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ELECTRICITY
becomes strongly positive when in contact with any of the substances given in the table, but the difference of e.m.f. of the two combinations, water-zinc and water-copper, is not equal to that of the combination copper-zinc.
Assume for the moment that the difference is zero, or, in other words, that water
Fig. 1.
is quite inert as regards contact e.m.f. and simply acts as a conductor. This is not actually the case, but a convenient assumption for the purpose of explaining the way a cell may give an e.m.f. in an external circuit. Let, in Fig. 1, Zn and Cu be a zinc and copper plate respectively, and let to these plates be fastened strips of copper for the attachment of the terminals A and B. The plates are
