Electricity

by W. H. McCormick

A “Flying Train”

Considerable interest was aroused last year by a model of a railway working upon a very remarkable system. This was the invention of Mr. Emile Bachelet, and the model was brought to London from the United States. The main principle upon which the system is based is interesting. About 1884, Professor Elihu Thompson, a famous American scientist, made the discovery that a plate of copper could be attracted or repelled by an electro-magnet. The effects took place at the moment when the magnetism was varied by suddenly switching the current on or off; the copper being repelled when the current was switched on, and attracted when it was switched off. Copper is a non-magnetic substance, and the attraction and repulsion are not ordinary magnetic effects, but are due to currents induced in the copper plate at the instant of producing or destroying the magnetism. The plate is attracted or repelled according to whether these induced currents flow in the same direction as, or in the opposite direction to, the current in the magnet coil. Brass and aluminium plates act in the same way as the copper plate, and the effects are produced equally well by exciting the magnet with alternating current, which, by changing its direction, changes the magnetism also. Of the two effects, the repulsion is much the stronger, especially if the variations in the magnetism take place very rapidly; and if a powerful and rapidly alternating current is used, the plate is repelled so strongly that it remains supported in mid-air above the magnet.

This repulsive effect is utilized in the Bachelet system ([Plate XV].). There are no rails in the ordinary sense, and the track is made up of a continuous series of electro-magnets. The car, which is shaped something like a cigar, has a floor of aluminium, and contains an iron cylinder, and it runs above the line of magnets. Along each side of the track is a channel guide rail, and underneath the car at each end are fixed two brushes with guide pieces, which run in the guide rails. Above the car is a third guide rail, and two brushes with guide pieces fixed on the top of the car, one at each end, run in this overhead rail. These guide rails keep the car in position, and also act as conductors for the current. The repulsive action of the electro-magnets upon the aluminium floor raises the car clear of the track, and keeps it suspended; and while remaining in this mid-air position it is driven, or rather pulled forward, by powerful solenoids, which are supplied with continuous current. We have referred previously to the way in which a solenoid draws into it a core of iron. When the car enters a solenoid, the latter exerts a pulling influence upon the iron cylinder inside the car, and so the car is given a forward movement. This is sufficient to carry it along to the next solenoid, which gives it another pull, and so the car is drawn forward from one solenoid to another to the end of the line. The model referred to has only a short track of about 30 feet, with one solenoid at each end; but its working shows that the pulling power of the solenoids is sufficient to propel the car.

PLATE XV.

Photo by

Record Press.

BACHELET “FLYING TRAIN” AND ITS INVENTOR.

To avoid the necessity of keeping the whole of the electro-magnets energized all the time, these are arranged in sections, which are energized separately. By means of the lower set of brushes working in the track guides, each of these sections has alternating current supplied to it as the car approaches, and switched off from it when the car has passed. The brushes working in the overhead guide supply continuous current to each solenoid as the car enters it, and switch off the current when the car has passed through. The speed at which the model car travels is quite extraordinary, and the inventor believes that in actual practice speeds of more than 300 miles an hour are attainable on his system.

CHAPTER XXX
ELECTRICITY IN WAR

One of the most striking features of modern naval warfare is the absolute revolution in methods of communication brought about by wireless telegraphy. To-day every warship has its wireless installation. Our cruiser squadrons and destroyer flotillas, ceaselessly patrolling the waters of the North Sea, are always in touch with the Admiral of the Fleet, and with the Admiralty at Whitehall. In the Atlantic, and in the Pacific too, our cruisers, whether engaged in hunting down the marauding cruisers of the enemy or in searching for merchant ships laden with contraband, have their comings and goings directed by wireless. Even before the actual declaration of war between Great Britain and Germany wireless telegraphy began its work. At the conclusion of the great naval review of July 1914, the Fleet left Portland to disperse as customary for manœuvre leave, but a wireless message was dispatched ordering the Fleet not to disperse. As no state of war then existed, this was a precautionary measure, but subsequent events quickly proved how urgently necessary it had been to keep the Fleet in battle array. Immediately war was declared Great Britain was able to put into the North Sea a fleet which hopelessly outnumbered and outclassed the German battle fleet.

