Question

in potentiometer why should the voltage of driver cell be higher than that of the cell whose emf is to be found?

Answer 1

Emf using potentiometers can be measured by finding a point on the wire which has a potential drop equal to that of the cell whose emf has to be measured and thereby taking the length corresponding to the null point. If the driver voltage is lower than the emf of the cell which has to be measured then you won’t be able to get a equal potential drop anywhere in the wire to make the deflection in the galvanometer zero. Thus in that case emf cannot be measured.