When the watt-hour meter is plugged into the tested circuit, alternating currents flow through the current coil and the voltage coil, the two alternating currents produce alternating magnetic flux in their cores, alternating magnetic flux through an aluminum disk, induction of eddy currents in the aluminum plate, and eddy current in the magnetic field, thus allowing the aluminum disc to be rotated by torque (active torque). The greater the power consumed by the load, the greater the current in the coil, the larger the eddy currents in the aluminum disk, the greater the torque of the aluminum disc. The size of the torque is proportional to the power consumed by the load. The greater the power, the larger the torque, the faster the aluminum plate rotates. When the aluminum disc rotates, it is also affected by the braking moment produced by the permanent magnet, the brake torque is opposite to the active moment; the braking torque is proportional to the speed of the aluminum disc, the faster the aluminum plate rotates, the greater the braking torque. When the active moment and braking torque reach the temporary balance, the aluminum plate will be Shiting. The energy consumed by the load is proportional to the conversion of the aluminum disc. When the aluminum disc rotates, the counter is driven to indicate the energy consumed. This is the simple process of working on a watt-hour meter.