Fig. 278 represents one of the light-producing machines. The electro-magnets are excited by a portion of the currents they themselves produce, they retaining sufficient residual magnetism to develop the currents. There is a pair of current-collectors on each side. This machine weighs 1,540 lbs., its height is 3 ft., and width 2 ft. It will produce a light having the intensity of 500 Carcel lamps, which may be doubled by increasing the speed. Fig. 279 is another form which is also adapted for illuminating purposes, and, when made with fewer coils, for electrotyping purposes also. There are in this also two sets of current-collectors, and by means of a connecting cylinder (seen at the base of the machine) the currents can be combined for quantity and for tension as may be required. This machine is only about 2 ft. square, and it produces a light equal to 200 burners; but this may be increased, as the following table shows:

The value of M. Gramme’s invention for electro-plating is proved by the fact of its adoption by Messrs. Christofle of Paris, whose electro-plating establishment is one of the largest in the world. This firm has no fewer than fourteen of these machines at work, and each is capable of depositing 74 ozs. of silver per hour. There is little doubt that the electric current will now soon be employed for reducing metals. Thus fine copper, which is worth 3s. or 4s. per lb., may perhaps be obtained at about the cost of ordinary copper; potassium, sodium, and aluminium at less than half their present price; and magnesium, calcium, and other rare metals at prices which will bring them into commercial use. The machine shown in Fig. 280 is intended for electro-plating and for general purposes: it supplies the means of readily and cheaply plating with copper, or with any other metal, such articles as steam pipes, boiler tubes, ship plates, guns, bolts, nails, marine engines, machinery, culinary vessels, cisterns, &c. The advantage of protecting iron or other material from corroding agents is obvious; and as iron coated with copper is available not only for useful, but also for artistic, purposes, as a cheap substitute for bronze, this invention will doubtless lead to a greatly extended application of bronzed iron in buildings and ornamental structures.

The machine well illustrates how mechanical work may be changed into electricity, and electricity caused to do work. The power required to drive the machine at a given speed is much less when no current is being drawn from it, than when the current is flowing. If the current from one machine is sent through the armature of another, the latter revolves, and may be made to do work. Thus power may be conveyed to a distance by electricity, with only the loss caused by the resistance of the conducting wires. If, when two machines are thus connected, the direction of rotation in the first one be suddenly reversed, the armature of the second will almost immediately stop, and then resume its motion in the opposite direction. A very interesting experiment can be performed when the circuit connecting the two machines is made to include a certain length of platinum wire. When both machines are in motion, the platinum exhibits no heating effects; but if the second machine be stopped by an assistant while the rotation of the first is continued, the wire is raised to a red heat. In this way it is shown that motion, electricity, and heat are related to each other, and are mutually convertible; for on the stopping of the second machine, the electricity being no longer used up, so to speak, in producing motion, has its power transformed into heat.

The Gramme machine has also been ingeniously employed for railway brakes on some of the Belgian lines; and it is applicable to telegraphy, where the cost of zinc, acids, batteries, &c., is a considerable item. It is impossible to predict the many applications for manufacturing purposes which will be made of electricity, now a cheap, reliable, and convenient mode has been discovered of producing currents of any required strength. Though by no means the first or only machine by which mechanical force can be converted into dynamical electricity, it shows an immense advance on any former one in the regularity of the action, and in the capability of being driven at a very high rate of speed without the inconvenient accompaniments of the heating of the conductors and destructive sparks at the movable contacts. There can be no doubt of the importance of this machine for use in lighthouses, and for metallurgical and chemical purposes, and the inventor believes the time will come when all large ocean-going vessels will carry an electric light at the masthead. The light would be sufficiently powerful to show rocks or land five or six miles ahead, and an additional safeguard of incalculable value would be thus provided for those “that go down to the sea in ships, that do business in great waters.”

ELECTRIC LIGHTING AND ELECTRIC POWER.

