What is potentiometer ? Give its constant and principle.

Explanation

(i) Potentiometer is used for the accurate measurement of potential differences, currrnt and resistances. It consists of ten meter long uniform wire of manganin or constant stretched in parallel rows each of 1 meter length on a wooden board.
(ii) The two free and 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 uis provided for making contact.
(iii) The principle of the potentiometer is illustrated. A steady current is maintained across the CD by a battery Bt. the battery, key and the potentiometer wire are connected in series forms the primary circuit.
(iv) The positive terminal of a primary cell of emf `xi` is connected to the point C and negative terminal is conneted to the joceky through a galvanoeter G and a high resistance HR. Thsi forms the secondary circuit.

(v) 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 `xi` then no current will flow through the galvanometer and it will show zero deflection.
(vi) 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 resistane per unit length of the wire.
Hence `xi=Irl`
(vii) Since I and r are constants, `xi prop 1`. the emf of the cell is directly proportional to the balancing length.