D.C. bridges are used for

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Measurement of resistance

Explanation: D.C. bridges, such as the Wheatstone bridge, are fundamental electrical instruments used specifically for the precision measurement of unknown resistances. They operate on the principle of a null-balance circuit where an unknown resistance is determined by balancing the bridge arms under steady-state direct current conditions.

Working Principle: The bridge consists of four arms forming a closed loop with a DC supply connected across one pair of nodes and a null detector (galvanometer) across the other. When the potential difference across the detector is zero, the bridge is balanced. At balance, the product of the resistances of opposite arms is equal, allowing for the precise calculation of an unknown resistor based on the ratio of known resistors.

Key Formulas: $R_x = R_3 \cdot \frac{R_2}{R_1}$ — Standard balance equation for a Wheatstone bridge; $\frac{P}{Q} = \frac{R}{S}$ — Condition of bridge balance where $S$ is the unknown

Key Points: D.C. bridges use a null-deflection method which is independent of the source voltage fluctuations.; The Wheatstone bridge is suitable for medium resistance measurements (1 $\Omega$ to 100 k$\Omega$).; Kelvin's Double Bridge is a modification of the Wheatstone bridge for low resistance measurements.

Advantages: High accuracy due to null-balancing method.; Measurement is independent of the detector sensitivity once balanced.

Disadvantages: Not suitable for high resistance measurements (requires megger).; Requires a stable DC source.

Applications: Calibration of resistors.; Strain gauge measurement systems.; Fault location in underground cables.

Additional Info: Option B (capacitance) and Option D (inductance) require A.C. bridges (like Maxwell or Schering bridge) because these components exhibit frequency-dependent reactance.. Option C is incorrect as bridges measure impedance/resistance, not current directly.

Answer: A is correct — D.C. bridges are specifically designed to determine unknown resistance values by achieving a balanced potential state in a Wheatstone configuration.

Exam Tip: Always remember that D.C. bridges cannot be used for AC parameters (L, C) because the reactive components do not produce a stable balance point under steady DC bias.

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