The working principle of watt-hour meter is: when the watt-hour meter is connected to the circuit under test, alternating current passes through the current coil and the voltage coil, and these two alternating currents generate alternating magnetic flux in their iron cores respectively; The alternating magnetic flux passes through the aluminum disk and induces eddy current in the aluminum disk; The eddy current is forced in the magnetic field, which makes the aluminum disk gain torque (active torque) and rotate. The greater the power consumed by the load, the greater the current flowing through the current coil, the greater the eddy current induced in the aluminum disk, and the greater the torque that makes the aluminum disk rotate. That is, 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 acted by the braking torque generated by the permanent magnet, which is opposite to the driving torque; The braking torque is proportional to the rotating speed of aluminum plate, and the faster the rotating speed of aluminum plate, the greater the braking torque. When the active torque and braking torque reach a temporary balance, the aluminum disk will rotate at a uniform speed. The electric 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 display the consumed electric energy. This is the simple process of watt-hour meter working.