Inductance comparison bridge is used to compute

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Unknown inductance and resistance

Explanation: An Inductance Comparison Bridge is a modified Wheatstone bridge circuit used to measure an unknown self-inductance by comparing it with a known standard inductance. Because practical inductors are not ideal and possess inherent winding resistance, the bridge simultaneously determines both the unknown inductance and its associated series resistance.

Working Principle: The bridge operates by balancing four arms: one containing the unknown inductor with its internal resistance, one containing a high-quality standard inductor with an adjustable resistor, and two non-inductive ratio arms. Under null conditions, the bridge is balanced such that the product of impedances of opposite arms is equal ($Z_1Z_4 = Z_2Z_3$). Since both arms contain resistance and inductance, balance is achieved by adjusting both a standard variable resistor and the variable standard inductor simultaneously.

Key Formulas: $L_x = L_s \frac{R_2}{R_3}$ — Balancing equation for inductance; $R_x = R_s \frac{R_2}{R_3}$ — Balancing equation for resistance

Key Points: The bridge requires a known standard inductor ($L_s$) which is often expensive and bulky.; Balance is achieved by adjusting $R_s$ and $L_s$ to nullify the detector current.; It is primarily used for the measurement of inductors with medium Q-factors.; Unlike the Maxwell Bridge, it uses a standard inductor rather than a capacitor.

Advantages: Balance equations are independent of the supply frequency.; Can be used to measure inductors over a wide range of values.

Disadvantages: Requires a high-quality standard variable inductor, which is costly.; More complex to construct than capacitance-based bridges like the Maxwell or Hay bridge.

Applications: Precision measurement of self-inductance in laboratories.; Standardizing inductors used in calibration circuits.

Additional Info: The bridge is essentially a variation of the Wheatstone bridge where resistances are replaced by complex impedances.. Option D is incorrect because capacitance measurement requires a Schering or Wien bridge, not an inductance comparison bridge.

Answer: A is correct — The inductance comparison bridge measures both the unknown inductance and its associated internal series resistance by balancing against a known standard inductor.

Exam Tip: Always remember that in AC bridges, the balance condition involves both magnitude and phase, requiring the adjustment of two variables (usually a resistor and an inductor/capacitor) to satisfy both real and imaginary components.

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