
the engine has started up.
In accumulator ignition the low-tension current from the accumulator
passes through an induction coil, and is thus transformed to
high-tension current. This current goes through a sparking plug, which
is fixed in the head of the cylinder. The sparking plug contains two
metal points separated by a tiny air gap of from about 1/30 to 1/50
inch. This gap provides the only possible path for the high-tension
current, so that the latter leaps across it in the form of a spark.
The spark is arranged to take place when the piston is at the top of
its stroke, that is, when the explosive mixture is at its maximum
compression, and the heat of the spark ignites the mixture, the
resulting explosion forcing down the piston with great power. In
practice it is found better as a rule to cause the spark to pass very
slightly before the piston reaches the extreme limit of its stroke. The
reason of this is that the process of igniting and exploding the charge
occupies an appreciable, though of course exceedingly small amount of
time. Immediately on reaching the top of its stroke the piston begins
to descend again, and if the spark and the top of the stroke coincide
in time the explosion does not take place until the piston has moved
some little distance down the cylinder, and so a certain amount of
power is lost. By having the spark a little in advance of the piston,
the explosion occurs at the instant when the piston begins to return,
and so the full force of the explosion is utilized.
In magneto ignition the current is supplied by a small dynamo. This
generates alternating current, and it is driven by the car engine.
The current is at first at low pressure, and it has to be transformed
to high-tension current in order to produce the spark. There are
two methods of effecting this transformation. One is by turning
the armature of the dynamo into a sort of induction coil, by giving
it two separate windings, primary and secondary; so that the dynamo
delivers high-tension current directly. The other method is to send
the low-tension current through one or more transformer coils, just
as in accumulator ignition. Accumulators can give current only for a
certain limited period, and they are liable consequently to run down at
inconvenient times and places. They also have the defect of undergoing
a slight leakage of current even when they are not in use. Magneto
ignition has neither of these drawbacks, and on account of its superior
reliability it has come into universal use.
In the working of quarries and mines of various kinds, and also in
large engineering undertakings, blasting plays a prominent part. Under
all conditions blasting is a more or less dangerous business, and it
has been the cause of very many serious accidents to the men engaged in
carrying it out. Many of these accidents are due to the carelessness
resulting from long familiarity with the work, but apart from this
the danger lies principally in uncertainty in exploding the charge.
Sometimes the explosion occurs sooner than expected, so that the men
have not time to get away to a safe distance. Still more deadly is
the delayed explosion. After making the necessary arrangements the
men retire out of danger, and await the explosion. This does not take
place at the expected time, and after waiting a little longer the men
conclude that the ignition has failed, and return to put matters right.
Then the explosion takes place, and the men are killed instantly or at
least seriously injured. Although it is impossible to avoid altogether
dangers of this nature, the risk can be reduced to the minimum by
igniting the explosives by electricity.
Electrical shot firing may be carried out in different ways, according
to circumstances. The current is supplied either by a dynamo or by a
battery, and the firing is controlled from a switchboard placed at a
safe distance from the point at which the charge is to be exploded, the
connexions being made by long insulated wires. The actual ignition is
effected by a hot spark, as in automobile ignition, or by an electric
detonator or fuse. Explosives such as dynamite cannot be fired by
simple ignition, but require to be detonated. This is effected by
a detonator consisting of a small cup-shaped tube, made of ebonite
or other similar material. The wires conveying the current project
into this tube, and are connected by a short piece of very fine wire
having a high resistance. Round this wire is packed a small quantity
of gun-cotton, and beyond, in a sort of continuation of the tube, is
placed an extremely explosive substance called “fulminate of mercury,”
the whole arrangement being surrounded by the dynamite to be fired.
When all is ready the man at the switchboard manipulates a switch, and
the current passes to the detonator and forces its way through the
resistance of the thin connecting wire. This wire becomes sufficiently
hot to ignite the gun-cotton, and so explode the fulminate of mercury.
The explosion is so violent that the dynamite charge is detonated, and
the required blasting carried out. Gunpowder and similar explosives
do not need to be detonated, and so a simple fuse is used. Electric
fuses are much the same as detonators, except that the tube contains
gunpowder instead of fulminate of mercury, this powder being ignited
through an electrically heated wire in the same way. These electrical
methods do away with the uncertainty of the slow-burning fuses formerly
employed, which never could be relied upon with confidence.
Enormous quantities of explosives are now used in blasting on a large
scale, where many tons of hard rock have to be removed. One of the most
striking blasting feats was the blowing up of Flood Island, better
known as Hell Gate. This was a rocky islet, about 9 acres in extent,
situated in the East River, New York. It was a continual menace to
shipping, and after many fine vessels had been wrecked upon it the
authorities decided that it should be removed. The rock was bored
and drilled in all directions, the work taking more than a year to
complete; and over 126 tons of explosives were filled into the borings.
The exploding was carried out by electricity, and the mighty force
generated shattered nearly 300,000 cubic yards of solid rock.
CHAPTER XXVIII
ELECTRO-CULTURE
About thirty years ago a Swedish scientist, Professor Lemström,
travelled extensively in the Polar regions, and he was greatly struck
by the development of the Polar vegetation. In spite of the lack of
good soil, heat, and light, he observed that this vegetation came to
maturity quicker than that of regions having much more favourable
climates, and that the colours of the flowers were remarkably fresh and
clear, and their perfumes exceptionally strong. This was a surprising
state of things, and Lemström naturally sought a clue to the mystery.
He knew that peculiar electrical conditions prevailed in these high
latitudes, as was shown by the wonderful displays of the Aurora
Borealis, and he came to the conclusion that the development of the
vegetation was due to small currents of electricity continually passing
backwards and forwards between the atmosphere and the Earth. On his
return to civilization Lemström at once began a series of experiments
to determine the effect of electricity upon the growth of plants,
and he succeeded in proving beyond all doubt that plants grown under
electrical influence flourished more abundantly than those grown in
the ordinary way. Lemström’s experiments have been continued by other
investigators, and striking and conclusive results have been obtained.
The air surrounding the Earth is always charged to some extent with
electricity, which in fine weather is usually positive, but changes to
negative on the approach of wet weather. This electricity is always
leaking away to the earth more or less rapidly, and on its way it
passes through the tissues of the vegetation. An exceedingly slow but
constant discharge therefore is probably taking place in the tissues
of all plants. Experiments appear to indicate that the upper part of a
growing plant is negative, and the lower part positive, and at any rate
it is certain that the leaves of a plant give off negative electricity.
In dull weather this discharge is at its minimum, but under the
influence of bright sunshine it goes on with full vigour. It is not
known exactly how this discharge affects the plant, but apparently it
assists its development in some way, and there is no doubt that when
the discharge is at its maximum the flow of sap is most vigorous.
Possibly the electricity helps the plant to assimilate its food, by
making this more readily soluble.
This being so, a plant requires a regular daily supply of uninterrupted
sunshine in order to arrive at its highest possible state of maturity.
In our notoriously variable climate there are many days with only
short intermittent periods of bright sunshine, and many other days
without any sunshine at all. Now if, on these dull days, we can perform
at least a part of the work of the sunshine, and strengthen to some
extent the minute currents passing through the tissues of a plant, the
development of this plant should be accelerated, and this is found
to be the case. Under electrical influence plants not only arrive at
maturity quicker, but also in most cases their yield is larger and of
finer quality.