It was mentioned in the last section that the introduction of so convenient and reliable a means of producing electrical currents as the Gramme machine, would cause electricity to be largely applied for illuminating and other purposes. The Gramme machine was first made in 1870, and it attracted much attention, as the principle of combining the currents was quite different from that used in previous magneto-electric machines. In fact, the Gramme machine yielded quite unexpected results, and the principle employed in it opened a new field. The development that has taken place in the applications of electricity within the twenty years since 1870 has been truly marvellous. The electric light appears to have been first used in lighthouses about 1862, and the machines by which the current was produced were, in principle, combinations of a great number of Clarke’s machines (see page 509). One such machine was invented by Mr. Holmes, and was used for the illumination of the South Foreland Lighthouse in 1862. Another similar form of still earlier invention had been set up in Paris as early as 1855,—not, indeed, for the purposes of illumination, but for a project which failed. Its arrangement had been originally suggested by a Belgian physicist in 1849; and the machine of 1855, having received certain improvements, afterwards became very well known by the name of the Alliance Company’s machine, or simply the Alliance machine. It is represented in its improved form in Fig. 280a. Here ranges of steel horse-shoe magnets will be observed, each magnet weighing about 40 lbs. and made of six plates of tempered steel, held together with screws. Each of the eight rows of magnets contains seven, and thus sixteen poles are presented at uniform distances, arranged in circles. Carried on the central axle are six discs, which revolve between the circles of sixteen poles, and on the circumference of each disc are sixteen equidistant bobbins or coils of insulated wire, so that the whole of the sixteen coils are opposite to the sixteen poles at the same moment. The extremities of the wires at the coils are connected with proper adjustments for gathering up the currents, and by means of these the coils may be arranged either for tension or for quantity, like the elements of a battery (page 494).

Wilde’s machine, which has been mentioned in page 511, is shown in fig. 280b. It will be observed that this consists of a small machine, M, with permanent steel magnets, and the current from these circulates through the coils of the electro magnets, A B. The arrangement of the armatures, bobbin, commutators, etc., is the same in both cases. But as a speed of 2,500 revolutions per minute was needed, it was necessary to keep the bearings, T T, from heating by causing cold water to circulate through them. Mr. Ladd arranged a machine on the same principle as Wilde’s, by suppressing the permanent magnets, but availing himself of the residual magnetism of the iron core to bring about the induction. A machine of this kind was shown at the Paris Exhibition of 1867, and people were quite astonished to see electrical power capable of producing a brilliant light developed by a small machine 2 ft. long, 1 ft. wide, and 9 in. high. But the great velocity of rotation, and the consequent heating of the bearings, left much to be desired before a really practical machine could be produced.

In the newest Siemens’ machine, represented in fig. 280c, the Gramme principle is made use of, as the revolving coil is of large diameter, and it consists of a copper cylinder, on which are wound a number of juxtaposed coils like those of a galvanometer. The revolving cylinder is surrounded by the poles of a system of electro-magnets excited by the whole of the induced current being passed through their coils. In a paper describing this machine, Siemens first made use of the term “dynamo-electric machine,” and this expression, contracted to the single word DYNAMO, has since been universally employed to designate machines of this kind. The modifications in the forms and arrangements of the different dynamos that have been invented in late years are endless, and every week patents are granted for further improvements and fresh combinations of the parts. It would be quite beyond the scope of this work to enumerate all the forms of the dynamo that have been favourably spoken of; but we shall content ourselves by adding a drawing of the Brush dynamo (Fig. 280d), which has been so largely used for electric lighting in the United States. In this dynamo we have a Gramme ring, but the number of coils on it is reduced to eight, the intervals being filled up with pieces of iron, and the ring revolves in a vertical plane between the poles of two double oblong electro-magnets, which are arranged with poles of the same name opposite to each other. The commutators shown in the nearer part convert the alternately reversed currents generated in the coils into a direct continuous one. They are formed with bundles of wires, as in the Gramme machine.

