A basic Wheatstone bridge circuit contains four resistances, a constant voltage input, and a voltage gage, as illustrated below.
For a given voltage input Vin, the currents flowing through ABC and ADC depend on the resistances, i.e.,
The voltage drops from A to B and from A to D are given by,
The voltage gage reading Vg can then be obtained from,
Now suppose that all resistances can change during the measurement. The corresponding change in voltage reading will be,