A Wheatstone bridge is an electrical circuit used to measure unknown resistances by balancing two legs of a bridge circuit. It consists of four resistors arranged in a diamond shape, with a voltage source connected across one diagonal and a galvanometer across the other. By adjusting the known resistances, the bridge can be balanced, allowing the calculation of the unknown resistance. This device was invented by Samuel H. Wheatstone in 1843 and is widely used in laboratories and industrial applications. It provides accurate measurements and is essential in various fields, including electronics, physics, and engineering.