Final answer:
It is true that the Thévenin voltage can be measured by measuring the open-circuit voltage across the load terminals using a high resistance voltmeter.
Step-by-step explanation:
The statement that the value of the Thévenin voltage can be measured experimentally by measuring the open-circuit voltage across the load terminals using a high resistance voltmeter is True. When measuring potential differences in a series circuit, the voltmeter is placed in parallel with the component across which the voltage is measured. This is critical because, as stated, measuring directly across a source of electromotive force (emf) such as a battery would also include the internal resistance of the battery (r), thus affecting the measurement. Therefore, the open-circuit voltage measured this way represents the Thévenin equivalent voltage of the entire network seen from the load terminals when the load is removed.
Furthermore, using a voltmeter with high resistance is vital because this ensures that very little current flows through the voltmeter, avoiding significant alterations to the circuit behavior. Voltmeters with high resistance effectively have a negligible impact on the circuit, enabling accurate measurements of the voltage.