XPost: alt.folklore.computers   
   From: dannyb@panix.com   
      
   In  hanco   
   k4@bbs.cpcn.com writes:   
      
   >Somewhere I heard (unconfirmed) that subway train lamps had a relay to peri   
   >odically switch polarity of the DC supply to the lamps back and forth.  FWI   
   >W, in the transit museum, some of the cars on exhibit remain still for long   
   > periods of time, and one of their interior fl. lights is dark.  Could that   
   > have something to do with the polarity not being switched?   
      
   Eyup. Back in roughly 1980 I had this really great booklet from   
   Westinghouse which described their lighting products. (Alas it's   
   gone to that great dustbin of history).   
      
   They had a section about powering fluorescents via DC and mentioned   
   the one-side darkening... and the importance of periodically   
   flipping the current flow direction.   
      
   The simple solution they recommended? Having a switch that   
   reversed the polarity each time it was turned on.   
      
   That stuck in my brane... and sometime later I was on   
   one of the retrofitted subway cars which had been   
   upgraded to fluorescent. And eyup, one of the tubes   
   on the destination signs was dark on one side.   
      
   We went over a switch which, at the time of the   
   subway system design and that type of car.. gave   
   us a couple of seconds of power outage (the battery   
   powered emergency lights came on).   
      
   When power was restored, the tube was fine...   
      
   Speaking of which, anyone know if the trains still use lead   
   acid storage batteries?   
      
      
   >When they rebuilt the RDG Blueliners, they put in fluorescent lights.  Whil=   
   >e standing in Reading Terminal, the flicker was very evident, although it w=   
   >asn't once the trains got moving.   
      
   >Old subway trains would briefly lose the lights when the train passed a 3rd=   
   > rail gap, which were frequent (usually near every station).   Modern train=   
   >s have battery supply so the lights stay on (though you can hear the HVAC w=   
   >hoose cease).  However, if the train is moving slowly over a crossover with=   
   > a longer 3rd rail gap, the lights will go out.   
      
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   		     dannyb@panix.com   
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