But the providing of a cheap and efficient source of current electricity, although an absolutely necessary step, would not have been capable of bringing about the present development of electric lighting, unless the appliances by which the current is made to manifest itself as light had not also been brought nearly to perfection. The conditions required to maintain a steady light from a current of electricity passing between carbon points have been already explained on page 497, and a representation of Dubosc’s electric lantern and regulator is shown. The regulator systems that have been invented since it became obvious that the light of the electric arc admitted of practical application on the large scale are very numerous. The earlier forms of regulator, which were used only for scientific purposes—such as lantern projections on screens, experiments on light, etc.—were complicated in their arrangements and uncertain in their action, for great variations in the light sometimes took place, and occasionally it would, indeed, be extinguished, and then again shine out as brightly as before. Nearly all the regulators that have come into use depend upon movements controlled by electro-magnetic actions produced automatically as the distance between the carbon changes. It would, however, lead us too far into the technicalities of the subject to explain minutely the mechanism of any particular form of the mechanical regulators, and the results depend so often upon the minute details, that it would be difficult to trace the action without a set of large and complete drawings. Perhaps the regulators that have been most used are those of Serrin, Siemens, Brush, Thomson, Houston and Edison. But nearly every inventor has produced different forms of his apparatus; Siemens, for instance, has patented eight or ten regulators. Fig. 280e shows the mechanism of one of the last named inventor’s regulators, in which the two actions required for the separation and approach of the carbons are determined respectively by the vibrations of the rocking lever, M Y L, actuated by the electro-magnet, E, and the simple weight of the upper carbon-holder, A A. When the lamp is not in circuit, the lever, L, is thrown back by a spring, the tension of which is regulated by the screw, R, so that the catch, Q, is disengaged from the wheel, I. The train of wheels is then free to revolve by action of the rack, A, supporting the weight of the upper carbon, until the motion stops by the carbons touching each other. Now let the lamp be connected up, and the current will pass from C, through the electro-magnet, the mass of the apparatus, and return by the wire connecting the lower carbon-holder with Z. The carbon points will glow, but the magnet then attracting M moves the lever, L, the piece, Q, engages the wheel I, pushing it one tooth forward. But this movement of the lever establishes a contact at X, so that the current abandons the electro-magnet, to pass the shorter way, and M being no longer attracted, the lever is pushed back by the spring, the contact at X is broken, and the magnet being again excited the lever turns as before, and Q pushes I round the space of another tooth. These alternating actions succeed each other with great rapidity, and effect the separation of the carbons through the train of wheels acting on the racks. These movements continue until, in a second or two, the separation of the carbons has become so great, that the current passing through the electro-magnet is no longer able to operate against the weight of the upper carbon-holder, and this happens when an arc of proper size is produced, this required result being brought about by proper adjustment of the parts of the apparatus, marked by the letters R, K and X. But as the carbons are consumed, the increase of the length of this arc further weakens the current, until the spring attached to the lever, L, prevails over the attractive force of the electro-magnet on M, and thus withdraws the catch, Q, altogether, when the wheels being free to turn, the weight operates to bring the carbons nearer together, until, with the lessened resistance, the energy of the current is restored, and Q again comes into play to arrest the approximating movement. It may be seen, from the above explanation, that this lamp is automatic; in other words, when it has once been properly adjusted, it is lighted by merely completing the circuit. For fixing the carbons properly in their holders there are, of course, other regulating screws. How very nearly perfection the automatic regulation of the arc electric lamp has been brought by such contrivances as these, will be obvious to all who have noticed the steadiness that has been attained in all the modern installations.

