Wheatstone bridge

A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. The primary benefit of a wheatstone bridge is its ability to provide extremely accurate measurements (in contrast with something like a simple voltage divider). Its operation is similar to the original potentiometer.

The Wheatstone bridge was invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. One of the Wheatstone bridge's initial uses was for the purpose of soils analysis and comparison.

In the figure, ${\displaystyle \scriptstyle R_{x}}$ is the unknown resistance to be measured; ${\displaystyle \scriptstyle R_{1},}$ ${\displaystyle \scriptstyle R_{2},}$ and ${\displaystyle \scriptstyle R_{3}}$ are resistors of known resistance and the resistance of ${\displaystyle \scriptstyle R_{2}}$ is adjustable. If the ratio of the two resistances in the known leg ${\displaystyle \scriptstyle (R_{2}/R_{1})}$ is equal to the ratio of the two in the unknown leg ${\displaystyle \scriptstyle (R_{x}/R_{3}),}$ then the voltage between the two midpoints (B and D) will be zero and no current will flow through the galvanometer ${\displaystyle \scriptstyle V_{g}.}$ If the bridge is unbalanced, the direction of the current indicates whether ${\displaystyle \scriptstyle R_{2}}$ is too high or too low. ${\displaystyle \scriptstyle R_{2}}$ is varied until there is no current through the galvanometer, which then reads zero.

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