The Voltaic Pile consists of copper and zinc plates, between which a piece of paper or cloth is placed, which has been soaked with salt water. Volta expected the electrical current to be a consequence from the direct contact between the two metal plates and thus seperated them by means of a piece of paper.
Volta, A. (1800). On the Electricity Excited by the Mere Contact of Conducting Substances of Different Kinds. Philosophical Transactions of the Royal Society of London, 90, 403-431.
Pera, M. (1992). The Ambigious Frog: The Galvani-Volta Controversy on Animal Electricity. Princeton (NJ): UP.
Pancaldi, G. (2003). Volta : science and culture in the Age of Enlightenment. Princeton: Princeton University Press.