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Page:Popular Science Monthly Volume 56.djvu/426

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412
POPULAR SCIENCE MONTHLY.

sired to reverse the car, h is brought back to the stop position, h is shifted to the reverse motion, and then h is again turned to the running position. When the controlling switch is provided with only one handle this is turned in one direction to run the car ahead, and in the opposite direction to run it backward, the graduations in velocity being obtained by placing the handle in positions intermediate between the stop position and the highest speed position.

As will be noticed, the wire d d branches at c and runs in both directions. Now, when the controller handles are both turned to the stop position the current from the trolley can get no farther than the ends of d in either switch, but if one of them is turned to the running position, the current at once passes to the wires in the cable e e e, and thus to the two motors. If the switches C C are in proper working order and there is no disarrangement of the wires leading to the motors or those within the latter, the current will obey the movements of the handle h, but under other conditions it may not. If such an emergency arises, the motorman reaches up to the hood and turns the safety switch a or b, and thus cuts the current off.

The force with which the motors turn the car wheels around depends upon the strength of the current; this is owing to the fact that the magnetic force is increased or decreased by variations in the current strength. If the current is doubled the magnetic force of the armature is nearly doubled, and so is that of the field magnet, therefore the pull between the poles is nearly four times as great. From this it will be seen that the force with which the car is pushed ahead can be increased enormously by a comparatively small increase in the strength of the current. If the current strength is doubled, the propelling force is practically quadrupled; and if the current is increased four times, the propelling force is made nearly sixteen times as great.

The speed at which the car runs depends upon the force that impels the current through the wire, and which is called electromotive force. The greater the electro-motive force, the higher the velocity. If the current passes from the wires in the cable e e e through each motor separately, and thence to the rails R, each machine will receive the effect of the whole electro-motive force of the current; but if after the current has passed through one motor it is directed through the other, then each machine will be acted upon by only one half the electro-motive force, and, as can be seen at once, the velocity in the first instance will be twice as great as in the last. This fact is taken advantage of in regulating the speed of the car, and controlling switches arranged so as to direct the current through the motors in this way are designated