Based on the diagram provided:
8.2.1. The voltmeter V₁ measures the potential difference across the ends of a conductor.
8.2.2. The voltmeter V₂ measures the electromotive force (emf).
8.2.3. The voltmeter V₂ measures the terminal potential difference.
To measure the terminal potential difference, the condition is that the switch (S) must be closed or in the "ON" position.