A potentiometer circuit is being used to find the unknown electromotive force (e.m.f.) of a cell X. In the circuit diagram, E is a cell whose e.m.f. is known precisely. QR is the potentiometer wire, and it has a movable contact S. Contact S is linked to a galvanometer and to cell X. Which requirement is not essential for determining the e.m.f. of X from this circuit?
- AThe e.m.f. of cell X must be lower than the e.m.f. of cell E.
- BThe internal resistance of cell X must be known.
- CThe lengths QS and QR must be determined accurately.
- DThe resistance of the wire QR must be proportional to its length.