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* Schering bridge is an AC bridge which is used for the measurement of unknown capacitance.* This is one of the most commonly used AC bridge. Schering bridge is shown in figure 1 under balance condition.

**Contents**show

## Introduction

From figure 1, there are four arms as

arm 1 = ab

arm 2 = ad

arm 3 = bc

arm 4 = dc

where

C_{1 }is an unknown value of capacitance to be measured.

r_{1 }is a series resistance representing loss in C1.

C_{2 }is a standard capacitor. It is an air or gas capacitor and hence loss-less capacitor.

R_{3} is a non–inductive resistance.

C_{4 }is a variable capacitor.

R_{4} is a variable non-inductive resistance.

Therefore, the impedances of arms 1, 2, 3 and 4 are respectively,

At balance condition,

Z_{1}Z_{4} = Z_{2}Z_{3}

Equating real and imaginary parts on both sides,

The independent variables in r_{1} and C_{1} are C_{4}, R_{4} respectively. Therefore, Balancing is done by adjustment of R_{4}, C_{4} with R_{3}, C_{2} fixed.

Dissipation factor (D) is given by

### Advantages of Schering Bridge

1. The balanced equation obtained is independent of frequency terms.

2. By using fixed values of C_{2}, R_{4}, the dial of R_{3} may be calibrated to read the capacitance (C_{1}) directly.

3. In case of fixed frequency, the dial of capacitor C_{4} can be calibrated to read dissipation factor directly as

D= ωC_{4}R_{4}.

### Disadvantage

There is a difficulty in obtaining balance as R_{3} appears in both equations.

### Application of Schering Bridge

This bridge is used for the measurement of the relative permittivity of dielectric materials.