Maxwell bridge

A Maxwell bridge (in long form, a Maxwell–Wien bridge) is a modification to a Wheatstone bridge used to measure an unknown inductance (usually of low Q value) in terms of calibrated resistance and capacitance. It is a real product bridge.

It uses the principle that the positive phase angle of an inductive impedance can be compensated by the negative phase angle of a capacitive impedance when put in the opposite arm and the circuit is at resonance; i.e., no potential difference across the detector and hence no current flowing through it. The unknown inductance then becomes known in terms of this capacitance.

With reference to the picture, in a typical application ${\displaystyle R_{1}}$ and ${\displaystyle R_{4}}$ are known fixed entities, and ${\displaystyle R_{2}}$ and ${\displaystyle C_{2}}$ are known variable entities. ${\displaystyle R_{2}}$ and ${\displaystyle C_{2}}$ are adjusted until the bridge is balanced.

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