Two cells of emf's approximately 5 V and 10 V are to be accurately compared using a potentiometer of length 400 cm.
 

1. The battery that runs the potentiometer should have a voltage of 8 V
2. The battery of the potentiometer can have a voltage of 15 V and R adjusted so that the potential drop across the wire slightly exceeds 10 V
3. The first portion of 50 cm of the wire itself should have a potential drop of 10 V
4. Potentiometer is usually used for comparing resistances and not voltages

(b) Hint: The potential drop along the wire of the potentiometer should be greater than the EMFs of cells.
In a potentiometer experiment, the emf of a cell can be measured, if the potential drop along the potentiometer wire is more than the emf of the cell to be determined. Here, values of emfs of two cells are given as 5 V and 10 V, therefore, the potential drop along the potentiometer wire must be more than 10 V.