Ans. Controllers having separate reversing cylinders are fitted with mechanical interlocks making it necessary to place lever in off position before reversing. [Illustration: Figs. 772 to 782.--Diagram of controller connections, illustrating the series parallel method of two motor control.] Two Motor Regulation.--With a two motor equipment, the controller becomes more complicated because it must be arranged to switch the motors in series or in parallel, so as to secure economy at half and full speed. The various connections of series-parallel regulation are shown in figs. 772 to 782. From these diagrams it is seen that the motors are first operated in series until all the resistance is cut out by the controller (figs. 772 to 777). The next point on the controller puts the two motors in parallel with some resistance in the circuit (fig. 778), which resistance is gradually short circuited on the remaining controller points, until at full speed all the resistance is cut out, the two motors remaining in parallel (figs. 778 to 782). Stopping a Motor.--If it be desired to stop a motor, the main switch is opened. As the armature of the motor continues to operate, due to its inertia, it generates an electromotive force which sends a current through the shunt connected field circuit and helps to maintain the field excitation. When the speed of the motor has decreased sufficiently so as not to endanger the motor should the main switch be thrown, the current in the series magnet becomes weakened, and the spring throws back the starting box arm. It should be noted that in stopping a motor having a starting box provided with a no voltage release simply open the main switch and do not touch the lever because otherwise, the self induced voltage of the field circuit may puncture the field winding or the insulation of the adjoining wires in the starting box. HAWKINS PRACTICAL LIBRARY OF ELECTRICITY IN HANDY POCKET FORM PRICE $1 EACH _They are not only the best, but the cheapest work published on Electricity. Each number being complete in itself. Separate numbers sent postpaid to any address on receipt of price. They are guaranteed in every way or your money will be returned. Complete catalog of series will be mailed free on request._ ELECTRICAL GUIDE, NO. 1 Containing the principles of Elementary Electricity, Magnetism, Induction, Experiments, Dynamos, Electric Machinery. ELECTRICAL GUIDE, NO. 2 The construction of Dynamos, Motors, Armatures, Armature Windings, Installing of Dynamos. ELECTRICAL GUIDE, NO. 3 Electrical Instruments, Testing, Practical Management of Dynamos and Motors. ELECTRICAL GUIDE, NO. 4 Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers, Storage Batteries. ELECTRICAL GUIDE, NO. 5 Principles of Alternating Currents and Alternators. ELECTRICAL GUIDE, NO. 6