Question

The AC voltage across a resistance can be measured using a :

• A. potentiometer
• B. moving coil galvanometer
• C. moving magnet galvanometer
• D. hot wire voltmeter

Answer 1

The Correct Option is D

Let's analyze the options to determine which instrument can be used to measure the AC voltage across a resistance:

1. Potentiometer: This device is typically used to measure DC voltage, as it relies on balancing lengths in a wire to determine voltage. It is not suitable for measuring AC voltage.
2. Moving Coil Galvanometer: This instrument is designed to measure direct current (DC) in a circuit by deflecting a coil in a magnetic field. It is not suitable for measuring AC voltage as AC voltage would cause the coil to oscillate rather than deflect steadily.
3. Moving Magnet Galvanometer: Similarly, this device is also more tailored towards DC measurements rather than AC. It works on the principle of electromagnetic induction.
4. Hot Wire Voltmeter: This instrument is specifically designed to measure AC voltage. It works by passing the current through a thin wire, which heats up due to the Joule effect. The thermal expansion is used to move a pointer, indicating the voltage value. It can measure RMS (Root Mean Square) values of AC voltage, making it suitable for the given task.

Therefore, the most suitable instrument for measuring the AC voltage across a resistance is the hot wire voltmeter.

Conclusion: The correct answer is the hot wire voltmeter as it accurately measures AC voltage by utilizing the heating effect of current.