Hey there,

A potentiometer is a device that is used to compare the emf (electromotive force) of two given primary cell, which is Daniel cell and leclanche cell.

Principle- when current flows constantly and the cross section area is uniform then the potential drop accross any length of the wire is directly proportional to that length.

E1/E2=L1/L2

Where L1 and L2 are the balancing lenth.

Hope it helps