From: eachus@Dr_No.mitre.org (Robert I. Eachus)
Newsgroups: alt.folklore.science
Subject: Re: Microwaved light bulb
Date: 19 Aug 92 15:56:43 GMT
	<1992Aug17.034822.5443@cco.caltech.edu>
	<1992Aug17.063922.22800@news.columbia.edu>
Organization: The Mitre Corp., Bedford, MA.


In article <1992Aug17.063922.22800@news.columbia.edu> lasner@watsun.cc.columbia.edu (Charles Lasner) writes:

   Can someone explain the Halogen operating principles over and above the
   conventional incandescant bulbs.  Also, I assume in the car case, this is
   also the so-called "quart-iodine" type bulb, not some other.

     Glad to, it's quartz-iodine by the way.  Halogen lamps are
variations on the same theme, but let's start with quartz-iodine.

     Put a tungsten filament in a quartz tube containing some iodine
crystals, and run a curent through it.  Get the filament hot enough
and it glows--lets use 3000 degrees Kelvin for a round number, which
is near the top of the operating range.  Some of the tungsten boils
off and accumulates on the quartz blocking the light, but when the
quartz gets to about 300 degrees centigrade, (Danger, Will Robinson,
do not touch...) the iodine will react with the tungsten to form
tungsten iodide.  The tungsten iodide "evaporates" and when it reaches
the filament it breaks down and redeposits the tungsten on the
filament.  (Warnings about not touching the quartz tube when cold have
to do with the oils on your skin.  The oil can react with the hot
quartz to devitrify it.  (Devitrify is a fancy way of saying crumble
to powder.)

     Bromine works also, but since it remains a gas in the tube,
the time for the "bromine cycle" to start is shorter.  However, when
the filament is really hot, the tungsten bromide can break down away
from the filament.  Therefore most halogen lamps contain a mixture of
iodine and bromine, while the really high end ones stick to iodine.
Also many halogen lamps nowadays are made with an envelope other than
quartz.

     Halogen headlights of course put the inner envelope, where the
reaction takes place, inside an outer glass envelope.  (The inner is
about one-inch long and half an inch in diameter.)  This of course
protects the very hot inner envelope from contamination, and the
unwary passerby from flying glass if the inner envelope explodes.
Quartz very seldom explodes, inferior substitutes often do when the
filament melts.  (That requires further explanation.  The filament is
actually a helix, and should last forever except that overvoltage can
cause sagging--and a turn to turn short.  Also the redeposited
tungsten can be uneven causing local hot spots.  In any case the end
of a QI bulb is usually local melting of the filament which then
touches the envelope.  If it cracks, well, in operation these things
are considerably above atmospheric pressure.)

   Further, is the 110V variety such as the Syvania CapSylite using
   the same principle or is there another effect at 110V as opposed to
   12V?

   No, the filament structure is different is all.  I'm not sure about
the small halogen flashlight bulbs, it seems possible to design an
inner envelope which gets hot enough, but why?  Most flashlight bulbs
fail from vibration well before tungsten darkening of the glass is
noticable.



--

					Robert I. Eachus

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