Hey’s Bridge is intended to measure an unknown inductance by comparison with a standard variable capacitance. This bridge measures unknown inductance in the range of 1mH to 100mH
L1 = R2 x R3 x C4

In this bridge we are having three sections. A built in oscillator section. Bridge section with four arms. The balance of bridge can be observed using a built-in audio amplifier with loudspeaker arrangement. This acts as a null detector.