WHAT MAKES THE TROLLEY CAR GO. 571 



them; thus the electrical connection is made and broken as the car 

 moves along. 



Most of the designs in which sectional conductors are used can 

 be placed much nearer to the surface of the street than the types 

 illustrated in Figs. 25 and 26, and this is a decided advantage, as it 

 greatly reduces the cost of construction. Any system that requires 

 an underground conduit, with the yokes F F to support the track, 

 can only be used by roads upon which the traffic is very great, for 

 the cost of construction would be such as to prohibit its use under 

 any other conditions, no matter how successful its operation might 

 be. For small roads with moderate traffic the question of first cost 

 is of paramount importance, and the only system that can offer a 

 satisfactory solution of the problem for these is one that does not 

 require an underground conduit. 



Although many patents have been taken out for systems simi- 

 lar to those described in the foregoing, nothing has been done prac- 

 tically with any of them except in an experimental way. Some are 

 in operation on small roads in out-of-the-way places, being intended 

 principally to illustrate the practicability of the system and thus 

 assist in promoting its introduction elsewhere, but the system that 

 has been adopted in a commercial way is one in which no attempt 

 is made to shield the conductor from moisture and water, and for 

 its successful operation dependence is placed entirely upon, the 

 proper drainage of the conduit. This system is well illustrated in 

 Fig. 27. The plow P carries upon its end two brushes, h h, which 

 are insulated from each other. These brushes rub against the con- 



e's:. ^-:^-: r^C^\'^.^r^?"t C .rv^^--^^ ^S>, . I^^-f^-r 



Fio. 29. — Ceoss-skction of Kailway Track provided with Third-rail Condtjotor. 



ductors a a, which are made of bars of channel iron and are well 

 insulated from the yokes F F and the conduit casing to which they 

 are attached by means of the supports c c. In the construction 

 shown in the figure the current comes from the generator through 

 one of the a bars and returns through the other, but both bars 

 can be used to conduct the current from the generator, in which 

 case the return can be effected through the track rails, just as in 

 the designs already considered. If both the bars a a are used to 

 convey the current from the power house the insulation between 



