Inductance is measured by Maxwell bridge.

Explanation:
To measure the inductance of an unknown inductor, a Maxwell bridge is used. The Maxwell bridge is a type of bridge circuit used to measure the value of an unknown inductor by comparing it with known values of resistance, capacitance, and inductance.

Maxwell bridge:
The Maxwell bridge consists of four arms - two arms with known resistors, one arm with a known capacitor, and one arm with the unknown inductor. The bridge is balanced by adjusting the value of the variable resistor in one of the arms. When the bridge is balanced, the ratio of the known resistors to the unknown resistor is equal to the ratio of the known capacitor to the unknown inductor.

Working principle:
The Maxwell bridge works on the principle of null balance. Initially, the bridge is unbalanced, and a null detector (such as a galvanometer) is used to detect the imbalance. By adjusting the variable resistor, the bridge is balanced, and the galvanometer shows zero deflection. When the bridge is balanced, the following condition is satisfied:

R1/R2 = C1/C2

Where R1 and R2 are the known resistors, and C1 and C2 are the known and unknown capacitance values, respectively.

Calculating inductance:
Once the bridge is balanced, the values of the known resistors, known capacitor, and variable resistor are recorded. From these values, the unknown inductance can be calculated using the formula:

L = R2 * C2 * (R1 + RV)

Where L is the unknown inductance, R2 is the known resistor, C2 is the known capacitor, R1 is the other known resistor, and RV is the value of the variable resistor at balance.

By measuring the values of the known components and the variable resistor at balance, the inductance of the unknown inductor can be accurately determined using the Maxwell bridge.

Owen bridge also used to measure inductance with low Q coils