An ingenious plan was devised by Jablochkoff for dispensing with all mechanism for regulating the distance of the carbons. This invention is known as the electric candle, and is of great interest from the fact that it was with this arrangement that the electric light was, for the first time, practically employed for street and theatre illumination. This was in 1878, when visitors to Paris, during the Exhibition, were astonished by the splendid displays in the Avenue de l’Opéra, at the shops of the Louvre, and at some of the theatres. Then it was shown, for the first time, that electric lighting was not merely a scientific curiosity, but a new and formidable rival to gas. The Jablochkoff candles were also subsequently used in the electric lamps on the Thames Embankment. The principle of the contrivance will be understood from fig. 280f. Two carbons, C and D, are placed parallel at a little distance apart, and the space between them is filled up with plaster of Paris, kaolin, or some similar material, through which the current will not pass, but which burns, fuses, volatilises, or crumbles away by the heat produced by the passage of the current between the two carbons. These carbons are, of course, fixed in insulated holders, and to start the candle a small tip of carbon paste is made to connect the carbons at the top. The Jablochkoff candles must be used with currents rapidly alternating in direction. The reason for this is, that otherwise one of the carbons (the positive one) would be consumed quicker than the other, and that would cause the distance between them to increase, until it became so great that the current would cease to pass, and the light would go out. In order to obtain such alternating currents with the Gramme machine, a special apparatus had to be devised to change its direct into alternately reversed currents; but, dynamos intended to supply electric lights are now made without commutators, and they supply rapidly succeeding currents in opposite directions. In certain types of dynamos, again, the armature coils are stationary, and it is the field magnets that are made to revolve, and in these cases, not even a sliding contact is required, but the end of the armature coils are directly and permanently connected with the main circuit. But as these dynamos are self-exciting, the electricity induced in a few of the armature coils is collected apart from the main circuit, and passed through the electro-magnets of the machine itself, after the alternate currents have, by means of a commutator, been converted into one direct continuous current.

The arc electric light, as used for the illumination of streets and public places, is too intense and concentrated to be pleasant to the eye, and therefore it has been found necessary to surround it by globes of enamelled glass, or of porcelain, or of ground glass, or of frosted glass. By these expedients for diffusing and softening the light, it is rendered much more acceptable, but this advantage is gained at the cost of a considerable loss of the whole illuminating power, a loss which is, probably, never less than 10 per cent., but is usually much greater. The globes used in Paris, with the Jablochkoff candles, were of enamelled glass, and the apparatus was arranged, as shown in Fig. 280g, where it is partly represented in section, and with a part of the globe broken off, in order to show one of the candles placed in the holder which connects it with the circuit. In each lamp several candles were mounted, in some cases four; but the lamps in the Place de l’Opéra held twelve. At first there were mechanical arrangements, automatic and otherwise, by which, when the candle was burned down the current could be turned on to another. But M. Jablochkoff afterwards discovered that there was really no need for such a mechanism. For when the whole of the candles are simultaneously and equally connected with the circuit conductors, it is found that one of them will more easily transmit the current than any one of the rest, and when that particular one has once been lighted by the heat developed, the current will pass almost entirely through the arc, any loss through the connecting strip of carbon, at the tops of the other candles, being quite insignificant. When the first of the candles has burnt down completely, until the insulating porcelain holder separates the carbons, the current will at once re-establish itself at the top of one of the remaining carbons, and so on, while one is left.

The arc electric light has not been brought to its present position without the expenditure of much care and ingenuity in the preparation of the carbons used for its production. When Davy first produced the voltaic arc, the electrodes he used were simply sticks of charcoal. These were very quickly consumed, and a more durable form of carbon was sought for. This was found by Foucault, who made use of rods sawn out of the carbonaceous residue left in the retorts in the process of making coal-gas. This substance was, however, by no means uniform or sufficiently pure, and the light obtained was consequently unsteady. Many experiments were made in preparing special carbons. Pounded coke, coke and charcoal, were mixed with syrup or tar into a paste, which was moulded and compressed, and then the sticks were kept in covered vessels at a high temperature for many hours. Acids were used for purification, and also alkalis, to remove silica. At the present time there are several manufacturers of electric light carbons who carry on extensive operations by processes which probably are very similar one to another, and which may well be represented by M. Carré’s, whose carbons have the highest reputation. M. Carré prefers a mixture of powdered coke, calcined lampblack, and a syrup made of sugar and gum. The whole is well mixed and incorporated, water being added from time to time to make up for loss by evaporation, and to give the paste the proper degree of consistence. The paste is then subjected to compression, by which it is forced through draw-holes, and the carbons, having been piled up in covered crucibles, are exposed for a certain time to a high temperature.

