Why does an ordinary moving coil ammeter used for DC, cannot be used to measure AC?

Hint: The magnitude and direction of alternating current change, and the mean value of ac throughout a whole cycle is zero. As a result, when linked in an AC circuit of any frequency, a moving coil ammeter will always read zero.

Complete answer:
Because the essence of a moving coil ammeter is to transfer electrical energy into mechanical energy by stimulating a coil. The amount of current in the coil now determines how much it is stimulated and creates a precise amount of torque, which is indicated on the display by a needle. The magnitude and direction of alternating current change, and the mean value of ac throughout a whole cycle is zero.

As a result, when linked in an AC circuit of any frequency, a moving coil ammeter will always read zero. This is why a standard moving coil ammeter for measuring DC cannot be used to measure AC, the ammeter is never able to achieve a specific value because it never reaches equilibrium.

Note: It should be noted that because the operation of induction type instruments is dependent on the production of torque due to the reaction between two magnetic fluxes with some phase difference between the flux of an AC magnet and the eddy current induced by this flux, the alternating current is measured by an induction ammeter. These instruments are only used to measure AC current.