🚀 go-pugleaf

In article <714@skipper.dfrf.nasa.gov> rando@skipper.dfrf.nasa.gov (Randy Brumbaugh) writes:
>A chemical process (the halogen cycle) is used to improve this 
>in these new fangled "halogen" lamps.  The tungsten chemically
>bonds with the halogen gas in the envelope.  When the gas
>contacts the hot filament, the chemical bond is broken, and 
>the tungsten is re-deposited in the filament.  These lamps
>won't last forever, because the tungsten isn't deposited
>at the same spot it came off the filament.

If a region starts to thin-out its resistivity increases,
increasing its power dissipation, increasing its heat.  The
rate of deposition is a monotonic function of the temperature,
so the hotter spots tend to _collect_ tungsten, fattening them.

The halogen cycle not only prevents rapid loss of filament
material, it repairs minor flaws in the filament and prevents
hot-spots due to these flaws.

Halogen lamps burn out (barring some extensive flaw) only
when one of the non-tungsten parts fails (the filament supports
weaken from temperature cycling, the way the filament would
if it weren't for the halogen cycle).

>Also, the quartz envelope shouldn't be touched by bare 
>hands -- oils on the skin will weaken the structure and may
>cause bubbles or lamp failure.

The oil causes a region of different thermal conductivity;
when the heat is applied this will cause a region of
different temperature which will cause a difference in
expansion which will cause a stress that will rupture the
envelope.  The oil doesn't "weaken the structure."  You can
wipe it off.

>Tungsten-halogen (T-H) lamps are now used almost exclusively
>for stage and television lighting applicatuions, where they

				--Blair
				  "Stage Nerds 'R' Us..."