A potentiometer wire of length L and a resistance r are connected in series with a battery of e.m.f. E0 and a resistance r1. An unknown e.m.f. E is balanced at a length I of the potentiometer wire. The e.m.f. E will be given by:
Current in the circuit having
voltage dropped in potentiometer wire of length L is:
since the balanced point is obtained at length I of the potentiometer wire, the emf of the cell E has to match the voltage dropped in length I of the wire.