As a practical illuminant for lighthouses, the arc electric light came into use many years ago (1862) as we have already seen. This was when the generator of the current was the magneto-electro machine; but, now, when this generator has developed into the modern dynamo, the cost of the electric supply has been enormously reduced, so that, power for power, electric lights may be worked at half the former cost, and with greater convenience and certainty. Light for light, electrical illumination is said to be far cheaper than gas. Again, the arc electric light has properties which have caused it to be employed, not only in every important lighthouse in England, France, Russia, America, and elsewhere, but most ships of war are provided with means of projecting a beam of electric light in any direction, in order that the presence of torpedo boats, etc., may be discovered at night, or harbours entered and signals made under circumstances when such operations would be otherwise impossible. It was by the use of the electric light that, in 1886, one of the Peninsular and Oriental Company’s steamers passed safely through the Suez Canal, at night, and the experiment was so satisfactory, that the canal authorities placed beacons and light-buoys to guide such vessels, as, being provided with electric apparatus, were enabled to hold their proper course between its banks. The use of projected beams for watching the movements of enemies, and for signalling to great distances in time of war, has been recognized by all the great military powers. The advantage of the electrical light in some mines, in subterranean and submarine operations and generally, in work that has to be carried on at night by large bodies of men, is constantly finding illustration. Few readers are unacquainted with the brilliant effect of the arc lamps in exhibitions, parks, &c.; at out of door fêtes, or applied to the illumination of fountains, such as those at the Paris Exhibition of 1889.

The arc lamps are used in series; that is, where there are a certain number of lamps to be supplied, the same electrical current circulates through the whole of them, and this, of course, must have force enough to overcome the resistance of the whole circuit. Thus, at each lamp, the intensity of the illumination must necessarily be very great. A solution was long sought to the problem of so dividing the current energy, that it might be made to produce lights, of moderate intensity, at a greater number of points. When Mr. Edison, shortly after having invented the phonograph, announced that he had solved the problem of the electric light division, there was a great panic amongst the holders of shares in gas companies, and a heavy fall in this kind of stock immediately occurred. As it turned out, the alarm was unnecessary, for gas was not to be superseded, immediately and definitely, by electricity. Nevertheless, it is by virtue of the principle that was contained in Edison’s invention, that electric lighting has assumed the wide-spread importance it has at the present day, and that it is now actually ousting gas as an illuminant in the business and domestic premises of our large towns, and in theatres, libraries, and other places of resort. The principle which has brought about this great development of electric illumination is that shown in a simple form in Fig. 261. It appears, however, that as early as 1841, a platinum wire, made incandescent with a battery current, was proposed as a source of light, and in 1845, carbon was used in the form of slender rods, by King, and also by J. W. Starr, in the United States. Both inventors inclosed their carbons in glass tubes, from which the air was exhausted, so that the carbon might not burn away. In the following year, Greener and Staite turned their attention to lamps of this kind, and, again, in 1849, Petrie worked on the same subject. After that, the problem ceased to engage attention, until, in 1873, a Russian man of science, named Lodyguine, took the matter up and patented a carbon incandescent lamp, which did not, however, prove a practical success, and although the idea was worked out in various ways by Konn, Reynier, Trouvé, and others, the apparatus they designed was, in every case, lacking in simplicity, and certainty of action. The Edison incandescent lamp, the announcement of the discovery of which so fluttered the gas companies, about 1878, was a reversion to the plan of an incandescent metallic wire. This wire was made of an alloy of platinum and iridium, which was adopted by Edison on account of the very high temperature required for its fusion. And in order to prevent the temperature from quite reaching that point, the wire was arranged in a spiral within which was a rod of metal that, by its dilatation with a certain temperature, caused a contact to be made which diverted part of the current through a shorter circuit, and thus lowered the temperature of the spiral to within the assigned limits. But the advantages presented by carbon over metallic conductors led Edison to attempt the formation of filaments by charring first slips of paper, afterwards slips of bamboo. About the same time Mr. J. W. Swan, of Newcastle-on-Tyne, was experimenting in the same direction, and, in the latter part of the year 1880, he exhibited the first incandescent lamps shown in England. Swan’s carbon filaments were prepared from cotton threads which had previously been steeped in dilute sulphuric acid, washed, and passed through draw holes to give them an uniform section. They are thus made perfectly homogeneous throughout, and, after having been wound on pieces of earthenware to the required shape, they are carbonized by packing in powdered charcoal and heating. These filaments are very thin, but solid and elastic. The arrangement of the lamp (see Fig. 280h) is extremely simple: the filament of carbon bent into a horse-shoe form, or turned so as to form a loop, is inclosed in a glass bulb of a globular or egg shape, about two inches in diameter. The extremities of the filament are connected in an ingenious manner to two platinum wires that pass outward through the glass into which they are fused, and terminate either in binding screws or in two small loops. The bulb is exhausted first by an ordinary air-pump, and then by a Sprengel mercurial pump, the current of electricity being sent through the filament during the last stages of the process, and finally the bulb is hermetically sealed. The light yielded by these lamps is mild and steady, and its intensity depends on the electric current sent through them; but this may, it is said, be carried as high as to make the light equal to that of twenty candles. Each horse power of force expended on the dynamo suffices to maintain ten of these lamps. At the Exhibition of Electrical Apparatus at Paris in 1881, the Swan lamp received the gold medal as being the best system in its class. The Swan and the Edison patents are now worked together by one Company, and the productions of this Company are very largely used, although there are several more or less modified systems of glow lamps prepared by other manufacturers.

