Volta himself supposed it to reside in the Contact of different metals.
Here was exerted his 'Electro-motive force,' which tore the combined electricities asunder and drove them as currents in opposite directions.
To render the circulation of the current possible, it was necessary to connect the metals by a moist conductor; for when any two metals were connected by a third, their relation to each other was such that a complete neutralisation of the electric motion was the result.

Volta's theory of metallic contact was so clear, so beautiful, and apparently so complete, that the best intellects of Europe accepted it as the expression of natural law.
Volta himself knew nothing of the chemical phenomena of the pile; but as soon as these became known, suggestions and intimations appeared that chemical action, and not metallic contact, might be the real source of voltaic electricity.

This idea was expressed by Fabroni in Italy, and by Wollaston in England.

It was developed and maintained by those 'admirable electricians,' Becquerel, of Paris, and De la Rive, of Geneva.

The Contact Theory, on the other hand, received its chief development and illustration in Germany.