A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance

Electrical resistance

The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

 by balancing two legs of a bridge circuit

Bridge circuit

A bridge circuit is a type of electrical circuit in which two circuit branches are "bridged" by a third branch connected between the first two branches at some intermediate point along them. The bridge was originally developed for laboratory measurement purposes and one of the intermediate...

, one leg of which includes the unknown component. Its operation is similar to the original potentiometer

Potentiometer (measuring instrument)

A potentiometer is an instrument for measuring the potential in a circuit. Before the introduction of the moving coil and digital volt meters, potentiometers were used in measuring voltage, hence the '-meter' part of their name...

. It was invented by Samuel Hunter Christie

Samuel Hunter Christie

Samuel Hunter Christie was a British scientist and mathematician.He studied mathematics at Trinity College, Cambridge where he was second wrangler. He was particularly interested in magnetism, studying the earth's magnetic field and designing improvements to the magnetic compass...

 in 1833 and improved and popularized by Sir Charles Wheatstone

Charles Wheatstone

Sir Charles Wheatstone FRS , was an English scientist and inventor of many scientific breakthroughs of the Victorian era, including the English concertina, the stereoscope , and the Playfair cipher...

 in 1843.
In the figure, is the unknown resistance to be measured; , and are resistors of known resistance and the resistance of is adjustable.