When measuring the potential difference across a single battery connected in series with another battery, the voltmeter will display the emf of the single battery, not the sum of the emfs of both batteries. This is because the voltmeter measures the voltage drop across the specific battery it is connected to, which is influenced by its internal resistance. The internal resistance of the battery can affect the reading, especially under load. Drawing out the circuit can help visualize the connections and clarify how the voltmeter interacts with the batteries. Understanding these principles is crucial for accurate measurements in electrical circuits.
