The direction of force exerted on moving coil depends on the direction of current flowing through moving coil. If the direction of magnetic field is kept constant, it produces unidirectional torque if the current is unidirectional in the moving coil. Thus the D.C. current when passed through the coil; unidirectional torque is produced as the direction of current is constant. But in case of A.C. the direction of current reverses in alternate half cycles of A.C. Hence force exerted on moving coil in positive half cycle acts in opposite direction to that in negative half cycle. This makes the average torque acting on the coil in one cycle to zero. Hence the meter cannot read A.C. quantities.