Still, it is certain that two different metals, when brought into contact, charge themselves, the one with positive and the other with negative electricity.

I had the pleasure of going over this ground with Kohlrausch in 1849, and his experiments left no doubt upon my mind that the contact electricity of Volta was a reality, though it could produce no current.

With one of the beautiful instruments devised by himself, Sir William Thomson has rendered this point capable of sure and easy demonstration; and he and others now hold what may be called a contact theory, which, while it takes into account the action of the metals, also embraces the chemical phenomena of the circuit.

Helmholtz, I believe, was the first to give the contact theory this new form, in his celebrated essay, Ueber die Erhaltung der Kraft, p.

45..