Wheatstone Bridge Half-Metre [vintage]

This is a 50cm sliding-wire wheatstone bridge produced by Bair & Tatlock of London. This example features a 50cm slide wire ruler across the centre, and surmounted by eleven brass terminal (one missing) nut knobs.
A Wheatstone bridge is designed to measure resistance in a conductor by comparing a wire with known resistance to one with unknown resistance. It is used to study the variation of resistance of a conductor with length, diameter and material.