In voltage measurement, the potentiometer is preferred to voltmeter because it

Analyze the question: It asks why a potentiometer is preferred to a voltmeter when measuring voltage (e.g., the e.m.f. of a cell).

Concept: A voltmeter has a finite resistance, so when connected across a cell it draws some current. That current causes a voltage drop across the cell's internal resistance, so the voltmeter reads the terminal p.d., which is slightly less than the true e.m.f.

A potentiometer uses the null (balance-point) method. At the balance point, the galvanometer shows zero deflection, which means no current flows through the cell or source being measured. The potentiometer therefore measures the true e.m.f. without disturbing the circuit.

Step-by-step check of the options:

• A. Wider range: Not true. Voltmeters can have multiple ranges, and a potentiometer's range is limited by the driver cell and wire.
• B. Consumes negligible current: True. At balance, no current is drawn from the source, so the measurement is accurate. In effect, it behaves like a voltmeter of infinite resistance.
• C. Is bulky: This is a disadvantage, not a reason to prefer it.
• D. Responds faster: Not true. A potentiometer needs time to find the balance point, so it is slower than a voltmeter.

Answer: B, because the potentiometer draws negligible (essentially zero) current at balance.

Exam tip: Remember "Potentiometer = Perfect, no current drawn at null." Whenever JAMB asks why a potentiometer is better than a voltmeter, think of no current drawn, so accurate e.m.f.