The function of a wattmeter is to measure what quantity?

• A. Voltage
• B. Power
• C. Resistance
• D. Current

Answer: (B)

Power is what a wattmeter shows—the rate at which electrical energy is delivered or consumed. In a DC circuit, power equals voltage times current, and a wattmeter effectively measures that product. In AC circuits, real power is V times I times the cosine of the phase angle between voltage and current (the power factor), so the wattmeter reads power in watts. The other instruments measure different quantities: a voltmeter reads voltage, an ammeter reads current, and an ohmmeter reads resistance. So the wattmeter is specifically used to indicate power, not voltage, current, or resistance.