Potentiometer and Null Measurement

A potentiometer uses a long, uniform resistance wire as a voltage scale. A steady driver current makes equal wire lengths have equal potential drops.

Move the jockey until the galvanometer shows zero. At this balance or null point, the test cell sends no current through the galvanometer branch.

For potential difference V across uniform wire length L, potential gradient is the potential drop per unit length.

At balance, V_l equals test emf ε. If two cells are balanced without changing driver current, k stays constant.

• Use a uniform wire and steady driver current.
• Connect correct polarity so the test emf opposes the wire drop.
• Maximum wire drop must be at least the test emf.
• Smaller k gives a longer balance length and greater sensitivity.

To find internal resistance, first measure open-circuit balance length l₁. Connect known resistance R across the cell and measure terminal-voltage balance length l₂.

The null method avoids loading error because the test cell supplies no current at balance.