Question

why is potentiometer not used to measure small emf ??

Answer 1

A potentiometer is to be used to measure the emf and internal resistance of a test cell. Emf using potentiometers is 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 ie considering very small emf. The driver voltage becomes 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.