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 similar to those described in the foregoing, nothing has been done practically 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, b b, which are insulated from each other. These brushes rub against the conductors
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