Galvani believed that the flow of charge from the nerve into the muscle caused the contractions. His fellow scientist at the University of Bologna, Alessandro Volta (1745-1807), recognized that Galvani’s frog leg was both a conductor and a detector of electricity. In 1791 he replaced the leg with paper soaked in salt water, a conductor, and used another means of detecting the electricity. He found that charges flowed only if the two metals touching the paper were different. The combination of two different metals separated by a conducting solution is called a galvanic cell after Galvani.

Volta went further. He found that the two metals that produced the greatest electrical effect were zinc and silver. In 1800 he stacked alternating disks of zinc and silver, separated by a card wetted with salt water. He found that this device, called the voltaic pile, was a continuous source of charge flow. Sir Humphrey Davy showed that the charge flow was due to a chemical reaction between the metals and the conductive solution in the cards.