wattmeters

Wattmeters are instruments that measure electrical power in watts. In DC circuits, a wattmeter directly measures P = VI. In alternating-current circuits, wattmeters measure real power P, equal to VI cos(phi), where phi is the phase angle between voltage and current. They are essential for assessing load, efficiency, and power quality.

Most traditional wattmeters use two electromagnetic coils: a current coil placed in series with the circuit and a potential coil connected across the supply. The movable element (a pointer on a calibrated scale or a rotating coil in an electrodynamic design) experiences a torque proportional to the instantaneous product of voltage and current, and after suitable averaging yields a reading proportional to real power.

Two main families exist: electrodynamic wattmeters (moving-coil) and electronic/digital wattmeters. The latter sample voltages and currents with sensors or transformers, perform a digital computation to obtain true power, reactive power, apparent power, power factor, and energy. Modern instruments may be true-RMS and able to handle non-sinusoidal waveforms, providing more accurate measurements in advanced power systems.

Applications include laboratory instrumentation, measurement of electrical load and efficiency, energy meters in utility networks, motor testing, and power quality analysis. Wattmeters are commonly used in both research and industry to characterize electrical devices, verify specifications, and support energy management and grid operations.