At the outset Germany had a number of cruisers in the Atlantic and the Pacific Oceans. Owing to the vigilance of our warships these vessels were unable to join the German Home Fleet, and they immediately adopted the rôle of commerce destroyers. In this work they made extensive use of wireless telegraphy to ascertain the whereabouts of British merchant ships, and for a short time they played quite a merry game. Prominent among these raiders was the Emden. It was really astonishing how this cruiser obtained information regarding the sailings of British ships. It is said that on one occasion she called up by wireless a merchant ship, and inquired if the latter had seen anything of a German cruiser. The unsuspecting merchantman replied that there was no such thing as a German warship in the vicinity. “Oh yes, there is,” returned the Emden; “I’m it!” and shortly afterwards she appeared on the horizon, to the great discomfiture of the British skipper. An interesting account of the escape of a British liner from another notorious raider, the Karlsruhe, has been given in the Nautical Magazine. The writer says:

“I have just returned home after a voyage to South America in one of the Pacific Steam Navigation Company’s cargo boats. When we left Montevideo we heard that France and Germany were at war, and that there was every possibility of Great Britain sending an ultimatum to Germany. We saw several steamers after leaving the port, but could get no information, as few of them were fitted with wireless and passed at some distance off. When about 200 miles east of Rio, our wireless operator overheard some conversation between the German cruiser Karlsruhe and a German merchant ship at anchor in Rio. It was clearly evident that the German merchant ship had no special code, as the conversation was carried on in plain German language, and our operator, who, by the way, was master of several languages, was able to interpret these messages without the slightest difficulty. It was then that we learned that Great Britain was at war. The German cruiser was inquiring from the German merchant ship what British vessels were leaving Rio, and asking for any information which might be of use. We also picked up some news of German victories in Belgium, which were given out by the German merchant ship. It was clearly evident that the Karlsruhe had information about our ship, and expected us to be in the position she anticipated, for she sent out a signal to us in English, asking us for our latitude and longitude. This our operator, under the instructions of the captain, declined to give. The German operator evidently got furious, as he called us an English ‘swine-hound,’ and said, ‘This is a German warship, Karlsruhe; we will you find.’ Undoubtedly he thought he was going to strike terror to our hearts, but he made a mistake.

“That night we steamed along without lights, and we knew from the sound of the wireless signals that were being flashed out from the German ship that we were getting nearer and nearer to her. Fortunately for us, about midnight a thick misty rain set in and we passed the German steamer, and so escaped. Our operator said that we could not have been more than 8 or 10 miles away when we passed abeam. Undoubtedly our wireless on this occasion saved us from the danger from which we escaped.”

Apparently little is known of the end of the Karlsruhe, but the Emden met with the fate she richly deserved; and fittingly enough, wireless telegraphy, which had enabled her to carry out her marauding exploits, was the means of bringing her to her doom. On 9th November 1914 the Emden anchored off the Cocos-Keeling Islands, a group of coral islets in the Indian Ocean, and landed a party of three officers and forty men to cut the cable and destroy the wireless station. Before the Germans could get to the station, a wireless message was sent out stating the presence of the enemy warship, and this call was received by the Australian cruisers Melbourne and Sydney. These vessels, which were then only some 50 miles away, were engaged, along with a Japanese cruiser, in escorting transports. The Sydney at once went off at full speed, caught the Emden, and sent her to the bottom after a short but sharp engagement. As the Emden fled at sight of the Australian warship, the landing party had not time to get aboard, and consequently were left behind. They seized an old schooner, provisioned her, and set sail, but what became of them is not known.

In land warfare field telegraphs play a very important part; indeed it is certain that without them the vast military operations of the present war could not be carried on. The General Headquarters of our army in France is in telegraphic communication not only with neighbouring French towns, but also with Paris and London. From Headquarters also run wires to every point of the firing-line, so that the Headquarters Staff, and through them the War Office in London, know exactly what is taking place along the whole front. The following extract from a letter from an officer, published by The Times, gives a remarkably good idea of the work of the signal companies of the Royal Engineers.

“As the tide of battle turns this way and the other, and headquarters are constantly moving, some means have to be provided to keep in constant touch with General Headquarters during the movement. This emergency is met by cable detachments. Each detachment consists of two cable waggons, which usually work in conjunction with one another, one section laying the line whilst the other remains behind to reel up when the line is finished with. A division is ordered to move quickly to a more tactical position. The end of the cable is connected with the permanent line, which communicates to Army Headquarters, and the cable detachment moves off at the trot; across country, along roads, through villages, and past columns of troops, the white and blue badge of the signal service clears the way. Behind the waggon rides a horseman, who deftly lays the cable in the ditches and hedges out of danger from heavy transport and the feet of tramping infantry, with the aid of a crookstick. Other horsemen are in the rear tying back and making the line safe. On the box of the waggon sits a telegraphist, who is constantly in touch with headquarters as the cable runs swiftly out. An orderly dashes up with an important message; the waggon is stopped, the message dispatched, and on they go again.”

