The Wheatstone Bridge

• In the Wheatstone Bridge shown in Figure, the resistance values of resistors R2, and R3 are known, while the resistance value of variable resistor R1 may be adjusted

 

• The resistance value of R1 is adjusted until the current reading of the ammeter connected between points A and B of the circuit becomes zero.
• When this happens, the bridge is said to be 'balanced', i.e., the voltages at points A and B are already equal, so the value of the unknown resistance may easily be calculated using voltage ratios:  

 Runknown / R3 = R1 / R2.  

 

• The equivalent resistance Rb of the circuit when it is balanced is just the resistance of the left leg (R1+R2) in parallel with the resistance of the right leg (R3+Runknown).
      
  Rb = [(R1+R2)(R3+Runknown)] / [R1 + R2 + R3 + Runknown].

   

  Alternatively, if the resistance values of R1, R2, and R3 are known but R1can not be adjusted, then the value of Runknown can still be calculated using Kirchhoff's Voltage Law.

   

  This set-up is often seen in strain gauges and resistance temperature detection circuits, since it is quicker to read a voltmeter than to manually adjust a resistor to balance the circuit.