A Maxwell bridge (in long form, a Maxwell-Wien bridge) is a type of Wheatstone bridge used to measure an unknown inductance in terms of calibrated resistance and capacitance. It is a real product bridge.
With reference to picture, in a typical application $R\_1$ and $R\_4$ are known fixed entities, and $R\_2$ and $C\_2$ are known variable entities. $R\_2$ and $C\_2$ are adjusted until the bridge is balanced.
$R\_3$ and $L\_3$ can then be calculated based on the values of the other components:
$R\_3\; =\; \{R\_1\; cdot\; R\_4\; over\; R\_2\; \}$
$L\_3\; =\; R\_1\; cdot\; R\_4\; cdot\; C\_2$
To avoid the difficulties associated with determining the precise value of a variable capacitance, sometimes a fixed-value capacitor will be installed and more than one resistor will be made variable.
The additional complexity of using a Maxwell bridge over simpler bridge types is warranted in circumstances where either the mutual inductance between the load and the known bridge entities, or stray electromagnetic interference, distorts the measurement results. The capacitive reactance in the bridge will exactly oppose the inductive reactance of the load when the bridge is balanced, allowing the load's resistance and reactance to be reliabily determined.