Wireless telegraphy too has its part to play in land war, and for field purposes it has certain advantages over telegraphy with wires. Ordinary telegraphic communication is liable to be interrupted by the cutting of the wire by the enemy, or, in spite of every care in laying, by the breaking of the wire by passing cavalry or artillery. No such trouble can occur with wireless telegraphy, and if it becomes necessary to move a wireless station with great rapidity, as for instance on an unexpected advance of the enemy, it is an advantage to have no wire to bother about. The Marconi portable wireless sets for military purposes are marvels of compactness and lightness, combined with simplicity. They are of two kinds, pack-saddle sets and cart sets. The former weigh about 360 lb., this being divided amongst four horses. They can be set up in ten minutes by five or six men, and require only two men to work them. Their guaranteed range is 40 miles, but they are capable of transmitting twice this distance or even more under favourable conditions. The cart sets can be set up in twenty minutes by seven or eight men, and they have a guaranteed range of from 150 to 200 miles.

It is obviously very important that wireless military messages should not be intercepted and read by the enemy, and the method of avoiding danger of this kind adopted with the Marconi field stations is ingenious and effective. The transmitter and the receiver are arranged to work on three different fixed wave-lengths, the change from one to another being effected quickly by the movement of a three-position switch. By this means the transmitting operator sends three or four words on one wave-length, then changes to another, transmits a few words on this, changes the wave-length again, and so on. Each change is accompanied by the sending of a code letter which informs the receiving operator to which wave-length the transmitter is passing. The receiving operator adjusts his switch accordingly, and so he hears the whole message without interruption, the change from one wave-length to another taking only a small fraction of a second. An enemy operator might manage to adjust his wave-length so as to hear two or three words, but the sudden change of wave-length would throw him out of tune, and by the time he had found the new wave-length this would have changed again. Thus he would hear at most only a few disconnected words at intervals, and he would not be able to make head or tail of the message. To provide against the possibility of the three wave-lengths being measured and prepared for, these fixed lengths themselves can be changed, if necessary, many times a day, so that the enemy operators would never know beforehand which three were to be used.

Wireless telegraphy was systematically employed in land warfare for the first time in the Balkan War, during which it proved most useful both to the Allies and to the Turks. One of the most interesting features of the war was the way in which wireless communication was kept up between the beleaguered city of Adrianople and the Turkish capital. Some time before war broke out the Turkish Government sent a portable Marconi wireless set to Adrianople, and this was set up at a little distance from the city. When war was declared the apparatus was brought inside the city walls and erected upon a small hill. Then came the siege. For 153 days Shukri Pasha kept the Turkish flag flying, but the stubborn defence was broken down in the end through hunger and disease. All through these weary days the little wireless set did its duty unfalteringly, and by its aid regular communication was maintained with the Government station at Ok Meidan, just outside Constantinople, 130 miles away. Altogether about half a million words were transmitted from Adrianople to the Turkish capital.

PLATE XVI.

(a) CAVALRY PORTABLE WIRELESS CART SET.

By permission of

Marconi Co. Ltd.

(b) AEROPLANE FITTED WITH WIRELESS TELEGRAPHY.

