Potentiometer is a simple device used to measure the electromotive force, potential difference across a resistor and internal resistance of a cell. It consists of a board where a tungsten or manganese wire is fitted on it. The wires have uniform cross-sectional area and of 10 m in length. The wires are joined in series by using thick copper strips. A metre scale is also attached on the wooden board. It works on the principle that the potential dropped between two points in a wire of uniform cross section is directly proportional to the distance between the points.
This video explains how to compare the emf’s of two given primary cells using a potentiometer.