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In article <7200013@silver>, commgrp@silver.bacs.indiana.edu writes:
> Old-style incandescent streetlamps were wired in series.  I always 
> wondered how such a system could work. I finally learned the method 
> which kept the lights on when a bulb burned out:
> 
> One hundred 120-v bulbs were wired in series and connected to a 12,000 
> volt line.  Above each lamp socket was a pair of spring-loaded 
> contacts separated by a piece of waxed paper.  If one bulb burns out, 
> the voltage goes up, the paper arcs through and burns away, forming a 
> short circuit.  The remaining bulbs get 1% more voltage.  This 
> ingenious method saved wire, transformers and switches.  The 
> maintenance crew had to be diligent and not let too many bulbs burn 
> out!

	I have not heard of any 12,000 volt systems, but "open circuit"
voltages of 2,000 to 4,000 volts were much more common.
	The key to making the series streetlighting system work is the
use of a constant current transformer.  As a result, as a bulb burned
out, the CURRENT across the system would remain the same, but the effective
end-to-end voltage decreased.  Therefore, the illumination level and
effective voltage aross each lamp remained the same.  When the current
transformer sensed an "open" (i.e., no current), the maximum secondary
voltage was in effect across the burned out lamp, thereby fusing the
shorting contact.
	The constant current transformer used for these series lighting
systems was rather characteristic: the transformer used a moveable core
which was balanced with a counterweight.  This "force-balance" type of
transformer created a constant current ratio.  The "current setting"
was varied by installing or removing weights on the core counterpoise.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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