Wheatstone Bridge, a device for measuring electrical resistance. One form of Wheatstone bridge is shown in the illustration. When the bridge is connected in an electrical circuit, part of the current flows to the object whose resistance is unknown (a light bulb in the illustration), and part flows to the resistor of known resistance. If more current flows through one side of the circuit than the other, the galvanometer shows the difference. The sliding contact is then moved along the wire until current flows equally along both sides of the bridge and the galvanometer shows zero.

When the bridge is thus balanced, the unknown resistance is calculated by a formula. The formula is: X = RD'/D (X is the unknown resistance. R is the known resistance. D is the distance from the key to the right end of the meter stick. D' is the distance from the key to the left end.)