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Page:Popular Science Monthly Volume 56.djvu/586

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570
POPULAR SCIENCE MONTHLY.

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

PSM V56 D0586 Washington dc street railway line with underground conductor.png
Fig. 28.—View of Street Railway Lines in Washington operated by Underground Conductor of Type shown in Fig. 27.

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