Introduction

To determine the self-inductance of an unknown coil.

Theory

This bridge circuit measures an inductance by comparison with variable standard self inductance. The connections for balance condition is shown in Fig. 1.

Fig 1: Circuit Diagram for Measurement of Self Inductance by Maxwell Bridge

Let, L₁= Unknown self Inductance of resistance R₁,

L₂= variable inductance of fixed resistance r₂,

R₂= variable resistance connected in series with inductor L₂,

R₃,R₄= known non inductive resistances,

At balance condition,

$$ (R_1 +j \omega L_1)*R_4 = (R_2 + r_2 + j \omega L_2)*R_3 ...(1) $$

Equating both the real and imaginary parts in eq.(1) and seperating them,

$$ L_1 =(\frac{R_3}{R_4})L_2 ...(2) $$

$$ R_1 = (\frac{R_3}{R_4})*(R_2+r_2) ...(3) $$

Resistors R₃ and R₄ are normally a selection of values from 10, 100, 1000 and 10,000Ω. r₂ is a decade resistance box.