WHEATSTONE BRIDGE

A widely used circuit for evaluating an unknown resistance is the Wheatstone Bridge, named after Sir Charles Wheatstone. It relies on comparing an  unknown resistance Rx with a variable resistance Ra, typically using a separate or an integrated resistance bank. The reference resistor Ra is varied until a galvanometer across the bridge reads zero. The ratio of Ra/Rx can be changed by setting the ratio R1/R2.

Bridge instruments were used by telephone engineers. Two configurations were applied for measuring the fault location in a broken or shorted telephone cable: the Murray loop and the Varley loop. I have two instruments with that functionality.