Which of the following is used to measure a current ?

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

Answer: (B)

Measuring current requires a device that carries the same current as the circuit. An ammeter is designed for this: it’s placed in series with the circuit so all current flows through it, and it has very low internal resistance to minimize altering the circuit, giving an accurate current reading. The other instruments have different roles: an ohmmeter checks resistance by sending a small current and measuring response to compute R, not for direct current measurement in a live circuit; a wattmeter determines power by measuring both voltage and current (and sometimes phase), so it’s not a direct current reader; a voltmeter measures voltage and is designed to draw very little current, so it cannot measure current by itself. So the instrument used to measure current is the ammeter.