The Wheatstone bridge is a ubiquitous circuit configuration that is useful in detecting small changes in resistance.

The basic circuit is the well-known Wien bridge oscillator where the R-C pairs are switched in and out of the feedback loop to achieve frequency range control. This was accomplished with rotary ...

You could use two matched resistors in part of the bridge, have an unknown ... If the power source is AC, you can also measure reactance using a similar circuit. As a simple thought experiment ...