A Wheatstone bridge consists of four resistors connected as shown. The voltage source can be applied across either A and D or B and C. The variable resistance is adjusted until the potential difference V0 between B and C is zero. This is called balancing the bridge. When the bridge is balanced, the products of the opposite resistors are equal.
As the value of the variable resistor is changed, the potential difference V0, as well as the current in this branch, will change proportionally.
The galvanometer ideally acts as a short circuit forcing points B and C to the same potential. When the bridge is not balanced a current will flow through the galvanometer due to this imbalance.
The Wheatstone bridge can be used in conjunction with many sensors to make ohmmeters, pressure gauges, anemometers, flow meters, and many other measuring devices. A thermistor, for example, may be placed in the circuit as the variable resistance. This device changes resistance with varying temperature. The bridge will unbalance with changes in temperature. Over a linear range, the current through the galvanometer will be proportional to this temperature change creating a "thermometer".