Which instrument is designed to minimize circuit disturbance when measuring current?

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

Answer: (A)

Measuring current with as little effect on the circuit as possible relies on using a device that has very low resistance and is placed in series with the circuit. An ammeter is designed for this purpose, with a small internal resistance so it drops only a tiny voltage and doesn’t significantly alter the current flowing through the circuit. That makes it the best choice for measuring current without disturbing the circuit’s behavior.

A voltmeter, by contrast, is built to have high resistance and is connected in parallel to measure voltage; using it to measure current would distort the circuit because it would not pass the same current as the load. A wattmeter measures power and combines both current and voltage, so its design isn’t focused on minimizing disturbance for a lone current measurement. An ohmmeter injects its own current to measure resistance and isn’t used in powered circuits, so it would disturb or damage the circuit if used to measure current directly.