Which instrument is used to measure resistance?

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

Answer: (C)

Resistance is the opposition to current flow. An ohmmeter is built to determine that opposition by applying a small test voltage (or current) to the component and measuring the resulting current (or voltage). From these measurements, it computes and displays the resistance in ohms using the relationship R = V / I. This direct use of the V and I relationship is why the ohmmeter is the instrument used to measure resistance.

In contrast, an ammeter measures current (placed in series), a voltmeter measures voltage (placed in parallel), and a wattmeter measures power (often combining voltage and current, and sometimes phase information). Because resistance is defined by the ratio of voltage to current, the device that directly assesses that ratio is the ohmmeter.

For accurate resistance readings, power should be off and the component should be isolated from other parallel paths, since other paths can affect the measurement.