Which of the following instruments is used to determine the accurate value of the electromotive force of a cell?

Analyze the question: We need the instrument that gives the accurate value of a cell's e.m.f.

Explain the concept:

• The e.m.f. of a cell is the potential difference across its terminals when no current is drawn from it (open circuit).
• A voltmeter draws a small current, because it has finite resistance. Some voltage is lost across the cell's internal resistance, so the reading is slightly lower than the true e.m.f.
• A potentiometer uses the null (balance-point) method. At the balance point, the galvanometer shows zero deflection, so no current flows from the cell being tested. The measured value is therefore the true e.m.f.

Go through the options:

• A. Voltmeter: Gives only an approximate value, since it draws current.
• B. Meter bridge: Used to compare or find resistances (Wheatstone bridge principle), not e.m.f.
• C. Voltameter: Used for electrolysis. It measures the amount of substance deposited, and is not a voltage-measuring device.
• D. Potentiometer: Gives an accurate e.m.f. through the null method, with no current drawn at balance.

Working: For a potentiometer wire with a uniform potential gradient, E₁/E₂ = l₁/l₂. Comparing the balance length of an unknown cell with that of a standard cell (known e.m.f.) gives the unknown e.m.f. accurately.

Answer: D. potentiometer

Exam tip: Remember "Potentiometer = Precise e.m.f." Likewise, "meter bridge = resistance" and "voltameter = electrolysis". These are common WASSCE and JAMB distractors.