Method

1. Measure Open Circuit Voltage:

• With the switch open, record the initial reading of the voltmeter, which measures the $emf$ $( \varepsilon)$ of the cell.

2. Close the Circuit and Measure Current and Voltage:

• Set the variable resistor to its maximum resistance, then close the switch. Record the terminal pd $V$ and the current $I$ from the ammeter.
• Open the switch after each reading to prevent the cell from discharging.

3. Reduce Resistance and Repeat:

• Decrease the resistance on the variable resistor incrementally, and repeat the measurements of $V$ and $I$.
• Obtain readings over a wide range of currents by continuing to decrease the resistance.

Safety

• Current Control: High currents can cause the wires and variable resistor to heat up. Use another resistor in series if necessary to limit current.
• Minimise Switch Closure Time: Only close the switch briefly to take each measurement, which prevents the cell from overheating and reduces potential changes in internal resistance due to discharge.

Key Concepts

• Electromotive Force (emf) $\varepsilon$: The maximum potential difference the cell can provide when no current flows (open circuit).
• Internal Resistance $r$ The resistance within the cell that causes the terminal pd to decrease as current flows.
• Equation Interpretation: The linear relationship $V = -rI + \varepsilon$ allows us to determine the internal properties of the cell by examining how the terminal pd changes with current.