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a voltaic or a secondary battery, supplying current to a small platinum wick or electropyre mounted in the hollow shell of a tiny silvered reflector. The current is leton by a press button and its strength can be graduated at will by help of a current indicator.

The tiny light can be directed upon a particular spot by means of the handle, and as it gives off little heat and no fumes it does not inconvenience the patient. Glass bulbs illuminated in this way have been inserted into the stomach of fishes and other semi-transparent animals to exhibit their internal structure; and an Austrian physiologist has designed an apparatus of the kind whereby the internal coatings of the human stomach can be inspected by inserting a little incandescent bulb and reflecting an image of the coating from a small mirror up a tube and out at the mouth.

The general advantages of the electric light over gas are many and various. In gas we are burning a foul distillation from coal, and consuming as well as polluting the air we breathe. Our supply of coal is exhaustible, but our supply of electricity is inexhaustible. In electricity we have a pure ethereal source of light destined to be the illuminant of the future. The arc lamps, it is true, while they yield a brilliant focus, give off very small traces of deleterious vapours, but arc lights are to be used out of doors, in streets, squares, and as head-lights on ships and locomotives, or as beacon-lights on headlands. The domestic light is the incandescent lamp, and in this we have a brilliant and mellow source which neither burns, nor taints, nor unduly heats the air. So like sunshine is it that it shows the most delicate colours in a very near approach to their daylight shades; and as wires can be led to places where oil or gas-pipes cannot reach, it is eminently adapted for decorative purposes. The pictures and hangings of a room are seen to advantage by it, and it can neither tarnish gilding nor blacken paint. Moreover, when it can be supplied at large, the cost will be even less than that of gas.



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


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 electricity, 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

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