The rapid development of aviation during the past few years has drawn attention to the necessity for some means of communication between the land and airships and aeroplanes in flight. At first sight it might appear that wireless telegraphy could be used for this purpose without any trouble, but experience has shown that there are certain difficulties in the way, especially with regard to aeroplanes. The chief difficulty with aeroplanes lies in the aerial. This must take the form either of a long trailing wire or of fixed wires running between the planes and the tail. A trailing wire is open to the objection that it is liable to get mixed up with the propeller, besides which it appears likely to hamper to some slight extent the movements of a small and light machine. A fixed aerial between planes and tail avoids these difficulties, but on the other hand its wave-length is bound to be inconveniently small. The heavy and powerful British military aeroplanes apparently use a trailing wire of moderate length, carried in a special manner so as to clear the propeller, but few details are available at present. A further trouble with aeroplanes lies in the tremendous noise made by the engine, which frequently makes it quite impossible to hear incoming signals; and the only way of getting over this difficulty appears to be for the operator to wear some sort of sound-proof head-gear. Signals have been transmitted from an aeroplane in flight up to distances of 40 or 50 miles quite successfully, but the reception of signals by aeroplanes is not so satisfactory, except for comparatively short distances. Although few particulars have been published regarding the work of the British aeroplanes in France, it seems evident that wireless telegraphy is in regular use. In addition to their value as scouts, our aeroplanes appear to be extremely useful for the direction of heavy artillery fire, using wireless to tell the gunners where each shell falls, until the exact range is obtained. In the case of airships the problem of wireless communication is much simpler. A trailing wire presents no difficulties, and on account of their great size much more powerful sets of apparatus can be carried. The huge German Zeppelin airships have a long freely-floating aerial consisting of a wire which can be wound in or let out as required, its full length being about 750 feet. The total weight of the apparatus is nearly 300 lb., and the transmitting range is said to be from about 120 to 200 miles.

Electricity is used in the navy for a great variety of purposes besides telegraphy. Our battleships are lighted by electricity, which is generated at a standard pressure of 220 volts. This current is transformed down for the searchlights, and also for the intricate systems of telephone, alarm, and firing circuits. The magazines containing the deadly cordite are maintained at a constant temperature of 70° F. by special refrigerating machinery driven by electricity, and the numerous fans for ventilating the different parts of the ship are also electrically driven. Electric power is used for capstans, coaling winches, sounding machines, lifts, pumps, whether for drainage, fire extinction, or raising fresh water from the tanks, and for the mechanism for operating boats and torpedo nets. The mechanism for manipulating the great guns and their ammunition is hydraulic. Electricity was tried for this purpose on the battle cruiser Invincible, but was abandoned in favour of hydraulic power. But though electricity is apparently out of favour in this department, it takes an extremely important share in the work of controlling and firing the guns; its duties being such as could not be carried out by hydraulic power.

The guns are controlled and fired from what is known as the fire-control room, which is situated in the interior of the ship, quite away from the guns themselves. The range-finder, from his perch up in the gigantic mast, watches an enemy warship as she looms on the horizon, and when she comes within range he estimates her distance by means of instruments of wonderful precision. He then telephones to the fire-control room, giving this distance, and also the enemy’s speed and course. The officer in charge of the fire-control room calculates the elevation of the gun required for this distance, and decides upon the instant at which the gun must be fired. A telephoned order goes to the gun-turret, and the guns are brought to bear upon the enemy, laid at the required elevation, and sighted. At the correct instant the fire-control officer switches on an electric current to the gun, which fires a small quantity of highly explosive material, and this in turn fires the main charge of cordite. The effect of the shell is watched intently from the fire-control top, up above the range-finder, and if, as is very likely, this first shell falls short of, or overshoots the mark, an estimate of the amount of error is communicated to the fire-control room. Due corrections are then made, the gun is laid at a slightly different elevation, and this time the shell finds its mark with unerring accuracy.

The range of movement, horizontal and vertical, of modern naval guns is so great that it is possible for two guns to be in such relative positions that the firing of one would damage the other. To guard against a disaster of this kind fixed stops are used, supplemented by ingenious automatic alarms. The alarm begins to sound as soon as any gun passes into a position in which it could damage another gun, and it goes on sounding until the latter gun is moved out of the danger line.

Since the outbreak of war the subject of submarine mines has been brought to our notice in very forcible fashion. Contrary to the general impression, the explosive submarine mine is not a recent introduction. It is difficult to say exactly when mines were first brought into use, but at any rate we know that they were employed by Russia during the Crimean War, apparently with little success. The first really successful use of mines occurred in the American Civil War, when the Confederates sank a number of vessels by means of them. This practical demonstration of their possibilities did not pass unnoticed by European nations, and in the Franco-German War we find that mines were used for harbour defence by both belligerents. It is doubtful whether either nation derived much benefit from its mines, and indeed as the war progressed Germany found that the principal result of her mining operations was to render her harbours difficult and dangerous to her own shipping. Much greater success attended the use of mines in the Russo-Japanese War, but all previous records shrink into insignificance when compared with the destruction wrought by mines in the present great conflict.

