Which of the following instrument is most accurately for comparing electromotive force (e.m.f) of two cells?

Analysis

The question asks for the most accurate instrument to compare the electromotive force (e.m.f.) of two cells. In physics, accuracy is determined by how well an instrument can measure a value without drawing current from the source, which would otherwise cause a voltage drop due to internal resistance.

Concept Explanation

• Voltmeter (Option A): A voltmeter has a finite internal resistance. When connected across a cell, it draws a small amount of current. This causes a potential drop across the internal resistance of the cell, meaning the voltmeter measures the terminal potential difference rather than the true e.m.f.
• Potentiometer (Option D): A potentiometer works on the null-point principle. When the circuit is balanced (the galvanometer shows zero deflection), no current flows from the cell being tested. Because no current is drawn, the potential difference measured is exactly equal to the e.m.f. of the cell. This makes it the most accurate method for comparing e.m.f.s.
• Galvanometer (Option C): A galvanometer is used to detect the presence of current, not to measure e.m.f. directly.
• Meter rule (Option B): A meter rule is a tool for measuring length, not electrical potential.

Step-by-Step Solution

1. Understand the requirement: To compare e.m.f. accurately, we must measure the potential difference when the cell is in an "open circuit" (no current flowing).
2. Evaluate the instruments: • Voltmeters are convenient but introduce errors because they draw current. • Potentiometers use a sliding contact to find a point where the potential difference of the cell is balanced by a potential drop across a wire, resulting in zero current flow.
3. Conclusion: Since the potentiometer draws no current at the balance point, it provides the most precise and accurate comparison of e.m.f.s.

Exam Tip

Whenever you see a question about "accuracy" in measuring e.m.f. or internal resistance, always look for the Potentiometer. Remember: Potentiometer = Null-point = No current drawn = High accuracy.