The electric machine was in action, and one of the attendants happening to approach or touch one of the frogs, the man as well as Madame Galvani observed that the limbs were violently agitated. Galvani was at once informed of this, and he made repeated experiments, which showed him that the convulsive movements only took place when a spark was drawn from the prime conductor of the electric machine, and while the nerve was touched by a conductor. Galvani then suspended a number of frogs to a railing by metal hooks, with a view to experiment upon them with atmospherical electricity. But the frogs’ limbs were again agitated when no electricity was apparent, and Galvani after some consideration came to the conclusion that the movement was owing to the position the animals assumed with reference to the metallic bodies. Thus when muscle and nerve were in contact with metallic bodies and connected by metal, the movements of the limbs were observable, and the greater the surface contact the greater was the convulsion. The philosopher next tried various metals, and discovered that the most powerful combination was zinc and silver.
Galvani, in 1791, published his discovery and his theory that the body acted as a Leyden jar, different parts being in a different state of electricity. No sooner were his deductions published than all Europe was in a ferment, and philosophers of all nations were discussing it. Fowler, Valli, Robison, Wells, Humboldt, etc., all were deeply interested, but none of them appear to have arrived at so correct conclusions as did Volta, the physician of Pavia. “Wherever frogs were to be found,” says Du Bois Reymond, “and where two different kinds of metal could be procured, everybody was anxious to see the mangled limbs of frogs brought to life in this wonderful way. Physiologists believed that at length they should realize their visions of a vital power, and physicians thought no cure was impossible.”
But notwithstanding the popular theory, Volta, in his letters to Carallo, while giving a full and clear account of the discovery made by Galvani and his own experiments, attacked and finally defeated the Professor. Volta quite upset Galvani’s Leyden-jar theory; Volta says that it was by accident that Mr. Galvani discovered the phenomenon, and by which he was more astonished than he ought to have been. Volta’s letters will be found in the Philosophical Transactions of the Royal Society (in French), and he attributes the effect to the metals which produced a small amount of electricity. He found that the nerve was acted upon on even parts of a muscle laid upon two different metals, and if those were united, a contraction took place.
“Many experiments were made in all parts of Europe,” says Doctor Roget, and “an opinion had been very prevalent that the real source of the power developed existed in the muscle and nerve which formed part of the circuit, and that the metals which composed the other part acted merely as the conductors by which that agency was transferred from the one to the other of these animal structures. But the discoveries of Volta dispelled the error, by proving that the sources of power were derived from the galvanic properties of the metals themselves when combined with certain fluids,” and decided that this principle was electricity. From this the “general fact” was deduced—viz., “that when a certain portion of a nerve which is distributed to any muscle is made part of a galvanic circuit, convulsions, generally of a violent and convulsive kind, are produced in that muscle.”
Volta at length made the discovery that when two metals were brought together electricity was developed, and by uniting a disc of copper and one of zinc, and subjecting them to the test of an Electroscope, he found positive and negative electricity developed in the zinc and copper respectively; so Volta came to the conclusion that each metal parted with electricity, and one became all “positive” and the other all “negative.” But when he came further to consider the possibility of building up a “pile” of these metal discs sufficiently strong to produce electric effects, he found that if his theory were correct he would lose from one side of the metal all he would gain from the other, and therefore he could never obtain more than the slight effect he had originally produced.
This was at first a difficulty apparently impossible to remove. It was so self-evident that the discs of metal, if placed in a pile in a series of pairs, would continually exercise like effects to the first pair of discs, that Volta was puzzled, and for some time he could not arrive at any reasonable solution. At last it struck him that if he placed between the discs some slow-conducting substance, the electricity would not pass from disc to disc, and the force developed or set in motion would be more powerful.
He made the experiment. The result was the Voltaic pile made in 1800, of which we give an illustration (fig. 219). A communication on the subject of Electricity by contact, written by M. Volta, is to be found in the Philosophical Proceedings for the year 1800.
Fig. 219.—Voltaic Pile.
Volta constructed the pile which bears his name, on the assumption that “every two heterogeneous bodies form a galvanic circle or arc in which electricity is generated.” The “pile” consisted of a number of discs of zinc and copper separated by discs of card soaked in water. This combination of metals separated by a bad conductor, developed considerable electricity, the “positive” going to the zinc at the top, and the “negative” turning to the opposite end. By touching the zinc and copper extremities simultaneously with wetted fingers we shall experience a shock. “I don’t need your frog,” Volta said, when he had proved his theory; “give me two metals and a moist rag, and I will produce your animal electricity. Your frog is nothing but a moist conductor, and in this respect it is inferior to my wet rag!”