Why the DC potentiometer method is more accurate than a direct voltmeter reading: identify the primary reason related to circuit loading.

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:

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)