A potentiometer and a driver cell of electromotive force (e.m.f.) 
are used to measure the e.m.f. of a new cell. A sliding contact at P is moved along a resistance wire QR until the reading on the galvanometer is zero.
What is an essential requirement for the e.m.f. of the new cell to be measured accurately?
The e.m.f. of the driver cell must be less than the e.m.f. of the new cell.
The galvanometer must have a large resistance.
The internal resistance of the new cell must be zero.
The resistance per unit length of the wire QR must be constant.
