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.