Page:Electricity (1912) Kapp.djvu/51

Rh

We have seen that no current due to contact e.m.f. can be produced in a circuit the members of which all belong to a series of contact e.m.f., and which therefore fall under the law that the potential difference between any two is equal to the sum of the potential differences of the intervening pairs. Whichever way we go round such a circuit the total e.m.f. is always zero. To get an e.m.f., and therefore a current in the circuit, we must find some conducting material which falls outside the series in the sense that it does not obey the law just stated. If the continuity of metallic contacts is interrupted by the interposition of such a material, then there will be no complete equilibrium, but a balance of e.m.f. in a definite direction and a current will result. Water is such a material; it