CHAPTER X.

ELECTRIC POWER.

We have already seen how the electric current can be made to produce light and distribute it over a considerable area. We have now to see how mechanical power can be transmitted by the same agency to great distances. Hitherto it has been the custom to convey power to a distance by means of compressed air or long belts and shafting; but in either case the distances have been comparatively short. Electricity, however, will travel as far as wire can lead it, and hence it offers us a mode of distributing power to a number of machines situated many miles apart, just in the same way as telegrams are now recorded or electric lamps fed.

It has long been known that the electric current flowing in a wire was capable of producing mechanical effects, and therefore of "transmitting power." The ordinary electric bell is, in fact, a case in point. Here the current passes through the coils of a small electromagnet, and thereby attracts the armature, which in turn strikes the clapper on the bell. The blow of the clapper is therefore an instance of power transmitted by the electric current, but, of course, on a very small scale. In the same way the printing of a Morse tele

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graph instrument is performed by electric power; and so also on an infinitesimal scale is the vibration set up by the current in the sounding-plate or diaphragm of the speaking telephone.

Moreover, many ingenious electric engines have been specially constructed by the earlier electricians to perform mechanical work when the electric current was passed through them; but in these cases the electric power was derived from the voltaic battery by the oxidation of zinc; and the steam-engine, in which power is got by the combustion of coal, proved far more economical. The consequence is that the electro-magnetic motors of Pixii, Dal Negro, and others, are interesting rather as curiosities of science. than as useful apparatus.

The improvement of the dynamo-electric machine effected by Gramme and others put into our hands a source of the electric current far more constant and powerful than the voltaic battery. With it mechanical power was directly employed to generate the electrieity, and this power could be obtained from the combustion of coal in the steam-engine. It was therefore no longer necessary to consume expensive zinc in the voltaic battery in order to produce the electricity required. Further it was found that complicated electric engines, like those of Pixii and Dal Negro, to turn the current at a distant place back again into power, were not essential either. In short, it was discovered that the action of the dynamo-electric machine was quite reversible, and that the current yielded by one Gramme machine when kept in motion by mechanical power would, if sent through a second Gramme machine, start and maintain it in motion.

This important fact, although predicted by Dr. W. Siemens in 1867, was only verified in a practical manner in 1873, at the International Exhibition of Vienna, where M. Hypolité Fontaine connected two Gramme dynamo-electric machines together by means of a wire circuit over 1,000 yards long. One of these machines was kept in motion by a gas-engine, and the current thus generated in its armature or bobbin was transmitted along the wires to the bobbin of the other machine, which, by a contrary action, it maintained in rotation. The rotating bobbin, by means of a pulley and belt, was caused to work a centrifugal pump. There the power derived in the first place from the combustion in the gas-engine was transmitted electrically over 1,000 yards of wire and made to pump some water.

The reversibility of the dynamo-electric machine, simple as the discovery seems to be, is pregnant with incalculable consequences to the world. It means something like a revolution in the conditions of labour, the methods of conveyance, and the prosperity of nations. At present, however, the prospect opened up by the transmission of electric power lies in the background, and the electric light engages most attention. Nevertheless, some important applications of the principle have already been made, not only abroad but in these islands.

The chief of these is the electric railway of Dr. Werner Siemens, which has been constructed in the suburbs of Berlin, between the Cadet School and Lichterfelde. The electric power for propelling the train is obtained from a stationary Siemens dynamo (such as we have described in Chapter VIII.) driven by

a steam-engine. The revolution of the bobbin of the dynamo, which is effected by the steam-engine, generates a powerful current of electricity which is conveyed along one rail of the track to the train, and back again by the other rail, thus completing the circuit. The iron rails, in fact, take the place of the wires in an ordinary electric lighting circuit from one pole of the bobbin of the generator to the other. In crossing from one rail to the other the current passes through the bobbin of a dynamo fixed to the bottom of the traincarriage which serves as a locomotive, and in doing so it rotates the bobbin, and this rotation is communicated to the wheels of the carriage by a simple gearing which turns the axletrees. The bobbin of the dynamo is put in electrical connection with the rail conveying the current to it by a metal brush which sweeps the rail, and after passing through the bobbin the current reaches the other rail, which acts as the return wire, completing the circuit.

This plan of utilising the rails for the circuit of the current is, however, not always satisfactory, especially in wet weather, when the track is flooded and the insulating supports of the rail conveying the current from the generator to the bobbin in the first place are wetted. The moisture spoils the insulation of the rail and the electricity leaks into the earth.

In the Electric Tramway, at the Paris Electrical Exhibition last year, Dr. Siemens led the current to the dynamo on the car by way of separate conductors erected on posts beside the track, after the manner of a telegraph line. Each of the conductors consisted of a brass pipe split throughout its length and carrying a little metal chariot, which ran along the ground in

the pipe and served to connect it by means of a flexible conductor to the electric motor on the axle of the driving car.

In the system devised by Professors Ayrton and Perry rails are employed as the conductors of the current; but the enormous leakage over the whole of a long line of railway is obviated by insulating the rails in sections. When the train is on a particular section the weight of the carriages bends the rails of that section into contact with a special conductor carrying the current, and thus only the section which the train is passing over is electrified. This section may be made so short that the leakage from it is inconsiderable. In this way the train helps itself to the current it requires as it goes along, and all the railway, both before and behind it, is unelectrified. A dynamo is fixed to every carriage, or it may be to every axle, and is supplied with current from the rails below through the wheels themselves as they press upon the rails beneath and bend them into contact with the conductors coming from the generator stationed at the end or some intermediate point of the route. In this way a high speed can be attained; and, moreover, an absolute block system is created, for only one train can be propelled on one section at a time.

Electric railways are likely to prove very useful for short local lines where the gradients are very steep or the atmosphere is apt to become close, as in the St. Gothard Tunnel and the Metropolitan District Railway. They are free from steam and coal smoke, or noise and cinders, and hence are peculiarly suitable for the interior of cities. Several schemes for building them are already promoted in this country,