USCG Exam PracticeHand/power tools, measuring instruments, and maintenance
A wattmeter reads true power in an AC circuit rather than apparent power because the instrument does which of the following?
- A. Measures only the voltage component of the circuit using a high-resistance coil in parallel
- B. Multiplies RMS voltage by RMS current without regard to phase angle
- ✓ Uses a current coil in series and a voltage coil across the load, producing deflection proportional to voltage, current, and the cosine of the phase angleCorrect
- D. Applies a high DC test voltage to determine the resistive component of the load
Why C is correct
The wattmeter uses two coils: a low-resistance current coil in series with the load and a high-resistance voltage coil across it. Deflection is proportional to the product of voltage, current, and the cosine of the phase angle, so it reads true power directly and automatically accounts for power factor.
Cited:NEETS Mod. 3 §1-4