**Comparison Bridge:**

There are two types of Comparison Bridge, Namely

- Capacitance Comparison Bridge
- Inductance Comparison Bridge

**1. Capacitance Comparison Bridge:**

Figure 11.18 shows the circuit of a capacitance comparison bridge. The ratio arms R_{1}, R_{2} are resistive. The known standard capacitor C_{3} is in series with R_{3}. R_{3} may also include an added variable resistance needed to balance the bridge. C_{x} is the unknown capacitor and R_{x} is the small leakage resistance of the capacitor. In this case an unknown capacitor is compared with a standard capacitor and the value of the former, along with its leakage resistance, is obtained. Hence.

The condition for balance of the bridge is

i.e.

Two complex quantities are equal when both their real and their imaginary terms are equal. Therefore,

i.e.

and

Since R_{3} does not appear in the expression for C_{x}, as a variable element it is an obvious choice to eliminate any interaction between the two balance controls.

**2. Inductance Comparison Bridge:**

Figure 11.20 gives a schematic diagram of an inductance comparison bridges. In this, values of the unknown inductance L_{x} and its internal resistance R_{x} are obtained by comparison with the standard inductor and resistance, i.e. L_{3} and R_{3}.

The equation for balance condition is

The inductive balance equation yields

and resistive balance equations yields

In this bridge R_{2} is chosen as the inductive balance control and R_{3} as the resistance balance control. (It is advisable to use a fixed resistance ratio and variable standards). Balance is obtained by alternately varying L_{3} or R_{3}. If the Q of the unknown reactance is greater than the standard Q, it is necessary to place a variable resistance in series with the unknown reactance to obtain balance.

If the unknown inductance has a high Q, it is permissible to vary the resistance ratio when a variable standard inductor is not available.