An electrodynamic instrument that contains two coils, one fixed and one movable, and produces a deflection equal to the square of the current passing through the coils, which are connected in series. It is frequently used as a standard for current or voltage measurements.

For use as a wattmeter, the fixed coil is connected in series with the load and the movable coil shunted across a high noninductive resistance. Transformer coupling to the current circuit may be used to isolate the instrument from the main current. The couple produced is then proportional to the wattage.