+ 14 POPULAR SCIENCE MONTHLY. 



the current passes through the armature coils. It is evident that 

 the direction of a car motor can not be reversed at the instant that 

 it is desired to have it act as a generator — that is, when it is de- 

 sired to put the brakes on; hence the direction of the current 

 through the armature is reversed. 



When a car is provided with magnetic brakes, the controlling 

 switches are so made that when the handle h is moved back to the 

 stop position it disconnects the motors from the trolley wire and at 

 the same time connects them with the magnetic brakes in such 

 a manner that they will act as generators and thus send current 

 through the coils of the latter. In order that the force mth 

 which the brakes are applied may be graduated, the controlling 

 switches are arranged so as to be moved several steps back of the 

 point which in the ordinary type of switch would be the final stop 

 position. When the handle h is placed on the first brake position 

 the current generated by the motors is not very strong, and as a 

 consequence the force of the brake is light, but sufficient to bring 

 the car to a stop in a reasonable distance. If a quicker stop is 

 desired the handle is moved to the second, third, or fourth brake 

 position, thus increasing the retarding force as much as may be 

 desired. Magnetic brakes are very desirable, as they save the car 

 wheels, and furthermore afford an additional safety in cases where 

 it is necessary to arrest the speed instantly. 



The position of the motors with reference to the truck and car 

 wheels is very well shown in Fig. 20, and also the manner in which 

 they are held in place. The covers of the openings through which 

 access to the commutator brushes is obtained are removed from 

 both motors, and in the forward one the top of- the commutator 

 and one of the brushes can be readily seen. The manner in which 

 the motors are suspended from the truck is not the same in this 

 figure as in those previously shown, but this is simply because the 

 machines are not made by the same concern, and each manufac- 

 turer has his own design. 



Fig. 21 shows the appearance of the interior of the controlling 

 switches C C, Figs. 18 and 19. It will be noticed that there are 

 two upright shafts, the ends of which project above the top of the 

 box. The handle h is placed upon the shaft to the left, and h on 

 that to the right. The first is the main controller, and the other 

 is the reversing switch. It will be noticed that the main con- 

 troller shaft carries a number of circular segments of different 

 lengths; these are so disposed that they come in contact with suit- 

 able stationary pieces as the handle h is turned around, and thus 

 vary the path of the current through the motors and the rheostats 

 in the manner required to effect the desired changes in the velocity 