Lemström used a large influence machine as the source of electricity
in his experiments in electro-culture. Such machines are very suitable
for experimental work on a small scale, and much valuable work has
been done with them by Professor Priestly and others; but they have
the great drawback of being uncertain in working. They are quite
satisfactory so long as the atmosphere remains dry, but in damp weather
they are often very erratic, and may require hours of patient labour to
coax them to start. For this reason an induction coil is more suitable
for continuous work on an extensive scale.
The most satisfactory apparatus for electro-culture is that used in the
Lodge-Newman method, designed by Sir Oliver Lodge and his son, working
in conjunction with Mr. Newman. This consists of a large induction
coil supplied with current from a dynamo driven by a small engine, or
from the public mains if available. This coil is fitted with a spark
gap, and the high-tension current goes through four or five vacuum
valve globes, the invention of Sir Oliver Lodge, which permit the
current to pass through them in one direction only. This is necessary
because, as we saw in Chapter VIII., two opposite currents are induced
in the secondary winding of the coil, one at the make and the other
at the break of the primary circuit. Although the condenser fitted in
the base of the coil suppresses to a great extent the current induced
on making the circuit, still the current from the coil is not quite
uni-directional, but it is made so by the vacuum rectifying valves.
These are arranged to pass only the positive current, and this current
is led to overhead wires out in the field to be electrified. Lemström
used wires at a height of 18 inches from the ground, but these were
very much in the way, and in the Lodge-Newman system the main wires
are carried on large porcelain insulators fixed at the top of poles
at a height of about 15 feet. This arrangement allows carting and all
other agricultural operations to be carried on as usual. The poles
are set round the field, about one to the acre, and from these main
wires finer ones are carried across the field. These wires are placed
about 30 feet apart, so that the whole field is covered by a network
of wires. The electricity supplied to the wires is at a pressure of
about 100,000 volts, and this is constantly being discharged into
the air above the plants. It then passes through the plants, and
so reaches the earth. This system may be applied also to plants
growing in greenhouses, but owing to the confined space, and to the
amount of metal about, in the shape of hot-water pipes and wires for
supporting plants such as vines and cucumbers, it is difficult to make
satisfactory arrangements to produce the discharge.
The results obtained with this apparatus at Evesham, in
Gloucestershire, by Mr. Newman, have been most striking. With
wheat, increases of from 20 per cent. to nearly 40 per cent. have
been obtained, and the electrified wheat is of better quality
than unelectrified wheat grown at the same place, and, apart from
electrification, under exactly the same conditions. In some instances
the electrified wheat was as much as 8 inches higher than the
unelectrified wheat. Mr. Newman believes that by electrification land
yielding normally from 30 to 40 bushels of wheat per acre can be made
to yield 50 or even 60 bushels per acre. With cucumbers under glass
increases of 17 per cent. have been obtained, and in the case of
strawberries, increases of 36 per cent. with old plants, and 80 per
cent. with one-year-old plants. In almost every case electrification
has produced a marked increase in the crop, and in the few cases
where there has been a decrease the crops were ready earlier than the
normal. For instance, in one experiment with broad beans a decrease
of 15 per cent. resulted, but the beans were ready for picking five
days earlier. In another case a decrease of 11½ per cent. occurred
with strawberries, but the fruit was ready for picking some days
before the unelectrified fruit, and also was much sweeter. In some of
the experiments resulting in a decrease in the yield it is probable
that the electrification was overdone, so that the plants were
over-stimulated. It seems likely that the best results will be obtained
only by adjusting the intensity and the duration of the electrification
in accordance with the atmospheric conditions, and also with the nature
of the crop, for there is no doubt that plants vary considerably in
their electrical requirements. A great deal more experiment is required
however to enable this to be done with anything like certainty.
Unlike the farmer, the market gardener has to produce one crop after
another throughout the year. To make up for the absence of sufficient
sunshine he has to resort to “forcing” in many cases, but unfortunately
this process, besides being costly, generally results in the production
of a crop of inferior quality. Evidently the work of the market
gardener would be greatly facilitated by some artificial substitute
for sunshine, to keep his plants growing properly in dull weather. In
1880, Sir William Siemens, knowing that the composition of the light
of the electric arc was closely similar to that of sunlight, commenced
experiments with an arc lamp in a large greenhouse. His idea was to add
to the effects of the solar light by using the arc lamp throughout the
night. His first efforts were unsuccessful, and he discovered that this
was due to the use of the naked light, which apparently contained rays
too powerful for the plants. He then passed the light through glass,
which filtered out the more powerful rays, and this arrangement was
most successful, the plants responding readily to the artificial light.
More scientifically planned experiments were carried out at the London
Royal Botanic Gardens in 1907, by Mr. B. H. Thwaite, and these showed
that by using the arc lamp for about five hours every night, a great
difference between the treated plants and other similar plants grown
normally could be produced in less than a month. Other experiments made
in the United States with the arc lamp, and also with ordinary electric
incandescent lamps, gave similar results, and it was noticed that the
improvement was specially marked with cress, lettuce, spinach, and
other plants of this nature.
In 1910, Miss E. C. Dudgeon, of Dumfries, commenced a series of
experiments with the Cooper-Hewitt mercury vapour lamp. Two greenhouses
were employed, one of which was fitted with this lamp. Seeds of various
plants were sown in small pots, one pot of each kind being placed in
each house. The temperature and other conditions were kept as nearly
alike as possible in both houses, and in the experimental house the
lamp was kept going for about five hours every night. In every case the
seeds in the experimental house germinated several days before those
in the other house, and the resulting plants were healthy and robust.
Later experiments carried out by Miss Dudgeon with plants were equally
successful.
From these experiments it appears that the electric arc, and still
more the mercury vapour lamp, are likely to prove of great value to
the market gardener. As compared with the arc lamp, the mercury vapour
lamp has the great advantage of requiring scarcely any attention, and
also it uses less current. Unlike the products of ordinary forcing by
heat, the plants grown under the influence of the mercury vapour light
are quite sturdy, so that they can be planted out with scarcely any
“hardening off.” The crop yields too are larger, and of better quality.
The wonderful effects produced by the Cooper-Hewitt lamp are certainly
not due to heat, for this lamp emits few heat rays. The results may
be due partly to longer hours worked by the plants, but this does
not explain the greater accumulation of chlorophyll and stronger
development of fibre.
Most of us are familiar with the yarn about the poultry keeper who
fitted all his nests with trap-doors, so that when a hen laid an
egg, the trap-door opened under the weight and allowed the egg to
fall through into a box lined with hay. The hen then looked round,
and finding no egg, at once set to work to lay another. This in turn
dropped, another egg was laid, and so on. It is slightly doubtful
whether the modern hen could be swindled in this bare-faced manner, but
it is certain that she can be deluded into working overtime. The scheme
is absurdly simple. Electric lamps are fitted in the fowl-house, and at
sunset the light is switched on. The unsuspecting hens, who are just
thinking about retiring for the night, come to the conclusion that the
day is not yet over, and so they continue to lay. This is not a yarn,
but solid fact, and the increase in the egg yield obtained in this way
by different poultry keepers ranges from 10 per cent. upwards. Indeed,
one poultry expert claims to have obtained an increase of about 40 per
cent.
