Fig. 26.—Underground Conduit with Inclosed Conductor.
Fig. 26 illustrates another type of inclosed conductor which at a first glance appears to be far superior to that just described, but upon closer investigation it is found to be not wholly free from objections that are difficult to overcome. The yoke F F, as in the design just described, is made wide enough to support upon its outer ends the track rails R R, and is cut away in the middle to an outline conforming with the shape of the conduit. The conductor that carries the current is located at d, being supported by the stands e. An elastic tube f is provided, which is water-tight and thus excludes moisture from its interior, within which the conductor d is located. On the top of tube f a flexible rail b is secured, and this connects with studs c, which are within the tube, as clearly shown in the drawing, and so situated that they may be forced down into contact with d. Normally these studs are separated from d, but when the car comes along, the wheel a, mounted upon the end of plow P, flattens the tube f and thus forces one or more of the studs c into contact with d. The distance between the studs c is such that at least two will always be in contact with d, thus insuring a continuous electrical connection with the motors so long as the plow is depressed.
The first impression upon looking at this design would be that it is entirely free from objections; for if we assume that the tube f is made of rubber, we can see it in our mind's eye springing up after the plow passes by and thus separating the contacts c from d, and at the same time yielding freely to the pressure of the wheel a. All of this is true, but rubber is not very durable when under such exposed conditions, and to maintain a length of several miles of it in a perfect state for even two or three years could not reasonably be expected; and if it became necessary to renew the tube oftener than this the cost of maintenance would be entirely too great. There is another objection, however, which is more serious, and that is that the conduit will gradually fill up with dirt, and this pressing against the rubber tube would force it out of shape, and thus cause the contacts c to bear permanently upon d, or else to become so far displaced that they would not touch it when depressed by the plow.
As the rubber tube can not be depended upon, inventors have sought to improve the construction by using sheet steel and making the tube flatter and much wider, so that a section of it would present an outline much resembling an elliptic carriage spring. Such a construction will meet the requirements as to strength and the retention of the contacts c in their proper position; but steel expands when warm and contracts when cooled, therefore a long tube would be stretched so much in winter that it might pull apart, while in summer it would be compressed and tend to buckle up and thus be forced out of place. These difficulties can be overcome by providing expansion joints at suitable intervals, so that they are not necessarily proof of the impracticability of devices based upon the principles involved in this design; they simply serve to forcibly bring to mind the fact that the path of the inventor of underground systems is not strewn with roses, no matter in what direction he may turn to find a solution of the problem.
Fig. 27.—Underground Conduit with Exposed Conductors.
The object in the designs Figs. 25 and 26 is to shield the conductor so that it will remain dry should the conduit be filled or partially filled with water. If water could be excluded from the conduit, the casing j c c, in the first figure, and the tube f, in the second one, would not be required, for there is no difficulty in providing an insulating support that will hold the conductor firmly in place and at the same time prevent the escape of the current; but as soon as moisture collects upon the surfaces of the insulating supports it acts as a conductor, and thus renders the insulation of little value. If water runs into the conduit in such quantities as to come in contact with the conductor, then the effect of the insulation is entirely destroyed; the aim of the inventors, therefore, is to provide means for preventing the accumulation of water or moisture around the conducting wire. It can be readily seen that the shorter the conductor the easier it is to protect it, and this fact has given rise to the development of a great number of designs classified as sectional conductors. In these, two conductors are used, one of which is continuous and so situated and insulated that it can not under any conditions be reached by either moisture or water. The other conductor is made in lengths that vary all the way from fifteen to two or three hundred feet. Normally, these short sections are not connected with the circuit—they are dead, as it is called—but when the car comes along, the plow, by acting upon suitable mechanism, establishes a connection between the continuous conductor and the portion of the sectional conductor that is directly under it, and in this way the current passes to the car. As soon as the car passes beyond a section of the sectional conductor, the connection between it and the continuous wire is broken automatically. Some of these arrangements depend upon mechanical devices, such as levers that are struck by the plow and thereby move a switch that closes a connection between the section and the continuous conductor, but in most instances the switch is operated by a magnet, which may be carried by the car or may be arranged so as to be energized as the car approaches it. Designs of this last type come under the head of electrically operated sectional conductor systems. There are other arrangements in which a magnet carried by the car attracts iron levers suitably disposed along the conduit, and these levers close switches that connect the section of conductor under the car with the continuous one. As the levers are actuated by the magnet, they only hold the switch closed while the latter passes over them; thus the electrical connection is made and broken as the car moves along.
Fig. 28.—View of Street Railway Lines in Washington operated by Underground Conductor of Type shown in Fig. 27.