The great advantages offered by electric glow lamps over gas-lights caused them to be speedily adopted by the most enterprising managers of theatres and places of amusement. Mr. D’Oyly Carte had the Savoy Theatre, in London, completely fitted up with these lamps in 1881. The light was soft and agreeable, it did away with the risks of fire both for the audience and the performers: for the footlights and scene-lights were also electric glow lamps, and the coolness of the house and greater purity of the air were at once appreciated. Several other London theatres have since adopted the incandescent electric lamps, and it is obvious that the system will become universal. In all ocean-going passenger steamers, electric lighting of the saloons and cabins is now the rule. No mode of illumination so readily adapts itself to the production of artistic and decorative effects as the glow lamps: for the covering glasses may be tinted of any required shade, and the lights may be placed in any position. Small glow lamps are occasionally used as personal adornments, when placed, for instance, as part of a lady’s head-dress amidst diamonds, a novel effect of great brilliancy is produced. It need hardly be said that in this application the wearer is not required to carry a dynamo about with her, for the electricity is supplied in a manner much more convenient for this purpose by a device presently to be described. For several years electric incandescent lamps, supplied by the like means, have been in action every night in the carriages of the trains running between London and Brighton, and more recently the Company have had electric reading lamps of five candle-power fitted up in the carriages of the main line trains. They are placed at the backs of the seats just above the passengers’ head. When anyone wishes to make use of one of these lamps, he places a penny in a slot, and then, on pressing a knob, the light appears, and at the end of half an hour it is automatically extinguished; but, of course, it can again be made to appear by another penny dropped in the slot, and so on every half-hour as long as may be required.