The ease with which a uniform temperature can be maintained by electric
heating has been utilized in incubator hatching of chickens. By means
of a specially designed electric radiator the incubator is kept at the
right temperature throughout the hatching period. When the chickens
emerge from the eggs they are transferred to another contrivance
called a “brooder,” which also is electrically heated, the heat being
decreased gradually day by day until the chicks are sturdy enough to
do without it. Even at this stage however the chickens do not always
escape from the clutches of electricity. Some rearers have adopted the
electric light swindle for the youngsters, switching on the light
after the chickens have had a fair amount of slumber, so that they
start feeding again. In this way the chickens are persuaded to consume
more food in the twenty-four hours, and the resulting gain in weight
is said to be considerable. More interesting than this scheme is the
method of rearing chickens under the influence of an electric discharge
from wires supplied with high-tension current. Comparative tests show
that electrified chickens have a smaller mortality and a much greater
rate of growth than chickens brought up in the ordinary way. It even is
said that the electrified chickens have more kindly dispositions than
their unelectrified relatives!
Possibly the high-tension discharge may turn out to be as beneficial
to animals as it has been proved to be for plants, but so far there is
little reliable evidence on this point, owing to lack of experimenters.
A test carried out in the United States with a flock of sheep is worth
mention. The flock was divided into two parts, one-half being placed
in a field under ordinary conditions, and the other in a field having
a system of overhead discharge wires, similar to those used in the
Lodge-Newman system. The final result was that the electrified sheep
produced more than twice as many lambs as the unelectrified sheep, and
also a much greater weight of wool. If further experiments confirm this
result, the British farmer will do well to consider the advisability of
electrifying his live-stock.
CHAPTER XXIX
SOME RECENT APPLICATIONS OF ELECTRICITY--AN ELECTRIC PIPE LOCATOR
One of the great advantages of living in a town is the abundant supply
of gas and water. These necessary substances are conveyed to us along
underground pipes, and a large town has miles upon miles of such pipes,
extending in all directions and forming a most complex network. Gas
and water companies keep a record of these pipes, with the object of
finding any pipe quickly when the necessity arises; but in spite of
such records pipes are often lost, especially where the whole face of
the neighbourhood has changed since the pipes were laid. The finding
of a lost pipe by digging is a very troublesome process, and even when
the pipe is known to be close at hand, it is quite surprising how
many attempts are frequently necessary before it can be located, and
its course traced. As may be imagined, this is an expensive business,
and often it has been found cheaper to lay a new length of pipe than
to find the old one. There is now an electrical method by which
pipe locating is made comparatively simple, and unless it is very
exceptionally deep down, a pipe never need be abandoned on account of
difficulty in tracing it.
The mechanism of an electric pipe locator is not at all complicated,
consisting only of an induction coil with battery, and a telephone
receiver connected to a coil of a large number of turns of thin copper
wire. If a certain section of a pipe is lost, and has to be located,
operations are commenced from some fitting known to be connected with
it, and from some other fitting which may or may not be connected with
the pipe, but which is believed to be so connected. The induction coil
is set working, and its secondary terminals are connected one to each
of these fittings. If the second fitting is connected with the pipe,
then the whole length of the pipe between these two points is traversed
by the high-frequency current. The searcher, wearing the head telephone
receiver, with the coil hanging down from it so as to be close to the
ground, walks to and fro over the ground beneath which the pipe must
lie. When he approaches the pipe the current passing through the latter
induces a similar current in the suspended coil, and this produces
a sort of buzzing or humming sound in the telephone. The nearer he
approaches to the pipe the louder is the humming, and it reaches its
maximum when he is standing directly over the pipe. In this way the
whole course of the pipe can be traced without any digging, even when
the pipe is 15 or 20 feet down. The absence of any sounds in the
receiver indicates that the second fitting is not on the required pipe
line, and other fittings have to be tried until one on this line is
found.
AN ELECTRIC ICEBERG DETECTOR
Amongst the many dangers to which ships crossing the Atlantic are
exposed is that of collision with icebergs. These are large masses of
ice which have become detached from the mighty ice-fields of the north,
and which travel slowly and majestically southwards, growing smaller
and smaller as they pass into warmer seas. Icebergs give no warning
of their coming, and in foggy weather, which is very prevalent in the
regions where they are encountered, they are extremely difficult to
see until they are at dangerously close quarters.
Attempts have been made to detect the proximity of icebergs by noting
the variations in the temperature of the water. We naturally should
expect the temperature of the water to become lower as we approach a
large berg, and this is usually the case. On the other hand, it has
been found that in many instances the temperature near an iceberg is
quite as high as, and sometimes higher than the average temperature of
the ocean. For this reason the temperature test, taken by itself, is
not at all reliable. A much more certain test is that of the salinity
or saltness of the water. Icebergs are formed from fresh water, and
as they gradually melt during their southward journey the fresh water
mixes with the sea water. Consequently the water around an iceberg is
less salt than the water of the open ocean. The saltness of water may
be determined by taking its specific gravity, or by various chemical
processes; but while these tests are quite satisfactory when performed
under laboratory conditions, they cannot be carried out at sea with
any approach to accuracy. There is however an electrical test which
can be applied accurately and continuously. The electrical conducting
power of water varies greatly with the proportion of salt present. If
the conductivity of normal Atlantic water be taken as 1000, then the
conductivity of Thames water is 8, and that of distilled water about
1/22. The difference in conductivity between normal ocean water and
water in the vicinity of an iceberg is therefore very great.
[Illustration:
_By permission of_] [_Dr. Myer Coplans._
FIG. 43.--Diagram of Heat-compensated Salinometer.]
The apparatus for detecting differences in salinity by measuring the
conductivity of the water is called a “salinometer,” and its most
perfect form, known as the heat-compensated conductivity salinometer,
is due to Dr. Myer Coplans. Fig. 43 shows a diagram of this interesting
piece of apparatus, which is most ingeniously devised. Two insulated
electrodes of copper, with platinum points, are suspended in a U-tube
through which the sea water passes continuously, as indicated in
the diagram. A steady current is passed through the column of water
between the two platinum points, and the conductivity of this column
is measured continuously by very accurate instruments. Variations in
the conductivity, indicating corresponding variations in the saltness
of the water, are thus shown immediately; but before these indications
can be relied upon the instrument must be compensated for temperature,
because the conductivity of the water increases with a rise, and
decreases with a fall in temperature. This compensation is effected by
the compound bars of brass and steel shown in the vessel at the right
of the figure. These bars are connected with the wheel and disc from
which the electrodes are suspended. When the temperature of the water
rises, the bars contract, and exert a pull upon the wheel and disc, so
that the electrodes are raised slightly in the U-tube. This increases
the length of the column of water between the platinum points, and so
increases the resistance, or, what amounts to the same thing, lowers
the conductivity, in exact proportion to the rise in temperature.
Similarly, a fall in temperature lowers the electrodes, and decreases
the resistance by shortening the column of water. In this way the
conductivity of the water remains constant so far as temperature is
concerned, and it varies only with the saltness of the water. Under
ordinary conditions a considerable decrease in the salinity of the
water indicates the existence of ice in the near neighbourhood, but the
geographical position of the ship has to be taken into account. Rivers
such as the St. Lawrence pour vast quantities of fresh water into the
ocean, and the resulting decrease in the saltness of the water within a
considerable radius of the mouth of the river must be allowed for.