Submarine mines may be divided into two classes; those for harbour defence, and those for use in the open sea. Harbour defence mines are almost invariably electrically controlled; that is, they are connected with the shore by means of a cable, and fired by an electric impulse sent along that cable. In one system of control the moment of firing is determined entirely by observers on shore, who, aided by special optical instruments, are able to tell exactly when a vessel is above any particular mine. The actual firing is carried out by depressing a key which completes an electric circuit, thus sending a current along the cable to actuate the exploding mechanism inside the mine. A hostile ship therefore would be blown up on arriving at the critical position, while a friendly vessel would be allowed to pass on in safety. In this system of control there is no contact between the vessel and the mine, the latter being well submerged or resting on the sea floor, so that the harbour is not obstructed in any way. This is a great advantage, but against it must be set possible failure of the defence at a critical moment owing to thick weather, which of course interferes seriously with the careful observation of the mine field necessary for accurate timing of the explosions. This difficulty may be surmounted by a contact system of firing. In this case the mines are placed so near the surface as to make contact with vessels passing over them. The observers on shore are informed of the contact by means of an electric impulse automatically transmitted along the cable, so that they are independent of continuous visual observation of the mined area. As in the previous system, the observers give the actual firing impulse. The drawback to this method is the necessity for special pilotage arrangements for friendly ships in order to avoid unnecessary striking of the mines, which are liable to have their mechanism deranged by constant blows. If the harbour or channel can be closed entirely to friendly shipping, the observers may be dispensed with, their place being taken by automatic electric apparatus which fires at once any mine struck by a vessel.

Shore-controlled mines are excellent for harbour defence, and a carefully distributed mine-field, backed by heavy fort guns, presents to hostile vessels a barrier which may be regarded as almost impenetrable. A strong fleet might conceivably force its way through, but in so doing it would sustain tremendous losses; and as these losses would be quite out of proportion to any probable gains, such an attempt is not likely to be made except as a last resort.

For use in the open sea a different type of mine is required. This must be quite self-contained and automatic in action, exploding when struck by a passing vessel. The exploding mechanism may take different forms. The blow given by a ship may be made to withdraw a pin, thus releasing a sort of plunger, which, actuated by a powerful spring, detonates the charge. A similar result is obtained by the use of a suspended weight, in place of plunger and spring. Still another form of mine is fired electrically by means of a battery, the circuit of which is closed automatically by the percussion. Deep-sea mines may be anchored or floating free. Free mines are particularly dangerous on account of the impossibility of knowing where they may be at any given moment. They are liable to drift for considerable distances, and to pass into neutral seas; and to safeguard neutral shipping international rules require them to have some sort of clockwork mechanism which renders them harmless after a period of one hour. It is quite certain that some, at least, of the German free mines have no such mechanism, so that neutral shipping is greatly endangered.

Submarine mines are known as ground mines, or buoyant mines, according to whether they rest on the sea bottom or float below the surface. Ground mines are generally made in the form of a cylinder, buoyant mines being usually spherical. The cases are made of steel, and buoyancy is given when required by enclosing air spaces. Open-sea mines are laid by special vessels, mostly old cruisers. The stern of these ships is partly cut away, and the mines are run along rails to the stern, and so overboard. The explosive employed is generally gun-cotton, fired by a detonator, charges up to 500 lb. or more being used, according to the depth of submersion and the horizontal distance at which the mine is desired to be effective. Ground mines can be used only in shallow water, and even then they require a heavier charge than mines floating near the surface. Mines must not be laid too close together, as the explosion of one might damage others. The distance apart at which they are placed depends upon the amount of charge, 500-lb. mines requiring to be about 300 feet apart for safety.

CHAPTER XXXI
WHAT IS ELECTRICITY?

The question which heads this, our final chapter, is one which must occur to every one who takes even the most casual interest in matters scientific, and it would be very satisfactory if we could bring this volume to a conclusion by providing a full and complete answer. Unfortunately this is impossible. In years to come the tireless labours of scientific investigators may lead to a solution of the problem; but, as Professor Fleming puts it: “The question—What is electricity?—no more admits of a complete and final answer to-day than does the question—What is life?”

From the earliest days of electrical science theories of electricity have been put forward. The gradual extension and development of these theories, and the constant substitution of one idea for another as experimental data increased, provide a fascinating subject for study. To cover this ground however, even in outline, would necessitate many chapters, and so it will be better to consider only the theory which, with certain reservations in some cases, is held by the scientific world of to-day. This is known as the electron theory of electricity.

