When a charged capacitor discharges through a resistor, the voltage decays exponentially with time. The time constant τ = RC determines how quickly this happens. By measuring τ for different resistance values, you can verify the linear relationship τ = RC and calculate the capacitance of the capacitor. A data logger makes this investigation particularly powerful, generating rich continuous voltage-time data for each resistance. This is an outstanding IB Diploma Physics HL Internal Assessment topic.