8. A 200 V d.c. series motor takes line current of 25 A when runs at 500 rpm. The armature resistance is 0.5 12 and the series field resistance is 0.3 2. If the load torque remains constant, find the value of the additional resistance to be inserted in series with the armature to reduce the speed to 250 rpm. [Ans: 3.6 2]