We have referred already, in [Chapter XXIV]., to atoms and electrons. All matter is believed to be constituted of minute particles called “atoms.” These atoms are so extremely small that they are quite invisible, being far beyond the range of the most powerful microscope; and their diameter has been estimated at somewhere about one millionth of a millimetre. Up to a few years ago the atom was believed to be quite indivisible, but it has been proved beyond doubt that this is not the case. An atom may be said to consist of two parts, one much larger than the other. The smaller part is negatively electrified, and is the same in all atoms; while the larger part is positively electrified, and varies according to the nature of the atom. The small negatively electrified portion of the atom consists of particles called “electrons,” and these electrons are believed to be indivisible units or atoms of negative electricity. To quote Professor Fleming: “An atom of matter in its neutral condition has been assumed to consist of an outer shell or envelope of negative electrons associated with some core or matrix which has an opposite electrical quality, such that if an electron is withdrawn from the atom the latter is left positively electrified.”

The electrons in an atom are not fixed, but move with great velocity, in definite orbits. They repel one another, and are constantly endeavouring to fly away from the atom, but they are held in by the attraction of the positive core. So long as nothing occurs to upset the constitution of the atom, a state of equilibrium is maintained and the atom is electrically neutral; but immediately the atom is broken up by the action of an external force of some kind, one or more electrons break their bonds and fly away to join some other atom. An atom which has lost some of its electrons is no longer neutral, but is electro-positive; and similarly, an atom which has gained additional electrons is electro-negative. Electrons, or atoms of negative electricity, can be isolated from atoms of matter, as in the case of the stream of electrons proceeding from the cathode of a vacuum tube. So far, however, it has been found impossible to isolate corresponding atoms of positive electricity.

From these facts it appears that we must regard a positively charged body as possessing a deficiency of electrons, and a negatively charged body as possessing an excess of electrons. In [Chapter I]. we spoke of the electrification of sealing-wax or glass rods by friction, and we saw that according to the nature of the substance used as the rubber, the rods were either positively or negatively electrified. Apparently, when we rub a glass rod with a piece of silk, the surface atoms of each substance are disturbed, and a certain number of electrons leave the glass atoms, and join the silk atoms. The surface atoms of the glass, previously neutral, are now electro-positive through the loss of electrons; and the surface atoms of the silk, also previously neutral, are now electro-negative through the additional electrons received from the glass atoms. As the result we find the glass to be positively, and silk to be negatively electrified. On the other hand, if we rub the glass with fur, a similar atomic disturbance and consequent migration of electrons takes place, but this time the glass receives electrons instead of parting with them. In this case the glass becomes negatively, and the fur positively electrified. The question now arises, why is the movement of the electrons away from the glass in the first instance, and toward it in the second? To understand this we may make use of a simple analogy. If we place in contact two bodies, one hot and the other cold, the hot body gives up some of its heat to the cold body; but if we place in contact with the hot body another body which is still hotter, then the hot body receives heat instead of parting with it. In a somewhat similar manner an atom is able to give some of its electrons to another atom which, in comparison with it, is deficient in electrons; and at the same time it is able to receive electrons from another atom which, compared with it, has an excess of electrons. Thus we may assume that the glass atoms have an excess of electrons as compared with silk atoms, and a deficiency in electrons as compared with fur atoms.

A current of electricity is believed to be nothing more or less than a stream of electrons, set in motion by the application of an electro-motive force. We have seen that some substances are good conductors of electricity, while others are bad conductors or non-conductors. In order to produce an electric current, that is a current of electrons, it is evidently necessary that the electrons should be free to move. In good conductors, which are mostly metals, it is believed that the electrons are able to move from atom to atom without much hindrance, while in a non-conductor their movements are hampered to such an extent that inter-atomic exchange of electrons is almost impossible. Speaking on this point, Professor Fleming says: “There may be (in a good conductor) a constant decomposition and recomposition of atoms taking place, and any given electron so to speak flits about, now forming part of one atom and now of another, and anon enjoying a free existence. It resembles a person visiting from house to house, forming a unit in different households, and, in between, being a solitary person in the street. In non-conductors, on the other hand, the electrons are much restricted in their movements, and can be displaced a little way but are pulled back again when released.”