A “FLYING TRAIN”
Considerable interest was aroused last year by a model of a railway
working upon a very remarkable system. This was the invention of
Mr. Emile Bachelet, and the model was brought to London from the
United States. The main principle upon which the system is based is
interesting. About 1884, Professor Elihu Thompson, a famous American
scientist, made the discovery that a plate of copper could be attracted
or repelled by an electro-magnet. The effects took place at the moment
when the magnetism was varied by suddenly switching the current on or
off; the copper being repelled when the current was switched on, and
attracted when it was switched off. Copper is a non-magnetic substance,
and the attraction and repulsion are not ordinary magnetic effects,
but are due to currents induced in the copper plate at the instant
of producing or destroying the magnetism. The plate is attracted or
repelled according to whether these induced currents flow in the same
direction as, or in the opposite direction to, the current in the
magnet coil. Brass and aluminium plates act in the same way as the
copper plate, and the effects are produced equally well by exciting
the magnet with alternating current, which, by changing its direction,
changes the magnetism also. Of the two effects, the repulsion is much
the stronger, especially if the variations in the magnetism take place
very rapidly; and if a powerful and rapidly alternating current is
used, the plate is repelled so strongly that it remains supported in
mid-air above the magnet.
This repulsive effect is utilized in the Bachelet system (Plate XV.).
There are no rails in the ordinary sense, and the track is made up
of a continuous series of electro-magnets. The car, which is shaped
something like a cigar, has a floor of aluminium, and contains an iron
cylinder, and it runs above the line of magnets. Along each side of
the track is a channel guide rail, and underneath the car at each end
are fixed two brushes with guide pieces, which run in the guide rails.
Above the car is a third guide rail, and two brushes with guide pieces
fixed on the top of the car, one at each end, run in this overhead
rail. These guide rails keep the car in position, and also act as
conductors for the current. The repulsive action of the electro-magnets
upon the aluminium floor raises the car clear of the track, and keeps
it suspended; and while remaining in this mid-air position it is
driven, or rather pulled forward, by powerful solenoids, which are
supplied with continuous current. We have referred previously to the
way in which a solenoid draws into it a core of iron. When the car
enters a solenoid, the latter exerts a pulling influence upon the iron
cylinder inside the car, and so the car is given a forward movement.
This is sufficient to carry it along to the next solenoid, which gives
it another pull, and so the car is drawn forward from one solenoid to
another to the end of the line. The model referred to has only a short
track of about 30 feet, with one solenoid at each end; but its working
shows that the pulling power of the solenoids is sufficient to propel
the car.
[Illustration: PLATE XV.
_Photo by_ _Record Press._
BACHELET “FLYING TRAIN” AND ITS INVENTOR.]
To avoid the necessity of keeping the whole of the electro-magnets
energized all the time, these are arranged in sections, which are
energized separately. By means of the lower set of brushes working
in the track guides, each of these sections has alternating current
supplied to it as the car approaches, and switched off from it when
the car has passed. The brushes working in the overhead guide supply
continuous current to each solenoid as the car enters it, and switch
off the current when the car has passed through. The speed at which the
model car travels is quite extraordinary, and the inventor believes
that in actual practice speeds of more than 300 miles an hour are
attainable on his system.
CHAPTER XXX
ELECTRICITY IN WAR
One of the most striking features of modern naval warfare is the
absolute revolution in methods of communication brought about
by wireless telegraphy. To-day every warship has its wireless
installation. Our cruiser squadrons and destroyer flotillas,
ceaselessly patrolling the waters of the North Sea, are always in touch
with the Admiral of the Fleet, and with the Admiralty at Whitehall. In
the Atlantic, and in the Pacific too, our cruisers, whether engaged in
hunting down the marauding cruisers of the enemy or in searching for
merchant ships laden with contraband, have their comings and goings
directed by wireless. Even before the actual declaration of war between
Great Britain and Germany wireless telegraphy began its work. At the
conclusion of the great naval review of July 1914, the Fleet left
Portland to disperse as customary for manœuvre leave, but a wireless
message was dispatched ordering the Fleet not to disperse. As no state
of war then existed, this was a precautionary measure, but subsequent
events quickly proved how urgently necessary it had been to keep the
Fleet in battle array. Immediately war was declared Great Britain was
able to put into the North Sea a fleet which hopelessly outnumbered and
outclassed the German battle fleet.
At the outset Germany had a number of cruisers in the Atlantic and the
Pacific Oceans. Owing to the vigilance of our warships these vessels
were unable to join the German Home Fleet, and they immediately adopted
the rôle of commerce destroyers. In this work they made extensive use
of wireless telegraphy to ascertain the whereabouts of British merchant
ships, and for a short time they played quite a merry game. Prominent
among these raiders was the _Emden_. It was really astonishing how this
cruiser obtained information regarding the sailings of British ships.
It is said that on one occasion she called up by wireless a merchant
ship, and inquired if the latter had seen anything of a German cruiser.
The unsuspecting merchantman replied that there was no such thing as
a German warship in the vicinity. “Oh yes, there is,” returned the
_Emden_; “I’m it!” and shortly afterwards she appeared on the horizon,
to the great discomfiture of the British skipper. An interesting
account of the escape of a British liner from another notorious raider,
the _Karlsruhe_, has been given in the _Nautical Magazine_. The writer
says:
“I have just returned home after a voyage to South America in one
of the Pacific Steam Navigation Company’s cargo boats. When we left
Montevideo we heard that France and Germany were at war, and that
there was every possibility of Great Britain sending an ultimatum to
Germany. We saw several steamers after leaving the port, but could get
no information, as few of them were fitted with wireless and passed
at some distance off. When about 200 miles east of Rio, our wireless
operator overheard some conversation between the German cruiser
_Karlsruhe_ and a German merchant ship at anchor in Rio. It was clearly
evident that the German merchant ship had no special code, as the
conversation was carried on in plain German language, and our operator,
who, by the way, was master of several languages, was able to interpret
these messages without the slightest difficulty. It was then that we
learned that Great Britain was at war. The German cruiser was inquiring
from the German merchant ship what British vessels were leaving Rio,
and asking for any information which might be of use. We also picked up
some news of German victories in Belgium, which were given out by the
German merchant ship. It was clearly evident that the _Karlsruhe_ had
information about our ship, and expected us to be in the position she
anticipated, for she sent out a signal to us in English, asking us for
our latitude and longitude. This our operator, under the instructions
of the captain, declined to give. The German operator evidently got
furious, as he called us an English ‘swine-hound,’ and said, ‘This is a
German warship, _Karlsruhe_; we will you find.’ Undoubtedly he thought
he was going to strike terror to our hearts, but he made a mistake.
“That night we steamed along without lights, and we knew from the sound
of the wireless signals that were being flashed out from the German
ship that we were getting nearer and nearer to her. Fortunately for
us, about midnight a thick misty rain set in and we passed the German
steamer, and so escaped. Our operator said that we could not have
been more than 8 or 10 miles away when we passed abeam. Undoubtedly
our wireless on this occasion saved us from the danger from which we
escaped.”
Apparently little is known of the end of the _Karlsruhe_, but the
_Emden_ met with the fate she richly deserved; and fittingly enough,
wireless telegraphy, which had enabled her to carry out her marauding
exploits, was the means of bringing her to her doom. On 9th November
1914 the _Emden_ anchored off the Cocos-Keeling Islands, a group of
coral islets in the Indian Ocean, and landed a party of three officers
and forty men to cut the cable and destroy the wireless station.
Before the Germans could get to the station, a wireless message was
sent out stating the presence of the enemy warship, and this call was
received by the Australian cruisers _Melbourne_ and _Sydney_. These
vessels, which were then only some 50 miles away, were engaged, along
with a Japanese cruiser, in escorting transports. The _Sydney_ at once
went off at full speed, caught the _Emden_, and sent her to the bottom
after a short but sharp engagement. As the _Emden_ fled at sight of the
Australian warship, the landing party had not time to get aboard, and
consequently were left behind. They seized an old schooner, provisioned
her, and set sail, but what became of them is not known.
