will or left indefinitely in the position which gives the speed desired.] When the speed of a motor driving a constant torque machine is reduced by inserting resistance in the armature circuit there is no corresponding reduction in current consumed. The motor runs more slowly simply because a part of the energy impelling it is shunted into the resistance and there dissipated in the form of heat. Hence, whether the motor be operating at full speed or half speed, the amount of current consumed is the same; the only difference being that in the one case all the energy taken from the line is expended in driving the motor while in the other case only one half is utilized for power, the other half being dissipated in the resistance. Speed regulation by armature resistance only is therefore open to two objections: 1, the difficulty of maintaining constant speed under varying load conditions, and 2, the necessity of wasting energy to secure speed reduction. These objections are, in part, offset by the fact that speed reduction by armature resistance may be applied to any motor of standard design and requires nothing more than the simplest and least expensive speed regulating rheostat. In cases where the motor will be operated nearly always at full speed, the difference in first cost of the installation may justify the use of the armature resistance method of control. As a rule, speed regulation by shunt field resistance is preferable. [Illustration: Fig. 762.--Cutler-Hammer compound speed regulator with no voltage and overload release; _regulation by combined armature and shunt field resistance_, designed to both decrease and increase the speed of a motor. Speed reduction is accomplished by inserting resistance in the armature circuit, the maximum amount of speed reduction obtainable with these controllers being 50 per cent. below normal. Speed increase is obtained by inserting resistance in the shunt field circuit, the maximum amount of speed increase obtainable with these controllers being 25 per cent. above normal.] Regulation by Shunt Field Resistance.--Since regulation by this method is for speeds above normal, a starter must be used to bring the motor up to its rated speed. Usually the starter is combined with the regulator, as shown in fig. 761, the device being called a _compound starter_. [Illustration: Figs. 763 to 765.--Holzer-Cabot shunt wound motor; diagrams showing connections and positions of index point for forward and reverse rotation.