Let us try to see now how an electric current is set up in a simple voltaic cell, consisting of a zinc plate and a copper plate immersed in dilute acid. First we must understand the meaning of the word ion. If we place a small quantity of salt in a vessel containing water, the salt dissolves, and the water becomes salt, not only at the bottom where the salt was placed, but throughout the whole vessel. This means that the particles of salt must be able to move through the water. Salt is a chemical compound of sodium and chlorine, and its molecules consist of atoms of both these substances. It is supposed that each salt molecule breaks up into two parts, one part being a sodium atom, and the other a chlorine atom; and further, that the sodium atom loses an electron, while the chlorine atom gains one. These atoms have the power of travelling about through the solution, and they are called ions, which means “wanderers.” An ordinary atom is unable to wander about in this way, but it gains travelling power as soon as it is converted into an ion, by losing electrons if it be an atom of a metal, and by gaining electrons if it be an atom of a non-metal.

Returning to the voltaic cell, we may imagine that the atoms of the zinc which are immersed in the acid are trying to turn themselves into ions, so that they can travel through the solution. In order to do this each atom parts with two electrons, and these electrons try to attach themselves to the next atom. This atom however already has two electrons, and so in order to accept the newcomers it must pass on its own two. In this way electrons are passed on from atom to atom of the zinc, then along the connecting wire, and so to the copper plate. The atoms of zinc which have lost their electrons thus become ions, with power of movement. They leave the zinc plate immediately, and so the plate wastes away or dissolves. So we get a constant stream of electrons travelling along the wire connecting the two plates, and this constitutes an electric current.

The electron theory gives us also a clear conception of magnetism. An electric current flowing along a wire produces magnetic effects; that is, it sets up a field of magnetic force. Such a current is a stream of electrons, and therefore we conclude that a magnetic field is produced by electrons in motion. This being so, we are led to suppose that there must be a stream of electrons in a steel magnet, and this stream must be constant because the magnetic field of such a magnet is permanent. The electron stream in a permanent magnet however is not quite the same as the electron stream in a wire conveying a current. We have stated that the electrons constituting an atom move in definite orbits, so that we may picture them travelling round the core of the atom as the planets travel round the Sun. This movement is continuous in every atom of every substance. Apparently we have here the necessary conditions for the production of a magnetic field, that is, a constant stream of electrons; but one important thing is still lacking. In an unmagnetized piece of steel the atoms are not arranged symmetrically, so that the orbits of their electrons lie some in one plane and some in another. Consequently, although the electron stream of each atom undoubtedly produces an infinitesimally small magnetic field, no magnetic effect that we can detect is produced, because the different streams are not working in unison and adding together their forces. In fact they are upsetting and neutralizing each other’s efforts. By stroking the piece of steel with a magnet, or by surrounding it by a coil of wire conveying a current, the atoms are turned so that their electron orbits all lie in the same plane. The electron streams now all work in unison, their magnetic effects are added together, and we get a strong magnetic field as the result of their combined efforts. Any piece of steel or iron may be regarded as a potential magnet, requiring only a rearrangement of its atoms in order to become an active magnet. In [Chapter VI]. it was stated that other substances besides iron and steel show magnetic effects, and this is what we should expect, as the electron movement is common to all atoms. None of these substances is equal to iron and steel in magnetic power, but why this is so is not understood.

This brings us to the production of an electric current by the dynamo. Here we have a coil of wire moving across a magnetic field, alternately passing into this field and out of it. A magnetic field is produced, as we have just seen, by the steady movement of electrons, and we may picture it as being a region of the ether disturbed or strained by the effect of the moving electrons. When the coil of wire passes into the magnetic field, the electrons of its atoms are influenced powerfully and set in motion in one direction, so producing a current in the coil. As the coil passes away from the field, its electrons receive a second impetus, which checks their movement and starts them travelling in the opposite direction, and another current is produced. The coil moves continuously and regularly, passing into and out of the magnetic field without interruption; and so we get a current which reverses its direction at regular intervals, that is, an alternating current. This current may be made continuous if desired, as explained in [Chapter IX].

Such, stated briefly and in outline, is the electron theory of electricity. It opens up possibilities of the most fascinating nature; it gives us a wonderfully clear conception of what might be called the inner mechanism of electricity; and it even introduces us to the very atoms of electricity. Beyond this, at present, it cannot take us, and the actual nature of electricity itself remains an enigma.