In the previous chapters we have dealt with electricity in charged bodies, or static electricity, and now we must turn to electricity in motion, or current electricity. In Chapter I. we saw that if a metal rod is held in the hand and rubbed, electricity is produced, but it immediately escapes along the rod to the hand, and so to the earth. In other words, the electricity flows away along the conducting path provided by the rod and the hand. When we see the word “flow” we at once think of a fluid of some kind, and we often hear people speak of the “electric fluid.” Now, whatever electricity may be it certainly is not a fluid, and we use the word “flow” in connexion with electricity simply because it is the most convenient word we can find for the purpose. Just in the same way we might say that when we hold a poker with its point in the fire, heat flows along it towards our hand, although we know quite well that heat is not a fluid. In the experiment with the metal rod referred to above, the electricity flows away instantly, leaving the rod unelectrified; but if we arrange matters so that the electricity is renewed as fast as it flows away, then we get a continuous flow, or current. Somewhere about the year 1780 an Italian anatomist, Luigi Galvani, was studying the effects of electricity upon animal organisms, using for the purpose the legs of freshly killed frogs. In the course of his experiments he happened to hang against an iron window rail a bundle of frogs’ legs fastened together with a piece of copper wire, and he noticed that the legs began to twitch in a peculiar manner. He knew that a frog’s leg would twitch when electricity was applied to it, and he concluded that the twitchings in this case were caused in the same way. So far he was quite right, but then came the problem of how any electricity could be produced in these circumstances, and here he went astray. It never occurred to him that the source of the electricity might be found in something quite apart from the legs, and so he came to the conclusion that the phenomenon was due to electricity produced in some mysterious way in the tissues of the animal itself. He therefore announced that he had discovered the existence of a kind of animal electricity, and it was left for his fellow-countryman, Alessandro Volta, to prove that the twitchings were due to electricity produced by the contact of the two metals, the iron of the window rail and the copper wire. [Illustration: FIG. 6.--Voltaic Pile.]