It consists of a wire of length 1m and of uniform cross sectional area stretched taut and clamped between two thick metallic strips bent at right angles, as shown. The metallic strip has two gaps across which resistors can be connected. The end points where the wire is clamped are connected to a cell through a key. One end of a galvanometer is connected to the metallic strip midway between the two gaps. The other end of the galvanometer is connected to a ‘jockey’.
The jockey is essentially a metallic rod whose one end has a knife-edge which can slide over the wire to make electrical connection.
R is an unknown resistance whose value to be determine. It is connected across one of the gaps. Across the other gap, we connect a standard known resistance S.
The jockey is connected to some point D on the wire, a distance l cm from the end A. The jockey can be moved along the wire. The portion AD of the wire has a resistance Rcml, where Rcm is the resistance of the wire per unit centimeter.
The portion DC of the wire similarly has a resistance Rcm (100-l ). The four arms AB, BC, DA and CD [with resistances R, S, Rcm l and Rcm(100-l)] form a Wheatstone bridge with AC as the battery arm and BD the galvanometer arm.
If the jockey is moved along the wire, then there will be one position where the galvanometer will show no current. Let the distance of the jockey from the end A at the balance point be l= l1. The four resistances of the bridge at the balance point then are R, S, Rcm l1 and Rcm(100–l1). The balance condition gives
Thus, once we have found out l1, the unknown resistance R is known in terms of the standard known resistance S by
Jockey is slide over AC wire that is why it is called Slide Wire Bridge
It cannot be used to measure very high and very low resistance.
Balance position of
not change with
interchange of cell and galvanometer. Meter
Wheatstone is most sensitive when resistance of all four arms is same.