Describe the potentiometer and explain its principle.

Explanation

Potentiometer is used for the accurate measurement of potential differences, current and resistances.
Description: It consists of ten meter long uniform wire of manganin or constantan (-) an stretched in parallel rows each of 1 meter length, on a wooden board. The two free ends A and B are brought to the same side and fixed to copper strips with binding screws. A meter scale is fixed parallel to the wire. A jockey is provided for making contact. The principle of the potentiometer is illustrated in Figure. A steady current is maintained across the wire CD by a battery Bt. The battery, key and the potentiometer wire are connected in series forms the primary circuit. The positive terminal of a primary cell of emf `xi` is connected to the point C and negative terminal is connected to the jockey through a galvanometer G and a high resistance HR. This forms the secondary circuit.

Procedure:Let contact be made at any point J on the wire by jockey. If the potential difference across CJ is equal to the emf of the cell & then no current will flow through the galvanometer and it will show zero deflection. CJ is the balancing length l. The potential difference across CJ is equal to Irl where I is the current flowing through the wire and r is the resistance per unit length of the wire.
Hence `xi = Irl" ...(1)"`
Principle : Since I and r and constants, `xi prop l`.
The emf of the cell is directly proportional to the balancing length.