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.

 

R1 Rvar = R2 R3

    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.

Some Applications:

    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".