A Wheatstone Bridge is a bridge circuit used for measuring an unknown electrical resistance by balancing two legs of its circuit, one of which contains the unknown resistance.  It was invented by Samuel Hunter Christie in 1833, but it was Sir Charles Wheatstone who improved and popularized it in 1843.

Figure 1 below shows a diagram of the Wheatstone Bridge.  

Figure 1.  The Wheatstone Bridge

In the Wheatstone Bridge shown in Figure 1, 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. 

Thus, the following equation applies when the ammeter reading becomes zero:

Runknown = R1R3 / 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).  Mathematically,

Rb = [(R1+R2)(R3+Runknown)] / [R1 + R2 + R3 + Runknown].

Alternatively, if the resistance values of R1, R2, and R3 are known but R1 can 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.

See Also:   Bridge Circuits;  More Articles