This device is a practical application of Wheatstone bridge used for measuring an unknown resistance and for comparing two unknown resistances.

It consists of one metre long straight and uniform wire manganin or constantan, stretched on a wooden board. These materials have low temperature coefficient of resistance and high resistivity.The ends of the wire are soldered to two thick L–shaped copper strips (negligible resistance), having binding terminals. Between these two there is another strip fixed such that there are two fixed gaps.

A resistance box (known resistance) and an unknown resistance are connected in the two fixed gaps respectively. A battery was connected between points of the two ends of L shaped strips through key.

One end of galvanometer is connected to a point which is a junction point of resistance box and unknown resistance. On the other end was connected to the jockey which can slide over the wire between the two L shaped strips, to balance the bridge.

A graduated scale runs parallel to this wire to measure the balancing length. Introduce a suitable value of Resistance and close the key K. Move the jockey on the wire AC to obtain the null point (i.e. zero reading of the galvanometer).