Question

How to compare the emf of 2 cells using a Potentiometer ?

Answer 1

Using a potentiometer, we can determine the emf of a cell by obtaining the balancing length l.  Here, the fall of potential along the length l of the potentiometer wire is equal to the emf of the cell, as no current is being drawn from the cell. 

then       E=kl; where k is a potential gradient along the wire.

thus it is possible to compare the emf of two  given cells by measuring the 

respective balancing lengths l1 and l2.

E1=kl1    and E2=kl2  (or)   E1/E2=l1/l2