An electrical circuit for measuring the value of a resistance. In the illustration, R1 is a resistance of unknown value, R2 is a fixed resistance of known value, R3 and R4 are variable resistances with known values. When no current flows between A and B the bridge is said to be balanced, the galvanometer registers no deflection, and R1/R2 = R3/R4. R1 can therefore be calculated. The Wheatstone bridge is used in various forms. In the metre bridge, a wire 1 metre long of uniform resistance is attached to the top of a board alongside a metre rule. A sliding contact is run along the wire, which corresponds to R3 and R4, until the galvanometer registers zero. Most practical forms use one or more rotary rheostats to provide the variation. The device was popularized though not invented by Sir Charles Wheatstone.