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 inter- 

 mediate 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 condi- 

 tions it may not. If such an emergency arises, the motorman 

 reaches up to the hood and turns the safety switch a or h, 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 mag- 

 net, 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 electro- 

 motive 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 



