3 88 



THE POPULAR SCIENCE MONTHLY. 



for making contact, and these wheels are just the length of a sec- 

 tion apart. When the leading wheel L is on a negative section, as 

 A 2 , the trailing wheel T is on a positive section, Ai, and vice versa, 



so that a circuit is 

 made between the 

 poles of the gen- 

 erator through the 

 rails, the two con- 

 tact wheels of a 

 train, and a wire 

 connecting these 

 wheels through 

 the motor M on the 

 locomotive, which 

 thus receives its 

 supply of elec- 

 tric energy. Of 

 course, the current 

 through the motor 

 is stopped and re- 

 versed each time 

 the contact wheels 

 pass from a posi- 

 tive to a negative 

 or a negative to a 

 positive section, but this makes no difference with the direction 

 in which the motor runs, nor does it injure the dynamo. We can 

 cause the locomotive to run backward, however, by altering the 

 positions of the commutator-brushes on the motor. In the case 

 of a single track, the positive sections Ax, A 3 , etc., would have to 



Fig. 5. — Mechanical Brake. 



Ai 



•** B4 



V 



&. 



OM 



*3 



X 



*2 



Si 



A* 



A 



[]• 





(JM, 



*-w 



Bz 



- Bi 



Fig. 6.— Diagram op the Cross-over Parallel System. 



be connected by a long wire instead of through the sections of 

 rail B 2 , B 4 , etc. This system requires that the sections shall all be 

 of equal length, which is sometimes inconvenient, as when broad 

 gorges have to be crossed, and at curved parts of the line. But 

 this difficulty can be overcome to some extent by employing a 

 " gravitation section " longer than the distance between the con- 

 tact wheels of a train. This section is constructed with a down- 

 ward slope, so that the weight of the train will propel it over the 

 part in which it receives no electric energy. 



