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POPULAR SCIENCE MONTHLY. 



is contiimous 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. ISTormally, these short sections are not 



Fig. 28. — View of Stkekt Railway Lines in Washington .operated bt Underground 

 CoNDCcTOR OK 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 con- 

 nection 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 



