When the watt-hour meter is connected to the circuit under test, there is alternating current flowing through the current coil and the voltage coil, and these two alternating currents produce alternating magnetic flux in their iron cores respectively. The alternating flux passes through the aluminum plate and induces eddy currents in the aluminum plate. The eddy current is subjected to force in the magnetic field, so that the aluminum disk gets torque (active moment) and rotates. The greater the power consumed by the load, the greater the current through the current coil, the greater the eddy current induced by the aluminum plate, and the greater the torque of rotation of the aluminum plate.

That is, the size of the torque is proportional to the power consumed by the load. The greater the power, the greater the torque, and the faster the aluminum disk rotates. When the aluminum disk rotates, it is subjected to the braking torque generated by the permanent magnet, and the braking torque is opposite to the active torque. The size of the braking torque is proportional to the speed of the aluminum disc, and the faster the aluminum disc rotates, the greater the braking torque. When the active moment and the braking moment reach a temporary balance, the aluminum disc will rotate at a constant speed. The electrical energy consumed by the load is proportional to the number of revolutions of the aluminum disk. When the aluminum plate rotates, it drives the counter to indicate the energy consumed. This is the simple process of how the energy meter works.