to charge still a little more electricity on the sphere, we must increase the electromotive force, or e.m.f. of the machine, by working it quicker; this will again raise the potential, but a point must eventually be reached when e.m.f. and potential again balance.
We have thus, as the limiting condition of the process of charging, equality between potential and e.m.f. It may be objected that this statement cannot have any physical meaning, because we are comparing two things which by their nature are different. Potential is of the nature of energy, whereas e.m.f. is only one of the factors which make up energy. When we pay our electric light bill, we pay, really, for energy, and not for current by itself; nor do we pay for e.m.f. by itself; nor for the product of the two. What we pay for is the product of three things, namely, current, e.m.f. and time. The electricity metre, which says how much we have to pay, takes account of all three factors, and gives the energy as the product of amperes, volts, seconds, or, as a mere matter of convenience, it gives it in kilovoltampere hours, the unit legalised by Act of Parliament, and known as the "Board of Trade Unit" of electrical energy. As a matter of strict logic it is there-
