Q. Why can a DC ammeter not read AC?
a) Dc ammeter can't measure ac because ac is changing continuously and periodically as well as a dc ammeter can just measure a constant current.
b) The usual moving coil dc ammeter is based on the torque generated on a current carrying loop in a magnetic field provided by a permanent magnet.
c) Ever since the current in an ac averages to zero it is changing too fast even at 60 Hz, the meter doesn't have time to respond to this because of the inertia of the coil. The average torques the coil experiences in a specified time interval is zero and hence there is no deflection.