Volta's battery or "pile" is the first "perpetual" electric motor, i.e.piece of apparatus capable of supplying a continuous current when included in a closed circuit. As Volta himself described it in the letter he wrote making it public, it consists of a series of copper or better, silver discs, placed one on top of each other and alternating with a like number of steel or better, zinc discs. between each pair of metal discs and the next there is a disc of cardboard or some other material soaked in salt or acidulated water. In his first version, at one end of the pile Volta added a container of salted or acidulated water. Tthis made it possible to put one hand in, which made sometimes very curious experiments to be carried out. To explain how the battery functions is rather complex, involving electrical and chemical phenomena. Volta placed most importance on the bi-metal contact and, although he observed some chemical effects, he remained attached to an electro-metallic explanation rather than a chemical one.