[Illustration: Fig. 770.--General Electric type K7 controller with cover open showing construction. The mechanism consists of a long spindle, carrying a number of heavy brass or gun metal segments, making contact for a longer or shorter time with a corresponding number of spring contacts. The spindle is provided at its upper end with a handle, and the various contacts are made by turning it through an arc of about 150°. For this method a moderate amount of resistance is employed. The first contact joins both motors and the full amount of resistance in series across the line, and as the motors are standing still, maximum current flows so that they exert their full torque. The moment they start to revolve, the current tends to fall, due to the generation of a reverse voltage; to prevent this and maintain a heavy current for some time, thus obtaining rapid acceleration, the resistance is arranged so that it can be gradually reduced, until at about the fourth notch the two motors are in series without resistance across the line. To increase still further the speed in the above type of controller, the series fields may be shunted, and then the next steps place the motors in parallel with the resistance.] Speed Regulation of Traction Motors.--The speed regulator for motors of this class is called a _controller_, and being located in an exposed place is enclosed in a metal casing. Controllers are designed to be used for starting, stopping, reversing, and regulating the speed of motors where one or more of these operations have to be frequently repeated. The controller used with a single motor equipment is practically the same as any other single motor starting box, excepting that the resistance has sufficient carrying capacity to be left in the circuit some time. When the motor is to operate at full speed all the resistance is cut out. To reverse, a reversing notch is placed in the armature or field circuit, but not in both. Ques. What provision is made to overcome the arc when the circuit is opened? Ans. A magnetic field is used with such polarity that it blows out the arc. [Illustration: Fig. 771.--Controller of the Rauch and Lang electric vehicles. It is of the flat radial type. Two movable copper leaf contacts of ample size make all commutations necessary to obtain the various speeds. Five speeds forward and reverse are provided.] Magnetic blow out coils are used on all controllers designed for 500 volt circuits, and on types designed for lower voltages requiring more than 60 amperes normal capacity. The coils are wound with either copper wire or flat strips of sufficient capacity to carry full load current continuously without undue heating, and after being wound they are treated with an insulating compound making them moisture proof. Ques. What provision is made to prevent reversal before bringing the controller lever to the "off" position?