# Test and Measurement World

## Test and Measurement World Terminology Section

One Stop For Your Test and Measurement Need

Introduction:
In Maxwell bridge, unknown inductance is measured by comparing with standard variable capacitance. It is much easier to obtain standard values of variable capacitors with acceptable degree of accuracy. The figure-1 depicts configuratio of Maxwell's inductance-capacitance bridge. The figure-2 depicts phasor diagram of Maxwell's Bridge. The unknown inductor L1 of effective resistance R1 in the branch AB is compared with the standard known variable capacitor C4 on arm CD. The other resistances R2, R3, and R4 are known as non-inductive resistors. The bridge is preferably balanced by varying C4 and R4, giving independent adjustment settings.

When bridge is balanced, equation is expressed as follows
➨(R1 + j*w*L1)/R3 = R2/(R4/(1+j*w*C4*R4)) ;
By equating real and imaginary parts, we will het
➨ L1 =C4*R2*R3 and R1 = R2 * (R3/R4)

### Benefits or advantages of Maxwell Bridge

Following are the benefits or advantages of Maxwell Bridge:
➨The balance equations are independent of each other, thus the two variables C4 and R4 can be varied independently.
➨Final balance equations are independent of frequency.
➨The unknown quantities can be denoted by simple expressions involving known quantities.
➨Balance equation is independent of losses associated with the inductor.
➨A wide range of inductance at power and audio frequencies can be measured.

### Drawbacks or disadvantages of Maxwell Bridge

Following are the disadvantages of Maxwell Bridge:
➨The bridge, for its operation, requires a standard variable capacitor, which can be very expensive if high accuracies are asked for. In such a case, fixed value capacitors are used and balance is achieved by varying R4 and R2.
➨This bridge is limited to measurement of low Q inductors (1< Q < 10).
➨Maxwell bridge is also unsuited for coils with very low value of Q (e.g., Q < 1). Such low Q inductors can be found in inductive resistors and RF coils. Maxwell bridge finds difficult and laborious to obtain balance while measuring such low Q inductors.