In land warfare field telegraphs play a very important part; indeed
it is certain that without them the vast military operations of the
present war could not be carried on. The General Headquarters of
our army in France is in telegraphic communication not only with
neighbouring French towns, but also with Paris and London. From
Headquarters also run wires to every point of the firing-line, so that
the Headquarters Staff, and through them the War Office in London,
know exactly what is taking place along the whole front. The following
extract from a letter from an officer, published by _The Times_, gives
a remarkably good idea of the work of the signal companies of the Royal
Engineers.
“As the tide of battle turns this way and the other, and headquarters
are constantly moving, some means have to be provided to keep in
constant touch with General Headquarters during the movement. This
emergency is met by cable detachments. Each detachment consists of two
cable waggons, which usually work in conjunction with one another, one
section laying the line whilst the other remains behind to reel up when
the line is finished with. A division is ordered to move quickly to
a more tactical position. The end of the cable is connected with the
permanent line, which communicates to Army Headquarters, and the cable
detachment moves off at the trot; across country, along roads, through
villages, and past columns of troops, the white and blue badge of the
signal service clears the way. Behind the waggon rides a horseman,
who deftly lays the cable in the ditches and hedges out of danger
from heavy transport and the feet of tramping infantry, with the aid
of a crookstick. Other horsemen are in the rear tying back and making
the line safe. On the box of the waggon sits a telegraphist, who is
constantly in touch with headquarters as the cable runs swiftly out. An
orderly dashes up with an important message; the waggon is stopped, the
message dispatched, and on they go again.”
Wireless telegraphy too has its part to play in land war, and for
field purposes it has certain advantages over telegraphy with wires.
Ordinary telegraphic communication is liable to be interrupted by the
cutting of the wire by the enemy, or, in spite of every care in laying,
by the breaking of the wire by passing cavalry or artillery. No such
trouble can occur with wireless telegraphy, and if it becomes necessary
to move a wireless station with great rapidity, as for instance on an
unexpected advance of the enemy, it is an advantage to have no wire to
bother about. The Marconi portable wireless sets for military purposes
are marvels of compactness and lightness, combined with simplicity.
They are of two kinds, pack-saddle sets and cart sets. The former weigh
about 360 lb., this being divided amongst four horses. They can be
set up in ten minutes by five or six men, and require only two men to
work them. Their guaranteed range is 40 miles, but they are capable
of transmitting twice this distance or even more under favourable
conditions. The cart sets can be set up in twenty minutes by seven or
eight men, and they have a guaranteed range of from 150 to 200 miles.
It is obviously very important that wireless military messages should
not be intercepted and read by the enemy, and the method of avoiding
danger of this kind adopted with the Marconi field stations is
ingenious and effective. The transmitter and the receiver are arranged
to work on three different fixed wave-lengths, the change from one to
another being effected quickly by the movement of a three-position
switch. By this means the transmitting operator sends three or four
words on one wave-length, then changes to another, transmits a few
words on this, changes the wave-length again, and so on. Each change
is accompanied by the sending of a code letter which informs the
receiving operator to which wave-length the transmitter is passing. The
receiving operator adjusts his switch accordingly, and so he hears the
whole message without interruption, the change from one wave-length to
another taking only a small fraction of a second. An enemy operator
might manage to adjust his wave-length so as to hear two or three
words, but the sudden change of wave-length would throw him out of
tune, and by the time he had found the new wave-length this would have
changed again. Thus he would hear at most only a few disconnected words
at intervals, and he would not be able to make head or tail of the
message. To provide against the possibility of the three wave-lengths
being measured and prepared for, these fixed lengths themselves can be
changed, if necessary, many times a day, so that the enemy operators
would never know beforehand which three were to be used.
Wireless telegraphy was systematically employed in land warfare for the
first time in the Balkan War, during which it proved most useful both
to the Allies and to the Turks. One of the most interesting features
of the war was the way in which wireless communication was kept up
between the beleaguered city of Adrianople and the Turkish capital.
Some time before war broke out the Turkish Government sent a portable
Marconi wireless set to Adrianople, and this was set up at a little
distance from the city. When war was declared the apparatus was brought
inside the city walls and erected upon a small hill. Then came the
siege. For 153 days Shukri Pasha kept the Turkish flag flying, but the
stubborn defence was broken down in the end through hunger and disease.
All through these weary days the little wireless set did its duty
unfalteringly, and by its aid regular communication was maintained with
the Government station at Ok Meidan, just outside Constantinople, 130
miles away. Altogether about half a million words were transmitted from
Adrianople to the Turkish capital.
[Illustration: PLATE XVI.
(_a_) CAVALRY PORTABLE WIRELESS CART SET.]
[Illustration:
_By permission of_ _Marconi Co. Ltd._
(_b_) AEROPLANE FITTED WITH WIRELESS TELEGRAPHY.]
The rapid development of aviation during the past few years has drawn
attention to the necessity for some means of communication between
the land and airships and aeroplanes in flight. At first sight it
might appear that wireless telegraphy could be used for this purpose
without any trouble, but experience has shown that there are certain
difficulties in the way, especially with regard to aeroplanes. The
chief difficulty with aeroplanes lies in the aerial. This must take the
form either of a long trailing wire or of fixed wires running between
the planes and the tail. A trailing wire is open to the objection
that it is liable to get mixed up with the propeller, besides which
it appears likely to hamper to some slight extent the movements of
a small and light machine. A fixed aerial between planes and tail
avoids these difficulties, but on the other hand its wave-length is
bound to be inconveniently small. The heavy and powerful British
military aeroplanes apparently use a trailing wire of moderate length,
carried in a special manner so as to clear the propeller, but few
details are available at present. A further trouble with aeroplanes
lies in the tremendous noise made by the engine, which frequently
makes it quite impossible to hear incoming signals; and the only way
of getting over this difficulty appears to be for the operator to wear
some sort of sound-proof head-gear. Signals have been transmitted
from an aeroplane in flight up to distances of 40 or 50 miles quite
successfully, but the reception of signals by aeroplanes is not so
satisfactory, except for comparatively short distances. Although few
particulars have been published regarding the work of the British
aeroplanes in France, it seems evident that wireless telegraphy is
in regular use. In addition to their value as scouts, our aeroplanes
appear to be extremely useful for the direction of heavy artillery
fire, using wireless to tell the gunners where each shell falls, until
the exact range is obtained. In the case of airships the problem of
wireless communication is much simpler. A trailing wire presents no
difficulties, and on account of their great size much more powerful
sets of apparatus can be carried. The huge German Zeppelin airships
have a long freely-floating aerial consisting of a wire which can
be wound in or let out as required, its full length being about 750
feet. The total weight of the apparatus is nearly 300 lb., and the
transmitting range is said to be from about 120 to 200 miles.
Electricity is used in the navy for a great variety of purposes
besides telegraphy. Our battleships are lighted by electricity,
which is generated at a standard pressure of 220 volts. This
current is transformed down for the searchlights, and also for the
intricate systems of telephone, alarm, and firing circuits. The
magazines containing the deadly cordite are maintained at a constant
temperature of 70° F. by special refrigerating machinery driven by
electricity, and the numerous fans for ventilating the different
parts of the ship are also electrically driven. Electric power is
used for capstans, coaling winches, sounding machines, lifts, pumps,
whether for drainage, fire extinction, or raising fresh water from the
tanks, and for the mechanism for operating boats and torpedo nets.
