Why can an electromagnetic system instrument be used for both AC and DC circuits, while magnetoelectric instruments are used in DC circuits only?

The electromagnetic field instrument, when the magnetic field is produced after the fixed loop is all current (or voltage), the dynamic and static iron core in the measuring mechanism are magnetized. The iron core produces torque under the action of electromagnetic force, and the movable part indicator indicates the size to be measured. When the measured current changes direction, the polarity of the moving and static core is changed at the same time, and the direction of the rotation torque remains unchanged. Therefore, the electromagnetic system instrument can be used for both direct current circuit and AC circuit. When the magnetoelectric meter enters sinusoidal alternating current, the deflection of the instrument can only reflect the average value of the instantaneous torque due to inertial influence of the movable part of the instrument. For sinusoidal alternating current, the average value of the torque in one cycle is zero, and the movable part of the instrument does not deflect. So magnetoelectric instruments can not be directly used in AC circuits.