What is a wattmeter?

• A. An instrument for measuring electrical power in watts
• B. A device to measure resistance
• C. A device to measure current
• D. A device to convert energy to light

Answer: (A)

A wattmeter measures electrical power in watts. In AC circuits this power equals the product of voltage, current, and the cosine of the phase angle between them (P = V I cos φ), so a wattmeter effectively combines the voltage and current information to determine how much real work is being done per second. Mechanically, it uses a current coil and a voltage (potential) coil so the deflection is proportional to the average product of instantaneous voltage and current. In DC circuits, where the waveforms are in phase, this reduces to P = V I, so the meter simply multiplies voltage and current. It’s distinct from tools that measure resistance (ohmmeter) or current alone (ammeter), or from devices that convert energy into light (like lamps). Some wattmeters can also indicate power factor or reactive power, but the basic idea is that the reading tells you how much real power is being used, in watts.