To maintain the electric light (whether arc or incandescent) quite steady, the greatest uniformity in the speed of the dynamo is essential; and if the prime mover by which it is worked, whether steam-engine, gas-engine, water-wheel, or turbine, is not perfectly regular in its action, the lights will fluctuate in brightness, and thus produce an effect which is very unpleasant. This is entirely obviated by the adjunct we have now to describe, which not only is most efficient as a regulator, but is, moreover, of still more importance by also providing the means of storing up the electrical energy in a portable form. The reader will have understood that in a voltaic cell the production of an electric current is the concomitant of a chemical union of substances within the cell (p. 493). Now, in the experiment shown in Fig. 263 (p. 498), it is the reverse of combination—namely, the decomposition of the water that is supposed to be effected under the influence of the current from a galvanic battery, and the poles are so connected that the direction of the current in the liquid while the decomposition is proceeding is from the wire in the O tube to that in the H tube. If the experiment be interrupted by removing the battery, and then putting a galvanometer (Fig. 258) in its place, the galvanometer will immediately indicate a current passing through the apparatus in a direction the reverse of the former one—that is, in the liquid it goes from H to O, and the volumes of the gases will slowly diminish while water is reproduced by imperceptible and gradual re-combination. Batteries can be made by joining up a series of arrangements like Fig. 263, consisting of nothing but strips of platinum surrounded by hydrogen and oxygen gases and the intervening acidified water. Analogous results are obtainable by cells containing other compounds with suitable metallic poles, for when decomposition has been effected through a series of such cells by a sufficiently powerful current from a primary battery, the series of cells will constitute, on removal of the primary battery, a secondary battery, for when the terminals of this are joined, the current will flow in the reversed direction while the separated parts of the original compounds are re-combining within the cells. These secondary batteries are called also polarisation batteries. A form of secondary battery was contrived some years ago (1859) by M. Gaston Planté, in which the current of the primary battery was made to act on plates of lead immersed in dilute sulphuric acid. The effect was to coat one of the lead plates of each pair with lead oxide; and in the action of the secondary battery this was reversed, and the plates gradually returned to their original condition, when, of course, the current ceased. Some improvements were made in the Planté battery by Faure, who coated one of a pair of very thin lead plates at once with a film of red oxide of lead, and used a layer of felt to separate it from the other plate. Such arrangements have been called “accumulators”; another term applied to them is “storage batteries”; but it is not to be supposed that in them electricity is stored or, so to speak, bottled up. They consist merely of such an arrangement of materials as that when a current (direct, not alternating) from a dynamo is passing, certain substances are placed in a position of chemical separation in such a manner that in re-combining an equable current of electricity is produced in the conductor externally uniting them. We need not notice some slight modifications of the Faure cells that have been lately introduced, as no new principle is involved. The light of incandescent lamps worked by the Faure accumulator is perfectly free from the fluctuations which may usually be noticed when the lamps are directly connected with the dynamo only. Even if the engine should stop altogether, the light may be maintained for hours. The accumulator has also the advantage of giving out the electric energy that may have been imparted to it days before; so that when a house is fitted up with an independent electric light installation, there is no necessity for running the dynamo all the time the lamps are in use, as two or three days weekly may suffice to charge all the accumulators. Then there is the portability of the accumulator, which permits electrical energy to be made use of in situations where dynamos and prime movers would be impossible. It is said that a large Faure cell weighing about 140 lbs. can receive and give out energy equal to one horse power for one hour. In the arrangement for the reading lamps in railway carriages referred to above, accumulators are placed under the seats; and it need hardly be said that when the electric light has been seen in a coiffure, a small Faure cell concealed about the wearer’s person has supplied the current. A very interesting and useful application of the accumulator is the portable electric light lamp for miners made by the Edison-Swan Company. It is simply an incandescent lamp protected by a strong glass cover attached to the side of a cylindrical case containing a four-celled accumulator. This lamp is provided with an ingenious contrivance by which the circuit would be interrupted, if by accident the outer glass cover of the lamp were broken. Let us now see what another new development of the applications of electricity gains by the use of accumulators by turning our attention to the electro-motor.