The mechanism for manipulating the great guns and their ammunition
is hydraulic. Electricity was tried for this purpose on the battle
cruiser _Invincible_, but was abandoned in favour of hydraulic power.
But though electricity is apparently out of favour in this department,
it takes an extremely important share in the work of controlling and
firing the guns; its duties being such as could not be carried out by
hydraulic power.
The guns are controlled and fired from what is known as the
fire-control room, which is situated in the interior of the ship,
quite away from the guns themselves. The range-finder, from his perch
up in the gigantic mast, watches an enemy warship as she looms on the
horizon, and when she comes within range he estimates her distance by
means of instruments of wonderful precision. He then telephones to the
fire-control room, giving this distance, and also the enemy’s speed
and course. The officer in charge of the fire-control room calculates
the elevation of the gun required for this distance, and decides upon
the instant at which the gun must be fired. A telephoned order goes to
the gun-turret, and the guns are brought to bear upon the enemy, laid
at the required elevation, and sighted. At the correct instant the
fire-control officer switches on an electric current to the gun, which
fires a small quantity of highly explosive material, and this in turn
fires the main charge of cordite. The effect of the shell is watched
intently from the fire-control top, up above the range-finder, and
if, as is very likely, this first shell falls short of, or overshoots
the mark, an estimate of the amount of error is communicated to the
fire-control room. Due corrections are then made, the gun is laid at a
slightly different elevation, and this time the shell finds its mark
with unerring accuracy.
The range of movement, horizontal and vertical, of modern naval guns
is so great that it is possible for two guns to be in such relative
positions that the firing of one would damage the other. To guard
against a disaster of this kind fixed stops are used, supplemented by
ingenious automatic alarms. The alarm begins to sound as soon as any
gun passes into a position in which it could damage another gun, and it
goes on sounding until the latter gun is moved out of the danger line.
Since the outbreak of war the subject of submarine mines has been
brought to our notice in very forcible fashion. Contrary to the general
impression, the explosive submarine mine is not a recent introduction.
It is difficult to say exactly when mines were first brought into
use, but at any rate we know that they were employed by Russia during
the Crimean War, apparently with little success. The first really
successful use of mines occurred in the American Civil War, when the
Confederates sank a number of vessels by means of them. This practical
demonstration of their possibilities did not pass unnoticed by European
nations, and in the Franco-German War we find that mines were used
for harbour defence by both belligerents. It is doubtful whether
either nation derived much benefit from its mines, and indeed as the
war progressed Germany found that the principal result of her mining
operations was to render her harbours difficult and dangerous to her
own shipping. Much greater success attended the use of mines in the
Russo-Japanese War, but all previous records shrink into insignificance
when compared with the destruction wrought by mines in the present
great conflict.
Submarine mines may be divided into two classes; those for harbour
defence, and those for use in the open sea. Harbour defence mines are
almost invariably electrically controlled; that is, they are connected
with the shore by means of a cable, and fired by an electric impulse
sent along that cable. In one system of control the moment of firing
is determined entirely by observers on shore, who, aided by special
optical instruments, are able to tell exactly when a vessel is above
any particular mine. The actual firing is carried out by depressing a
key which completes an electric circuit, thus sending a current along
the cable to actuate the exploding mechanism inside the mine. A hostile
ship therefore would be blown up on arriving at the critical position,
while a friendly vessel would be allowed to pass on in safety. In
this system of control there is no contact between the vessel and the
mine, the latter being well submerged or resting on the sea floor,
so that the harbour is not obstructed in any way. This is a great
advantage, but against it must be set possible failure of the defence
at a critical moment owing to thick weather, which of course interferes
seriously with the careful observation of the mine field necessary for
accurate timing of the explosions. This difficulty may be surmounted
by a contact system of firing. In this case the mines are placed so
near the surface as to make contact with vessels passing over them. The
observers on shore are informed of the contact by means of an electric
impulse automatically transmitted along the cable, so that they are
independent of continuous visual observation of the mined area. As in
the previous system, the observers give the actual firing impulse.
The drawback to this method is the necessity for special pilotage
arrangements for friendly ships in order to avoid unnecessary striking
of the mines, which are liable to have their mechanism deranged by
constant blows. If the harbour or channel can be closed entirely to
friendly shipping, the observers may be dispensed with, their place
being taken by automatic electric apparatus which fires at once any
mine struck by a vessel.
Shore-controlled mines are excellent for harbour defence, and
a carefully distributed mine-field, backed by heavy fort guns,
presents to hostile vessels a barrier which may be regarded as almost
impenetrable. A strong fleet might conceivably force its way through,
but in so doing it would sustain tremendous losses; and as these losses
would be quite out of proportion to any probable gains, such an attempt
is not likely to be made except as a last resort.
For use in the open sea a different type of mine is required. This must
be quite self-contained and automatic in action, exploding when struck
by a passing vessel. The exploding mechanism may take different forms.
The blow given by a ship may be made to withdraw a pin, thus releasing
a sort of plunger, which, actuated by a powerful spring, detonates
the charge. A similar result is obtained by the use of a suspended
weight, in place of plunger and spring. Still another form of mine
is fired electrically by means of a battery, the circuit of which is
closed automatically by the percussion. Deep-sea mines may be anchored
or floating free. Free mines are particularly dangerous on account of
the impossibility of knowing where they may be at any given moment.
They are liable to drift for considerable distances, and to pass into
neutral seas; and to safeguard neutral shipping international rules
require them to have some sort of clockwork mechanism which renders
them harmless after a period of one hour. It is quite certain that
some, at least, of the German free mines have no such mechanism, so
that neutral shipping is greatly endangered.
Submarine mines are known as _ground_ mines, or _buoyant_ mines,
according to whether they rest on the sea bottom or float below the
surface. Ground mines are generally made in the form of a cylinder,
buoyant mines being usually spherical. The cases are made of steel,
and buoyancy is given when required by enclosing air spaces. Open-sea
mines are laid by special vessels, mostly old cruisers. The stern of
these ships is partly cut away, and the mines are run along rails
to the stern, and so overboard. The explosive employed is generally
gun-cotton, fired by a detonator, charges up to 500 lb. or more being
used, according to the depth of submersion and the horizontal distance
at which the mine is desired to be effective. Ground mines can be used
only in shallow water, and even then they require a heavier charge
than mines floating near the surface. Mines must not be laid too close
together, as the explosion of one might damage others. The distance
apart at which they are placed depends upon the amount of charge,
500-lb. mines requiring to be about 300 feet apart for safety.
CHAPTER XXXI
WHAT IS ELECTRICITY?
The question which heads this, our final chapter, is one which must
occur to every one who takes even the most casual interest in matters
scientific, and it would be very satisfactory if we could bring this
volume to a conclusion by providing a full and complete answer.
Unfortunately this is impossible. In years to come the tireless labours
of scientific investigators may lead to a solution of the problem; but,
as Professor Fleming puts it: “The question--What is electricity?--no
more admits of a complete and final answer to-day than does the
question--What is life?”
From the earliest days of electrical science theories of electricity
have been put forward. The gradual extension and development of these
theories, and the constant substitution of one idea for another as
experimental data increased, provide a fascinating subject for study.
To cover this ground however, even in outline, would necessitate many
chapters, and so it will be better to consider only the theory which,
with certain reservations in some cases, is held by the scientific
world of to-day. This is known as the _electron_ theory of electricity.
