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“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.

([Larger])

THE MARCONI TRANSATLANTIC WIRELESS STATION AT GLACE BAY, NOVA SCOTIA

Drawing by Irene Sutcliffe

“ROMANCE OF REALITY” SERIES

ELECTRICITY

BY
W. H. McCORMICK

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

LIST OF PLATES

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Fig. 10.—Diagram to illustrate the Seebeck effect.

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.

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].

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.

PLATE II.

(a) EXPERIMENT TO SHOW MAGNETIC INDUCTION.

(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.

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.

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

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.

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.

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].

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.

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.

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.

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.

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.

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.

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.