Hay’s bridge for Measurement of Inductance

In the previous article, we have learned about Maxwell bridge. It is a modification of Maxwell’s bridge. The circuit diagram for Hay’s Bridge is shown below. It uses a resistance in series with a standard capacitor.

Fig. Hay’s Bridge

L1=unknown resistance having a resistance R1

R2, R3, R4=known non-inductive resistance

C4=standard capacitor

At balance,

e1e4 = e2e3

Separating real and imaginary term from above equation, we get

By solving the above two equations, we get

The Q factor of the coil is given by


1) provides very simple expression for unknown inductance for high Q coils. This bridge is suitable for coils having Q > 10.

2) simple expression of Q factor.

3) From the expression of Q-factor, it is clear that for high Q factor the value of resistance R4 should be small.


1) This bridge is suitable for measurement of high Q inductors.

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