Under the conditions of maximum field strength, as with switch S on point 1, the torque will be greatest for any given current strength and the reverse voltage also greatest at any given speed. The current through the armature of the motor, to perform any given work, will thus be a minimum, as well as the speed at which the motor has to run, in order to develop sufficient reverse voltage to permit this current to flow. Regulation of speed by varying the field strength is limited in range of action, since the field saturation point is soon reached, moreover, with too low a field strength, armature reaction produces excessive field distortion, sparking, etc. [Illustration: Fig. 757.--Speed regulation of series motor by cutting out sections of the field winding. In this method the field winding is tapped at several points, dividing the coil into sections and the leads from these points are connected a multi-point switch of the type that would be used on a rheostat. By moving the lever S, to the left or right, the current will flow through one or more sections of the field winding, thus decreasing or increasing the ampere turns and thereby providing means of regulation.] [Illustration: Fig. 758.--Speed regulation of a series motor by the method of short circuiting sections of the field winding. It will be seen that there are seven different positions for the contact springs on the barrel contacts. A. represents the armature and brushes, little A, B, and C, the divided field magnet coils, L the line connection, and G the earth connection. The diagram shows the connections for trolley car operation.] Ques. How is the speed of shunt and compound motors varied with respect to the normal speed in the two methods? Ans. The first method (variable resistance in armature circuit) reduces the speed _below_ the normal or rated speed of the machine, while the second method increases the speed _above_ the normal. In the first method the amount of speed reduction depends partly upon the amount of resistance introduced into the armature circuit, and partly upon the load. In the second method the amount of speed increase depends entirely upon the amount of resistance placed in the shunt winding circuit. Eighty-five per cent. is about the maximum speed reduction obtainable by armature resistance but so great a reduction is seldom satisfactory since comparatively slight increases in the load will cause the motor to stall. Shunt field regulation may be obtained up to any point for which the motor is suited, the only limitation in this case being the maximum speed at which the motor may be safely operated. It should be remembered, however, that speed increase by shunt field weakening increases the current in proportion to the increase in speed, and care should be taken not to overload the armature.