SOLUTION

Moving Iron (MI) instruments can be used for both AC and DC measurements:

• AC Measurement: When used with AC, the AC current constantly changes direction (positive and negative cycles). The moving iron element is attracted or repelled by the magnetic field. Crucially, the force acting on the iron is proportional to the square of the current (I²). Because squaring any number (positive or negative) results in a positive value, the force is always in one direction, regardless of the AC current’s instantaneous polarity. It allow the instrument to measure the RMS (Root Mean Square) value of the AC signal.
• DC Measurement: When used with DC, the current is constant, producing a steady magnetic field. The iron piece deflects proportionally to the square of this constant current, giving a stable reading.