What characteristic allows a wattmeter to measure both AC and DC power?

A wattmeter is designed to measure electrical power accurately, regardless of whether the current is alternating current (AC) or direct current (DC). The key characteristic that allows for this versatility is its ability to measure the direction of electrical flow.

In the context of power measurement, the wattmeter typically consists of two coils: one for voltage and one for current. The interaction between these coils fundamentally relies on the flow of current, which can be in either direction. For AC, the current reverses direction periodically, while for DC, the current flows in a single, consistent direction. The wattmeter effectively translates these directional flows into usable power readings for both types of current.

This capacity to detect the direction of electrical flow ensures that the wattmeter can measure the instantaneous power and integrate it over time to yield total power—be it AC or DC. The calibration of the scale, the specific type of resistors used, and the frequency of the power source are important factors for accurate readings but do not inherently contribute to the wattmeter's fundamental ability to measure both AC and DC power. The critical factor remains the measurement of directional flow, enabling the wattmeter to function across different types of electrical systems.