Which of the following meters must use internal power to check a load or circuit?

• A. Voltmeter
• B. Multimeter
• C. Ohmmeter
• D. Ammeter

Answer: (C)

Measuring resistance requires a test current to be pushed through the unknown component in order to observe the resulting voltage and compute R. An ohmmeter provides that by using its own internal power source (a small battery) to drive a known current through the resistance and then measures the voltage drop to display the resistance. Because of this built‑in power to energize the test, the ohmmeter is the instrument that must use internal power to check a load or circuit.

A voltmeter, by contrast, is designed to measure voltage with very high input impedance and does not source current into the circuit. An ammeter is placed in series to measure current and is designed to have low resistance so it minimally affects the circuit; it also does not source a test current to the load. A multimeter can measure resistance in its ohmmeter mode, but that mode relies on the same internal source to energize the test, which is why the instrument that must power the test is the ohmmeter.