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 the brushes _b b_ is not required. To avoid the accumulation of water in the conduit the drain _G_ is provided with outlets _d_, located at suitable points. Although this system is the simplest that can be devised for use in streets or public highways its construction is very costly, so much so that it can only be used in cities where the traffic is so great as to require the running of cars on short headway; and, furthermore, it can not be operated with any degree of success except in municipalities where there is a good sewage system. During the summer months it is liable to be more or less impaired by heavy showers, but the trouble in such cases is only temporary. In winter time snowstorms are liable to affect it in the same way, especially if, after a heavy fall, a warm wave comes along and produces a rapid thaw.