We have referred already, in Chapter XXIV., to atoms and electrons.
All matter is believed to be constituted of minute particles called
“atoms.” These atoms are so extremely small that they are quite
invisible, being far beyond the range of the most powerful microscope;
and their diameter has been estimated at somewhere about one millionth
of a millimetre. Up to a few years ago the atom was believed to be
quite indivisible, but it has been proved beyond doubt that this is
not the case. An atom may be said to consist of two parts, one much
larger than the other. The smaller part is negatively electrified,
and is the same in all atoms; while the larger part is positively
electrified, and varies according to the nature of the atom. The small
negatively electrified portion of the atom consists of particles called
“electrons,” and these electrons are believed to be indivisible units
or atoms of negative electricity. To quote Professor Fleming: “An atom
of matter in its neutral condition has been assumed to consist of an
outer shell or envelope of negative electrons associated with some
core or matrix which has an opposite electrical quality, such that if
an electron is withdrawn from the atom the latter is left positively
electrified.”
The electrons in an atom are not fixed, but move with great velocity,
in definite orbits. They repel one another, and are constantly
endeavouring to fly away from the atom, but they are held in by the
attraction of the positive core. So long as nothing occurs to upset
the constitution of the atom, a state of equilibrium is maintained
and the atom is electrically neutral; but immediately the atom is
broken up by the action of an external force of some kind, one or more
electrons break their bonds and fly away to join some other atom. An
atom which has lost some of its electrons is no longer neutral, but is
electro-positive; and similarly, an atom which has gained additional
electrons is electro-negative. Electrons, or atoms of negative
electricity, can be isolated from atoms of matter, as in the case of
the stream of electrons proceeding from the cathode of a vacuum tube.
So far, however, it has been found impossible to isolate corresponding
atoms of positive electricity.
From these facts it appears that we must regard a positively charged
body as possessing a deficiency of electrons, and a negatively charged
body as possessing an excess of electrons. In Chapter I. we spoke of
the electrification of sealing-wax or glass rods by friction, and we
saw that according to the nature of the substance used as the rubber,
the rods were either positively or negatively electrified. Apparently,
when we rub a glass rod with a piece of silk, the surface atoms of
each substance are disturbed, and a certain number of electrons leave
the glass atoms, and join the silk atoms. The surface atoms of the
glass, previously neutral, are now electro-positive through the loss
of electrons; and the surface atoms of the silk, also previously
neutral, are now electro-negative through the additional electrons
received from the glass atoms. As the result we find the glass to
be positively, and silk to be negatively electrified. On the other
hand, if we rub the glass with fur, a similar atomic disturbance and
consequent migration of electrons takes place, but this time the glass
receives electrons instead of parting with them. In this case the glass
becomes negatively, and the fur positively electrified. The question
now arises, why is the movement of the electrons away from the glass in
the first instance, and toward it in the second? To understand this we
may make use of a simple analogy. If we place in contact two bodies,
one hot and the other cold, the hot body gives up some of its heat to
the cold body; but if we place in contact with the hot body another
body which is still hotter, then the hot body receives heat instead of
parting with it. In a somewhat similar manner an atom is able to give
some of its electrons to another atom which, in comparison with it,
is deficient in electrons; and at the same time it is able to receive
electrons from another atom which, compared with it, has an excess of
electrons. Thus we may assume that the glass atoms have an excess of
electrons as compared with silk atoms, and a deficiency in electrons
as compared with fur atoms.
A current of electricity is believed to be nothing more or less
than a stream of electrons, set in motion by the application of
an electro-motive force. We have seen that some substances are
good conductors of electricity, while others are bad conductors or
non-conductors. In order to produce an electric current, that is a
current of electrons, it is evidently necessary that the electrons
should be free to move. In good conductors, which are mostly metals,
it is believed that the electrons are able to move from atom to atom
without much hindrance, while in a non-conductor their movements are
hampered to such an extent that inter-atomic exchange of electrons is
almost impossible. Speaking on this point, Professor Fleming says:
“There may be (in a good conductor) a constant decomposition and
recomposition of atoms taking place, and any given electron so to speak
flits about, now forming part of one atom and now of another, and anon
enjoying a free existence. It resembles a person visiting from house
to house, forming a unit in different households, and, in between,
being a solitary person in the street. In non-conductors, on the other
hand, the electrons are much restricted in their movements, and can be
displaced a little way but are pulled back again when released.”
Let us try to see now how an electric current is set up in a simple
voltaic cell, consisting of a zinc plate and a copper plate immersed in
dilute acid. First we must understand the meaning of the word _ion_.
If we place a small quantity of salt in a vessel containing water, the
salt dissolves, and the water becomes salt, not only at the bottom
where the salt was placed, but throughout the whole vessel. This means
that the particles of salt must be able to move through the water.
Salt is a chemical compound of sodium and chlorine, and its molecules
consist of atoms of both these substances. It is supposed that each
salt molecule breaks up into two parts, one part being a sodium atom,
and the other a chlorine atom; and further, that the sodium atom loses
an electron, while the chlorine atom gains one. These atoms have the
power of travelling about through the solution, and they are called
_ions_, which means “wanderers.” An ordinary atom is unable to wander
about in this way, but it gains travelling power as soon as it is
converted into an ion, by losing electrons if it be an atom of a metal,
and by gaining electrons if it be an atom of a non-metal.
Returning to the voltaic cell, we may imagine that the atoms of the
zinc which are immersed in the acid are trying to turn themselves into
ions, so that they can travel through the solution. In order to do
this each atom parts with two electrons, and these electrons try to
attach themselves to the next atom. This atom however already has two
electrons, and so in order to accept the newcomers it must pass on
its own two. In this way electrons are passed on from atom to atom of
the zinc, then along the connecting wire, and so to the copper plate.
The atoms of zinc which have lost their electrons thus become ions,
with power of movement. They leave the zinc plate immediately, and so
the plate wastes away or dissolves. So we get a constant stream of
electrons travelling along the wire connecting the two plates, and this
constitutes an electric current.
The electron theory gives us also a clear conception of magnetism. An
electric current flowing along a wire produces magnetic effects; that
is, it sets up a field of magnetic force. Such a current is a stream
of electrons, and therefore we conclude that a magnetic field is
produced by electrons in motion. This being so, we are led to suppose
that there must be a stream of electrons in a steel magnet, and this
stream must be constant because the magnetic field of such a magnet is
permanent. The electron stream in a permanent magnet however is not
quite the same as the electron stream in a wire conveying a current. We
have stated that the electrons constituting an atom move in definite
orbits, so that we may picture them travelling round the core of the
atom as the planets travel round the Sun. This movement is continuous
in every atom of every substance. Apparently we have here the necessary
conditions for the production of a magnetic field, that is, a constant
stream of electrons; but one important thing is still lacking. In an
unmagnetized piece of steel the atoms are not arranged symmetrically,
so that the orbits of their electrons lie some in one plane and some
in another. Consequently, although the electron stream of each atom
undoubtedly produces an infinitesimally small magnetic field, no
magnetic effect that we can detect is produced, because the different
streams are not working in unison and adding together their forces.
In fact they are upsetting and neutralizing each other’s efforts. By
stroking the piece of steel with a magnet, or by surrounding it by a
coil of wire conveying a current, the atoms are turned so that their
electron orbits all lie in the same plane. The electron streams now all
work in unison, their magnetic effects are added together, and we get
a strong magnetic field as the result of their combined efforts. Any
piece of steel or iron may be regarded as a potential magnet, requiring
only a rearrangement of its atoms in order to become an active magnet.
