A potentiometer and a driver cell of electromotive force (e.m.f.) $E$ 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?
1 )
The e.m.f. of the driver cell must be less than the e.m.f. of the new cell.
2 )
The galvanometer must have a large resistance.
3 )
The internal resistance of the new cell must be zero.
The resistance per unit length of the wire $QR$ must be constant.
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