Introduction to working of a potentiometer:
Potentiometer is an instrument used for measurement of potential difference. It consists of a ten meter long uniform wire of manganin or constant and stretched in ten segments, each of one metre length.The segments are stretched parallel to each other on a horizontal wooden board, The ends of the wire are fixed to copper strips with binding screws. A metre scale is fixed on the board, parallel to the wire. Electrical contact with wires is established by pressing the jockey J.
Connections of potentiometer to discuss its working :
Let us discuss the working of potentiometer and the relation obtained as the result.
Working of Potentiometer
A battery Bt is connected between the ends A and B of a potentio-meter wire through a key K. A steady current I flows through the potentiometer wire as shown in figure.This forms the primary circuit. A primary cell E is connected in series with the positive terminal at A of the potentiometer, a galvanometer, high resistance and jockey. This forms the secondary circuit.
If the potential difference between A and J is equal to the emf of the cell, no current flows through the galvanometer. It shows zero deflection. AJ is called the balancing length. If the balancing length is l, the potential difference across
AJ =Irl where r is the resistance per unit lemngth of the potentiometer wire and I is the current in the primary circuit.
E = Irl,
since I and r are constants
Therefore the emf of the cell is directly proportional to its balancing length. This is the working of potentiometer.
Uses of Potentiometer
It is used to measure the potential difference .
The emf( electromotive force ) of two cells can be compared using a potentiometer.
The difference of potentials between two terminals of a cell in an open circuit is called the electromotive force emf of a cell.
Comparison of emfs of two given cells is given by the equation
If emf of one cell(E1) is known the emf of the other cell (E2) can be calculated using the relation: