## Measurement of Self Inductance By Hay's Bridge:

The Hay's bridge is a modification of Maxwell's bridge. The connection diagram and the phasor diagram for Hay's Bridge are shown in the below figure. Hay's bridge uses resistance in series with the standard capacitor (unlike Maxwell's bridge which uses resistance in parallel with the capacitor).

Let                    L1unknown inductance having a resistance R1
R2, R3, R4 = known non-inductive resistance,
and                  C4 = standard capacitor.

 Measurement of Self Inductance By Hay's Bridge

The expressions for the unknown inductance and resistance contain the frequency term. Therefore it appears that the frequency of the source of supply to the bridge must be accurately known. This is not true for the inductance when a high Q coil is being measured as is explained below:

For a value of Q greater than 10, the term (1/Q)² will be smaller than 1/100 and can be neglected.

Therefore the above equation reduces to

L1 = R2 R3 C4
which is the same as for Maxwell's bridge

1. Hay's bridge gives a very simple expression for unknown inductance for high Q coils and is suitable for coils having Q > 10.
2. Hay's bridge also gives a simple expression for Q factor.
3. If we examine the expression for Q factor

Q = 1/Ï‰C₄R₄

We find that the resistance R4 appears in the denominator and hence for high Q coils its value should be small. Thus Hay's bridge requires only a low-value resistor for R4, whereas Maxwell's bridge requires a parallel resistor, R4, of very high value.