In Chapter VI. it was stated that other substances besides iron and
steel show magnetic effects, and this is what we should expect, as the
electron movement is common to all atoms. None of these substances is
equal to iron and steel in magnetic power, but why this is so is not
understood.
This brings us to the production of an electric current by the dynamo.
Here we have a coil of wire moving across a magnetic field, alternately
passing into this field and out of it. A magnetic field is produced,
as we have just seen, by the steady movement of electrons, and we may
picture it as being a region of the ether disturbed or strained by the
effect of the moving electrons. When the coil of wire passes into the
magnetic field, the electrons of its atoms are influenced powerfully
and set in motion in one direction, so producing a current in the
coil. As the coil passes away from the field, its electrons receive a
second impetus, which checks their movement and starts them travelling
in the opposite direction, and another current is produced. The coil
moves continuously and regularly, passing into and out of the magnetic
field without interruption; and so we get a current which reverses its
direction at regular intervals, that is, an alternating current. This
current may be made continuous if desired, as explained in Chapter IX.
Such, stated briefly and in outline, is the electron theory of
electricity. It opens up possibilities of the most fascinating nature;
it gives us a wonderfully clear conception of what might be called the
inner mechanism of electricity; and it even introduces us to the very
atoms of electricity. Beyond this, at present, it cannot take us, and
the actual nature of electricity itself remains an enigma.
INDEX
Accumulators, 38, 90.
Alarms, electric, 120.
Alternating currents, 71, 75.
Amber, discovery of, 2.
Ampère, 33.
Arc lamp, 93.
Armature, 68.
Atlantic cable, 145.
Atom, 287.
Aurora borealis, 25.
Automatic telephone exchange, 165.
Aviation and “wireless,” 280.
Bachelet “flying” train, 271.
Bastian heater, the, 110.
Battery, voltaic, 33.
Bell telephone, the, 156.
Bells and alarms, electric, 116.
Blasting, 256.
Bunsen cell, 223.
Cable-laying, 150.
Cables, telegraph, 144.
Cell, voltaic, 29.
Clocks, electric, 124.
Coherer, the, 183.
Commutator, 70.
Compass, magnetic, 52.
Condenser, 63.
Conductors, 6.
Conduit system, 83.
Convectors, 109.
Cookers, electric, 110.
Creed telegraph, 137.
Crookes, Sir W., 230.
Current, electric, 30.
Daniell cell, 31, 223.
Davy, Sir Humphry, 93.
Detector, in wireless telegraphy, 188, 198.
Diamond-making, 113.
Duplex telegraphy, 139.
Dussaud cold light, 106.
Dynamo, 66.
Edison, Thomas A., 103.
Electric cookers, 110.
Electric heating, 109.
Electric motor, 66.
Electric lighting, 70, 75, 93.
Electricity, early discoveries, 1;
nature of, 287.
Electro-culture, 258.
Electrolysis, 224.
Electro-magnets, 58.
Electron, 287.
Electroplating, 213.
Electrophorus, the, 11.
Electrotyping, 213.
Faraday, 66.
Finsen light treatment, 243.
Franklin, Benjamin, 19.
Frictional electricity, 2.
Furnace, electric, 111.
Galvani, 27.
Galvanometer, 59.
Glass, 4.
Goldschmidt system, 197.
_Great Eastern_, the, 148.
Half-watt lamp, 105.
Heating by electricity, 109.
Hughes printing telegraph, 136.
Iceberg detector, 267.
Ignition, electric, 253.
Incandescent lamps, 103.
Induction, 9.
Induction coil, 61.
Ion, 291.
Kelvin, Lord, 152.
Korn’s photo-telegraph, 174.
Lamps, electric, 93.
Leclanché cell, 32, 116.
Lemström’s experiments in electro-culture, 258.
Lepel system, 196.
Leyden jar, 15, 181.
Light, 23.
Lighting, electric, 75, 93.
Lightning, 1, 19, 23.
Lightning conductors, 25.
Lindsay, wireless experiments, 180.
Lodge, Sir Oliver, 260.
Machines for producing static electricity, 9.
Magnetic poles, 50.
Magnetism, 44, 56, 291.
Marconi, 186, 195.
Medicine, electricity in, 241.
Mercury-vapour lamp, 99.
Microphone, 159.
Mines, submarine, 283.
Mines, telephones in, 169.
Mono-railway, 89.
Morse, telegraph, 130;
experiments in wireless telegraphy, 180.
Motor, electric, 66.
Motor-car, electric, 91.
Navy, use of wireless, 274;
of electricity, 282.
Negative electricity, 5.
Neon lamps, 102.
Non-conductors, 6.
Ohm, 33.
Oil radiator, 110.
Ozone, 23, 247.
Ozone ventilation, 249.
Petrol, motor, ignition in, 253.
Photographophone, the, 173.
Pile, voltaic, 28.
Pipe locator, 266.
Plant culture, electric, 258.
Polarization, 31.
Pollak-Virag telegraph, 137.
Positive electricity, 5.
Poulsen, Waldemar, 171, 197.
Poultry, electro-culture of, 264.
Power stations, 75.
Preece, wireless experiments, 180.
Primary and secondary coils, 62.
Radiator, 109.
Railways, electric, 87;
use of wireless, 211.
Resistance, 33.
Röntgen rays, 228, 242.
Searchlights, 98.
Ships, use of wireless, 206.
Siphon recorder, the, 252.
Sparking plug, 154.
Static electricity, 7.
Stations, wireless, 204.
Steinheil telegraph, 130.
Submarine telegraphy, 144.
Submarine telephony, 169.
Surface contact system, 83.
Telefunken system, 196.
Telegraph, the, 128, 144, 171, 179, 203.
Telegraphone, 171.
Telephone, the, 154, 171, 179, 201.
Telephone exchange, 160.
Thermopile, 36.
Thermostat, 121.
Thunderstorms, 22, 194.
Trains, electric, 87;
the Bachelet, 271.
Tramways, electric, 78, 83.
Trolley system, 83.
Tubes for X-rays, 233.
Tuning in wireless telegraphy, 191.
Tungsten lamps, 104.
Volt, 33.
Voltaic electricity, 28, 129, 290.
War, electricity in, 274;
telegraph in, 277.
Water, electrolysis of, 38.
Water-power, 81.
Waves, electric, 181, 191, 199.
Welding, electric, 114.
Welsbach lamp, 103.
Wheatstone and Cooke telegraphs, 130.
Wimshurst machine, 12.
Wireless telegraphy and telephony, 179, 203, 270, 280.
Wires, telegraph, 141.
X-rays, 231, 242.
MORRISON & GIBB LIMITED, EDINBURGH
5/15 2½
Transcriber’s Notes
Punctuation, hyphenation, and spelling were made consistent when a
predominant preference was found in the original book; otherwise they
were not changed.
Simple typographical errors were corrected; unbalanced quotation
marks were remedied when the change was obvious, and otherwise left
unbalanced.
Illustrations in this eBook have been positioned between paragraphs and
outside quotations. In versions of this eBook that support hyperlinks,
the page references in the List of Plates lead to the corresponding
illustrations. (There is no list of the other illustrations.)
Plate VIII., “Typical Electric Locomotives,” listed as being on page
90, was not in the original book and therefore not in this ebook.
The index was not checked for proper alphabetization or correct page
references.
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