At the Vienna Exhibition of 1873, the Gramme Company showed two of their machines, and it is said that when one of these machines was at rest, a workman connected the ends of two covered copper wires with the other machine, thinking that these were placed to carry the current from that machine when in movement. Everybody was surprised when, without any power from the machinery, the ring was soon in rapid rotation. These wires were in fact joined up to the other Gramme machine which was already in action, and it was the current from this that set the former in motion. There is no reason why this story should not be perfectly true, although there are good reasons for believing that the electro-motor was the result of no such accidental circumstance. The attractions and repulsions between the poles of electro-magnets was soon seen to supply an available source of motive power, and the subject has been already mentioned on page 518. Professor Jacobi, of St. Petersburg, seems to have been the first who constructed an electro-magnetic engine, the exciting power being the current supplied by a voltaic battery. This was in 1834, and in a few years afterwards the Professor applied his engine to a small paddle-wheel boat, 28 feet long, which was electrically propelled for several days, but at a slow speed. The engine in this case was virtually a magneto-electric machine worked backwards, that is, instead of applying power to turn the machine and so produce a current of electricity, the current was supplied by the battery and produced power. In 1850, an electro-motor of five horse power was shown by an American, Mr. Page, the principle of which may be illustrated by supposing a reversal of the action represented in Fig. 271, thus: if, instead of producing currents by moving the magnet, C, in and out of the coil, A B, we substitute a battery for D, we can, by alternating the direction of the current through the coil, cause a reciprocating motion of the magnet, C, and this again may be described as a magneto-electric machine worked backward. It was soon recognized that no practical electro-motor was adequate to the production of such high powers as the steam engine supplies, and that the cost must necessarily many times exceed that of steam power. But certain advantages, nevertheless, pertained to the electro-motor in certain positions, as instance in safety, and where a small force only was occasionally required. Now, when the Gramme machine was invented to supply currents of electricity under conditions much more favourable than the magneto-electric machines it superseded, and at a cost vastly less than that of any voltaic battery, it is highly improbable that the relation of the new current generator to the production of electro-motive power would long be overlooked.

The electro-motor may, therefore, be considered simply as a dynamo worked backward, and almost any form of dynamo may in this way be used as an electro-motor, that is, a current being supplied either from a battery or from a dynamo, the motor converts the electrical energy into mechanical energy. Any dynamo that supplies a direct and continuous current can thus be used; but there are certain conditions which make it desirable to somewhat modify the proportions and arrangement of the several parts when the machine is for motor purposes.

In general, any source of current may be used, but in the applications of the electro-motor there are chiefly two methods in practice of supplying the current. The one takes the current from a dynamo in motion, the other from an accumulator which has previously been “charged” by a dynamo.

Both of these methods are used in the familiar and interesting application of the electro-motor to the propulsion of carriages on tramways and railways. For the latter, indeed, an attempt was made half-a-century ago on the Edinburgh and Glasgow railway, to employ the force of an electro-magnetic machine actuated by a battery. This was in 1842, and although this electric locomotive was fitted up completely, it did not attain a speed of more than four miles an hour. The weight with the batteries, carriage, etc., exceeded five tons. But in the recent inventions which have been in practical operation in many places, it is found quite easy to dispense with any current producer on the electric locomotive itself, for the electricity is supplied by a fixed dynamo and the current is transmitted along the line by a conductor from which a sliding contact conveys it to the electro-motor, which is attached to the framework of the carriage and acts on the driving axles of the wheels directly or by toothed gear. In such cases the return current is carried either by another conductor or by the rails themselves. In another arrangement one rail conveys the current to the locomotive and the other returns it. When the rails are so used they have, of course, to be insulated from the ground and laid with special electrical contact pieces joining their consecutive lengths, and all the carriage wheels have to be insulated, so that the currents shall flow only through the coils of the electro-motor. A railway on this system has been worked at Berlin for some time, and a short tramway on the same plan has lately been opened at Brighton. The Bessborough and Newry Electric Railway (Ireland) uses a single separate conductor three miles long, and the power is supplied at a very small cost from a dynamo station near the middle of the line, where water power is taken advantage of to drive a large turbine. Quite recently electric propulsion has been adopted on some of the short tunnel lines in London, and it is quite probable that ultimately the system will be adopted throughout the whole course of the underground railways, with the view of obtaining a purer and more agreeable atmosphere.