The resistance can be measured most accurately by

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Examoogle · Accepted Answer

Bridge method

Explanation: The bridge method (such as the Wheatstone bridge) is the most accurate technique for measuring resistance because it relies on the principle of a null-balance condition. Unlike deflection-type instruments, bridge circuits are independent of meter calibration and source fluctuations, making them ideal for high-precision measurements.

Working Principle: The Wheatstone bridge consists of four arms forming a closed circuit. An unknown resistance $R_x$ is placed in one arm and compared against precision variable resistors. When the galvanometer shows zero deflection (null balance), the potential difference across the galvanometer is zero, and the ratio of resistances is equal, i.e., $\frac{R_1}{R_2} = \frac{R_x}{R_3}$.

Key Formulas: $R_x = R_3 \cdot \frac{R_1}{R_2}$ — Determining unknown resistance at balance; $\Delta R = R \cdot \frac{\Delta I}{I}$ — Sensitivity relation

Key Points: Bridge circuits operate on a ratio-metric comparison, eliminating errors from source voltage variations.; The sensitivity of the bridge depends on the galvanometer sensitivity and the applied bridge voltage.; Bridges are classified into DC bridges (e.g., Wheatstone for medium resistance) and AC bridges (e.g., Maxwell/Hay for inductance).; Null methods are inherently more accurate than deflection methods.

Advantages: Extremely high accuracy compared to direct deflection methods; Independent of supply voltage fluctuations; Minimal dependency on internal resistance of the indicating instrument

Disadvantages: Requires manual balancing, making it slower than digital multimeters; Not suitable for continuous monitoring of rapidly changing resistances

Applications: Laboratory-grade resistance measurements; Strain gauge measurement circuits; Calibration of standard resistors

Additional Info: Voltmeter-Ammeter method is susceptible to errors due to the internal resistance of the meters.. Multimeters are convenient but lack the precision required for calibration standards.. Megger is specialized for high resistance (insulation) measurement and uses a ratio-coil mechanism rather than bridge balance.

Answer: B is correct — The bridge method utilizes a null-deflection technique which provides superior accuracy by comparing the unknown resistance against known precision resistors.

Exam Tip: Remember that for low resistance measurements, the Kelvin Double Bridge is used, while for very high resistance, the Megger or Loss of Charge method is preferred.

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