series of contact e.m.f., and be subjected to the law that the sum of its potential differences to zinc on the one side and to copper on the other side is equal to the potential difference between zinc and copper. We thus have a perfect balance of e.m.f.'s set up by the direct contact between the two discs and the indirect contact via the joining wire. Since the e.m.f.'s are in equilibrium, no current can flow. If it were possible to upset this equilibrium on one side or the other, then we could produce a current, and that is actually done by an arrangement of substances, some of which fall outside the series of contact e.m.f.'s. Such arrangements are called "voltaic cells." A familiar example is the so-called Leclanché cell (named after its inventor), which is found in almost every household for the working of electric bells.
Before entering on a study of voltaic cells it will be convenient to amplify the series on p. 40 by the definite statement of the e.m.f. to be obtained with any combination of the metals. The figures in the following table represent experimental results obtained by Ayrton and Perry, and recorded in Whetham's Practical Electricity—
