Describe on Metre Bridge - QS Study
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Metre bridge is one form of Wheatstone’s bridge. It consists of thick strips of copper, of negligible resistance, fixed to a wooden board.

There are two gaps G1 and G2 between these strips. A uniform manganin wire AC of length one metre whose temperature coefficient is low, is stretched along a metre scale and its ends are soldered to two copper strips. An unknown resistance P is connected in the gap G1 and a standard resistance Q is connected in the gap G2 (Figure). A metal jockey J is connected to B through a galvanometer (G) and a high resistance (HR) and it can make contact at any point on the wire AC. Across the two ends of the wire, a Leclanche cell and a key are connected.


Fig: Metre bridge

Adjust the position of metal jockey on metre bridge wire so that the galvanometer shows zero deflection. Let the point be J. The portions AJ and JC of the wire now replace the resistances R and S of Wheatstone’s bridge. Then

P/Q = R/S = r AJ / r JC

where r is the resistance per unit length of the wire.

so, P/Q = AJ/JC = l1 / l2

where AJ = l1 and JC = l2

so, P = Q (l1 / l2)

Though the connections between the resistances are made by thick copper strips of negligible resistance, and the wire AC is also soldered to such strips a small error will occur in the value of (l1 / l2) due to the end resistance. This error can be eliminated, if another set of readings are taken with P and Q interchanged and the average value of P is found, provided the balance point J is near the midpoint of the wire AC.