Some of the experiments which he devised for this purpose were extremely ingenious, and show how thoroughly empirical were his methods and how modern his scientific spirit. In the course of his experiments he found that a difference in the metals of which the arc was composed, when used for the purpose of eliciting the so-called animal electricity, made a great difference in the electrical phenomena observed and in the amount of muscle twitchings obtained. In one brilliant series of experiments, moreover, he showed that, even when the metallic portions touching nerve and muscle were identical, there might still be distinct electrical phenomena, if only an artificial difference in temperature of the end of the metallic arc were produced. Volta was even able to demonstrate that such minute physical differences as the filing of one end of the metallic arc used might give rise to small currents of electricity. In the midst of these experiments, he came to the realization that two portions of metal of different kinds, separated by a moist non-conducting material, might be made to produce a constant current of electricity for some time. More than this, however, he found that discs of metal of different kinds might be piled on top of one another with intervening discs of moist cloth, and so produce proportionately stronger currents as more and more of the metal plates were employed. This was the origin of the voltaic pile, as it has been called--the first battery for the production at will of regular currents of electricity of definite strength.[19] While engaged at this he succeeded in demonstrating what has come to be known as Volta's basic experiment; namely, that two plates of metal of different kinds become electrically excited merely by contact. This was practically the beginning of the great advance in applied electricity which ushered in our modern electrical era. It seems a simple matter now, looking back over the century that has elapsed since then, to have taken the successive steps that Volta did for the construction of his electrical pile and for the demonstration of the principle of contact electricity. Groping, as he was, in the dark, however, it took him three years to make the progress that we have described in a few words. How great his discoveries appeared, even to the most distinguished of his scientific contemporaries, can best be judged from an expression of one of the greatest of French electrical scientists, Arago, who declared "Volta's pile the most wonderful instrument that has ever come from the hand of man, not excluding even the telescope or the steam-engine."