State the principle of a potentiometer.

Explanation

Potentiometer: it is a device which is used to find the internal resistance of a cell or to compare the emfs of two cells.
Construction: It consists of a long wire XY, say 4m in length, adjusted on a wooden board in the form of four parallel wires each 1m long. Actually these four pieces of wire (each 1m long) which are connected to one another by thick copper strips so as to form a single wire of length 4m. An electric circuit consisting of a battery of e.m.f E, an ammeter A, a rheostat `R_(h)` and a key K is connected between the ends X and Y. A constant current is allowed to pass through the wire XY. A jockey J is connected to terminal X through a galvanometer G. A wooden metre scale is provided on the board parallel to the wire. Comparison of emfs of two cells: