Maxwell bridge

A Maxwell 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 inductance or resistance and capacitance.

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 (an AC voltmeter or ammeter)) and hence no current flowing through it.

can then be calculated based on the values of the other components: 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.

It is also unsuited for the coils with low Q values, less than one, because of balance convergence problem.

Unfortunately, for real components, this relationship is not linear, and using a derived or calculated value in place of a measured one can lead to serious inaccuracies.

A practical issue in construction of the bridge is mutual inductance: two inductors in propinquity will give rise to mutual induction: when the magnetic field of one intersects the coil of the other, it will reinforce the magnetic field in that other coil, and vice versa, distorting the inductance of both coils.

To minimize mutual inductance, orient the inductors with their axes perpendicular to each other, and separate them as far as is practical.

Similarly, the nearby presence of electric motors, chokes and transformers (like that in the power supply for the bridge!)

Before those limits are approached, the ESR of the capacitor will likely have significant effect, and have to be explicitly modeled.