expended in the same proportion as those effects are produced; and
hence arises the impossibility of obtaining by their agency a
perpetual effect; or in other words a perpetual motion. But the
electro-motive force, ascribed by Volta to the metals, when in
contact, is a force which, as long as a free course is allowed to
the electricity it sets in motion, is never expended, and continues
to be excited with undiminished power in the production of a
never-ceasing effect. Against the truth of such a supposition the
probabilities are all but infinite.' When this argument, which he
employed independently, had clearly fixed itself in his mind,
Faraday never cared to experiment further on the source of
electricity in the voltaic pile. The argument appeared to him
'to remove the foundation itself of the contact theory,' and he
afterwards let it crumble down in peace.[1]

Footnote to Chapter 7

[1] To account for the electric current, which was really the core
of the whole discussion, Faraday demonstrated the impotence of the
Contact Theory as then enunciated and defended. 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