Why the DC potentiometer method is more accurate than a direct voltmeter reading: identify the primary reason related to circuit loading.
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Ait loads the circuit to the maximum extent
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Bit loads the circuit moderately
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Cit does not load the circuit at all (null-balance measurement)
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Dit uses a centre-zero galvanometer instead of voltmeters
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Eit averages out the noise by integration
Answer
Correct Answer: it does not load the circuit at all (null-balance measurement)
Explanation
Introduction / Context:DC potentiometers determine an unknown electromotive force (EMF) by balancing it against a known, adjustable reference drop along a calibrated resistor. At balance (null), no current is drawn from the unknown source, which eliminates loading error that occurs with finite-impedance voltmeters.
Given Data / Assumptions:
- Null-balance condition is achieved (galvanometer shows zero deflection).
- Reference supply and wire are properly standardized with a known cell.
Concept / Approach:
With no current drawn from the unknown EMF at balance, the measured value equals its open-circuit value. A voltmeter, even a high-impedance one, draws some current and thus can slightly reduce the measured voltage of a high-source-resistance circuit. Therefore, potentiometers provide superior accuracy for small or delicate sources.
Step-by-Step Solution:
Connect unknown source to the potentiometer via a galvanometer.Adjust the sliding contact until galvanometer indicates zero (null current).Read EMF using the calibrated length × current (or directly the scale).Since i_unknown = 0 at null, loading error = 0.Verification / Alternative check:
Comparisons with high-quality digital voltmeters show agreement; deviations arise only from calibration errors, not loading, validating the principle.
Why Other Options Are Wrong:
- Options (a) and (b) contradict the null method.
- (d) Centre-zero galvanometer is an indicator, not the reason for accuracy.
- (e) Potentiometers do not integrate to average noise.
Common Pitfalls:
- Assuming “infinite” DVM input means identical accuracy—source loading can still matter for very high source resistance or microvolt-level sources.
Final Answer:
it does not load the circuit at all (null-balance measurement)