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Unknown capacitance value is obtained by
Comparison with standard
Using a tuned detector
Using capacitance of other ratio arms
Using a vibration galvanometer
Comparison with standard
Quick Summary: An unknown capacitance is determined by comparing it against a known standard capacitor within a bridge circuit, such as a De Sauty bridge or a Schering bridge. The balance condition of the bridge occurs when the ratio of impedances of the arms results in a null detector reading, allowing the unknown value to be calculated relative to the standard.
An unknown capacitance is determined by comparing it against a known standard capacitor within a bridge circuit, such as a De Sauty bridge or a Schering bridge. The balance condition of the bridge occurs when the ratio of impedances of the arms results in a null detector reading, allowing the unknown value to be calculated relative to the standard.
CxтАЛ=CsтАЛтЛЕR1тАЛR2тАЛтАЛ тАФ General balance equation for a simple comparison bridge where CsтАЛ is the standard capacitor.
Z=j╧ЙC1тАЛ тАФ Complex impedance representation of a capacitor.
In an AC bridge, four arms are connected in a loop with an AC excitation source. When the bridge is balanced, the product of impedances of opposite arms is equal: Z1тАЛZ4тАЛ=Z2тАЛZ3тАЛ. By using a standard calibrated capacitor in one arm, the bridge is adjusted by varying resistive elements until the detector shows zero deflection, satisfying the equilibrium condition related to the unknown capacitance.
AC bridges are the primary instruments for measuring passive components like C, L, and R.
A standard capacitor is required because capacitance measurements are inherently relative rather than absolute.
Null detectors are used to indicate the bridge balance state, ensuring high sensitivity.
High precision and sensitivity.
Independent of supply voltage fluctuations when null-balance is achieved.
Requires a known standard capacitor (which may drift over time).
Measurement is sensitive to frequency and stray capacitance.
Precision capacitance standards calibration.
Measurement of dielectric loss (dissipation factor) using Schering bridges.
Option B (Tuned detector) is an instrument used to detect the null point, not the reference for measurement.
Option D (Vibration galvanometer) is a type of null detector used at low frequencies.
A is correct тАФ Unknown capacitance is determined by null-balance comparison against a known standard capacitor in an AC bridge.
Always verify if the bridge circuit is balanced for both resistive and reactive components, as capacitor leakage requires a parallel or series resistance balance.