of contact e.m.f., are causing a current to flow. In this primitive form the arrangement is, however, very imperfect.
As soon as the current flows, there come into play some secondary actions, which cause the contact e.m.f. between zinc and liquid, which is in a forward direction, to decrease, and that between liquid and copper, which is in a backward direction, to increase. The contact e.m.f. causes the current to flow, but as soon as the current flows, this current itself reduces the contact e.m.f. This reaction may be illustrated in a homely way by saying appetite causes a man to eat, but when eating he loses his appetite.
This interdependence between cause and effect is observable in all physical processes, and in its bearing upon the relation between electric currents and mechanical forces it has been formulated by Lenz, and is known as Lenz's law. Here we have to do not with mechanical, but with chemical forces. The current, in passing through the liquid, decomposes it, sending oxygen to the zinc, which is dissolved, and hydrogen to the copper, where it forms a coating and introduces an additional counter e.m.f. of contact. This process is technically termed "polarisation"
