The wheatstone bridge:

You have already solved resistor networks using Ohm's law and Kirchoff's laws.  In this unit we are going to look at a special arrangement of series and parallel resistors called a Wheatstone Bridge.

The Wheatstone Bridge circuit and other similar variants were widely used in test equipment to determine the value of an unknown resistor by comparison with other resistors whose values are accurately known.

The normal arrangement in a Wheatstone bridge used for resistance measurement is for two resistors, usually R₁ and R₂, to be fixed and of known value and R₄ to be an accurate variable resistor adjusted by means of a calibrated dial.  The resistor R­₃ is then the unknown whose value is to be measured.