Wheatstone bridge

A wheatstone bridge is used to measure an unknown resistance by balancing two legs of a 'bridge circuit', one leg of which includes the unknown component.

...insert a circuit diagram...

The ratio of the two resistances in the known leg will be equal to the ration of the two in the unknown leg when no current flows between the two midpoints. This difference measurement can be made extremely sensitive. So if the 3 known components are known to high accuracy, the unknown resistance can be determined to high accuracy.

The Wheatstone bridge illustrates the concept of a difference measurement, and how it allows extremely accurate measurements to be made.