19. Electro-Magnet on Starting Box.—As was explained under electric motors, a starting box is simply a series of resistance coils r, r, r, r, r, in [Fig. 49]. When the motor is not in use the switch l rests upon the point 1 and no electric current passes. When the switch is moved to point 2, the current entering at a passes to the pivot of the switch and up the metal strip l to the point 2, then around the series of coils, r, r, r, r, r, to the post b and thence back to the generator. As the switch is moved to the right, the current passes through less and less of this resistance until, when it reaches point 7, all the coils of resistance are "cut out," that is, they are not in the path of the current. Now the motor has reached its full speed and is developing enough counter-electro-motive force to protect itself against too much current. Through a shunt, however, a portion of the current passes from a to b around the electro-magnet e, the two poles of which are presented to the metal strip l, which must be of iron. This magnet holds the switch over so long as the current is on, but when the current is cut off, by opening a switch in the line wire, e ceases to be a magnet and l is carried back to point 1 by a spring. Thus an extra resistance must always be in circuit when the motor is first started. Those who start motors are expected to move the lever l of the starting box slowly from point to point, pausing a second or two on each to give the motor time to acquire proper speed for its protection. How too great a current would "burn out" a motor will be explained later.
The motor man handles a lever for starting his car, which works like that of the "starting box." His "starting box," however, is called a "controller." Although it accomplishes the same result as the starting box it has a wholly different and vastly more complex mechanism than that already described.
The elevator boy, who runs our electric elevator, handles a lever which also does the same thing through far different mechanism. Indeed, in his case electro-magnets are used to prevent him from cutting out resistance too fast if he should move his lever too quickly.
20. Starting Switches for Electric Elevators.—The motor man has to be instructed particularly how he should handle the lever of his controller, and he is trusted to follow his directions to some extent, however lacking in intelligence and integrity he may be. But the elevator boy receives scarcely any instructions about his machine, and, indeed, his machine has been constructed pretty nearly "foolproof." It will automatically correct his errors of management. If he throws the handle from one extreme to the other, all resistance cannot be thrown out instantly, but this is accomplished by a series of electro-magnets closing one switch after another and thus cutting out resistance gradually.
21. Arc Lamp Feed.—As will be explained later, an arc lamp must have its carbons touching one another when the current is first thrown on, and then the carbons must be drawn apart from a quarter to half an inch. The upper carbon is lifted away from the lower one by a portion of the current passing by means of a shunt around an electro-magnet.
Fig. 50
