A separately excited $DC$ motor runs at $1000$ rpm on no load when its armature terminals are connected to a $200$V DC source and the rated voltage is applied to the field winding. The armature resistance of this motor is $1 \Omega$. The no-load armature current is negligible. With the motor developing its full load torque, the armature voltage is set so that the rotor speed is $500$ rpm. When the load torque is reduced to $50$% of the full load value under the same armature voltage conditions, the speed rises to $520$ rpm. Neglecting the rotational losses, the full load armature current (in Ampere) is ____________.