This paper is an examination of the efficiency of the rheostatic speed control of a motor when the load is such that the torque is a definite function of the speed. It is shown that, so long as the torque decreases with speed, the controller losses for a given reduction in speed may be considerably less than in the case of a motor working against a constant load torque; also, the greater the reduction in speed, the greater will be the advantage of rheostatic control in the case of such loads. The case of both direct-current and induction motors is considered, and it is shown that, with the latter motor, the inefficiency due to controller losses may be less important than the lowering of the power factor.