**Maxwell Bridge Theory:**

Maxwell Bridge theory, shown in Fig. 11.21, measures an unknown inductance in terms of a known capacitor. The use of standard arm offers the advantage of compactness and easy shielding. The capacitor is almost a loss-less component. One arm has a resistance R_{1} in parallel with C_{1}, and hence it is easier to write the balance equation using the admittance of arm 1 instead of the impedance.

The general equation for bridge balance is

i.e.

where

From Eq. (11.14) we have

Equating real terms and imaginary terms we have

Also

Maxwell Bridge Theory is limited to the measurement of low Q values (1 – 10). The measurement is independent of the excitation frequency. The scale of the resistance can be calibrated to read inductance directly.

**Advantages and Disadvantages:**

The Maxwell bridge using a fixed capacitor has the disadvantage that there is an interaction between the resistance and reactance balances. This can be avoided by varying the capacitances, instead of R_{2} and R_{3}, to obtain a reactance balance. However, the bridge can be made to read directly in Q.

The bridge is particularly suited for inductances measurements, since comparison with a capacitor is more ideal than with another inductance. Commercial bridges measure from 1 – 1000 H, with ± 2% error. (If the Q is very large, R_{1} becomes excessively large and it is impractical to obtain a satisfactory variable standard resistance in the range of values required).