How the emf of two cells are compared using potentiometer ?

With suitable circuit diagram, show how emfs of two cells can be compared using a potentiometer.

State the working principle of potentiometer explain with the help of circuit diagram how the emf of two primary cells are compared by using the potentiometer.

The e.m.f. of two cells can be compared by

Explain the principle of a potentiometer. How will you compare the e.m.f. of two primary cells by using potentiometer? Explain with proper circuit diagram.