Wheatstone Bridge

A Wheatsone Bridge is a useful circuit arrangement that allows accurate measurement of resistance.

A Wheatstone bridge is "balanced" by adjusting resistors R3 and R4 so that no current flows through the Galvanometer. Then, the voltage at points A and B in the circuit is the same. Voltage across R1 is the same as across R3 and the voltage across Rx is the same as across R4. Current through R1 and Rx is the same and current through R3 and R4 is the same.

Since points A and B have a common potential we can write

I1R1 = I3R3

Likewise, we can write

I1Rx = I3R4

Dividing one equation by the other, we have

We can replace the variable resistors R3 and R4 with a slidewire with variable length L3 and L4,

Kirchoff's Rules

Internal Resistance

Return to Ch 19, DC Circuits

(c) Doug Davis, 2002; all rights reserved