Bridge Circuit Operation:
The bridge operates by placing Rx in the circuit, as displays in Figure, and then adjusting R3 so in which all current flows by the arms of the bridge circuit. While this condition exists, there is no current flow by the ammeter, and the bridge is said to be balanced. Whenever the bridge is balanced, the currents by each of the arms are exactly proportional. They are equal if R1 = R2. Most of the time the bridge is constructed so in which R1 = R2. While this is the case, and the bridge is balanced, and then the resistance of Rx is the similar as R3, or Rx = R3.
When balance exists, R3 will be equal to the unknown resistance, even if the voltage source is unstable or is not accurately known. A classical Wheatstone bridge has various dials used to vary the resistance. At one the bridge is balanced, the dials could be read to search the value of R3. Bridge circuits could be used to measure resistance to tenths or even hundredths of a percent accuracy. While used to measure temperature, a few Wheatstone bridges along with precision resistors are accurate to about + 0.1°F.