If, therefore, a magnetic needle is suspended at the American end, it will respond to every electrical current, and to any interruption, renewal, or reversal of that current established in England. The needle may thus be made to swing to the right or left, by forming or reversing a current through the wire; and it will return to its position whenever the current is interrupted, and repeat its movement whenever the current is renewed. In fact it may be made to move like the arm of the old-fashioned telegraph, or of a railway signal. It only remains to have a machine by which the operator can form and interrupt currents rapidly, and a code by which certain movements of the needle stand for certain letters of the alphabet, and you have the electric telegraph.
There are many ingenious applications of the machinery, but in principle they all resolve themselves into transformations of energy. Chemical energy is transformed into electric energy, and that again into mechanical work in moving the needle.
The telephone is another instance of similar transformations. Here spoken words create vibrations of the air, which cause corresponding vibrations in a thin plate or disc of metal at one end, which are conveyed by intermediate machinery to a similar disc at the other end, whose vibrations cause similar vibrations in the air, reproducing the spoken words at a distance which may be a great many miles from the speaker.
The great inventions of modern science which have so revolutionised society are all instances of the laws of the conservation of energy. Man makes the powers of nature available for his purposes by transforming them backwards and forwards, now into one, now into another form of energy, as required for the result he wishes to attain. He wants mechanical power to pump water or drive a locomotive or steamboat: he gets it from the steam-engine, by transforming the energy of heat in coal, which came ages ago from the energy of chemical action produced by the sun’s rays in the green leaves of growing plants. He wants to send messages in a few seconds across the Atlantic: he does it by transforming chemical energy into electricity in a voltaic battery, sending its vibrations along a conducting wire, and converting it at the far end into mechanical power, making a magnetic needle turn on its axis and give signals. If, instead of sending a message, he wants to hold a conversation at a distance, he invents the telephone, by which sound-vibrations of air are transformed into vibrations of a disc, then into electric currents, then into vibrations of a distant disc, and finally back again to spoken words. Or, if he wants light, he turns electricity into it by tipping the poles of his battery with carbon and bringing them close together.
The latest inventions of electrical science—the dynamo and the accumulator—afford remarkable instances of this convertibility of one primitive energy into different forms. In the instance just quoted of obtaining light from electricity by the voltaic battery, the cost has hitherto proved an obstacle to its adoption. The electrical energy is all obtained from the transformation of the heat produced in the cells by the chemical action on the metal used, which is commonly zinc. Now, the heat of combination of zinc with oxygen is only about one-sixth of that of coal, while the cost of zinc is about twenty times as great. Theoretically, therefore, energy got by burning zinc costs 120 times as much as that got by burning coal. Practically the difference is not nearly so great, for there is very little loss of energy in the battery by the process of conversion, while the best steam-engine cannot convert into work as much as twenty per cent, of the heat energy in the coal consumed. Still, after making every allowance, the cost of energy from zinc remains some twenty times as great as from coal, so that unless some process is found for obtaining back the zinc as a residual product, there is no prospect of this form of electricity being generally available for light or for mechanical power.
The dynamo is an instrument invented for the mechanical generation of electricity by taking advantage of the principle that electrical energy is produced by moving magnets near coils of wire, or coils of wire near magnets. A current is thus started by induction, and, once started, the mechanical power exerted in making the magnet or coils revolve is continually converted into electricity until the accumulated electrical energy becomes very powerful. The original energy comes of course from the coal burned in the steam-engine which makes the magnet or coils revolve.
The principle of the conservation of energy is well illustrated by the fact that as the dynamo generates an electric current if made to revolve, conversely it may be made to revolve itself if an electric current is sent through it from an exterior source. It is, therefore, available not only as a source of light in the former case, but as a direct source of mechanical power in the latter. It is on this principle that electric engines are constructed and electric railways are worked. Here also it is a question of cost and convenience, for you can only get electricity enough either to light a street or to drive an engine, by an original steam-engine or other motive power to work the dynamo, and a system of conducting wires to convey the electricity to the place where the light or power is wanted. Where the motive power is supplied by nature, as in the case of tidal or river currents or waterfalls, it is quite possible that power may be obtained in this way to compete with that obtained directly from the steam-engine; but there are as yet considerable practical difficulties to be overcome in the transmission of any large amount of energy for long distances.
To overcome some of these difficulties the accumulator has been invented, which affords yet another remarkable instance of the transformation of energy. It consists of two lead plates immersed in acidulated water. When a strong electrical current is sent through the water, it decomposes it, the oxygen going to one lead plate and the hydrogen to the other. The oxygen attacks the lead plate to which it goes, forming peroxide of lead; while the hydrogen reduces any oxide in the other plate, producing pure lead, and leaving a film of surplus hydrogen on the surface. The charging current is then reversed, so that the latter plate is now attacked and the former one reduced, when the current is again reversed. By continuing this process the surfaces of both lead plates become porous, so that they present a large surface, and can therefore hold a great deal of peroxide of lead. The charging current being now broken, the oxygen which has been forcibly separated from the liquid seeks to recombine with hydrogen; and if the two lead plates are joined by a wire, this effort of the oxygen generates an electrical current in the opposite direction to the original one, which is the current utilised. Electricity is thus stored up in a portable box, where it can be kept till wanted, when it is drawn out by connecting the plates, and as a large amount of energy has been accumulated the current which is produced lasts for a considerable time.
Unfortunately accumulators are bulky, heavy, and expensive, and nearly half the energy of the original charging current is lost in obtaining the reversed or working current. They are therefore not as yet adapted for general use, though perfectly capable of supplying either light or motive power, for both which purposes they have been successfully applied in special cases. The future both of electric power and electric lighting is now reduced entirely to a question of cost; and though it is hard to beat gas and the steam-engine, with cheap coal, and air and water for nothing, it is possible that by using natural sources of power to move dynamos, and by obtaining zinc back as a residual product in batteries, electricity may in certain cases carry the day.