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From: feg@moss.ATT.COM (Forrest Gehrke,2C-119,7239,ATTBL)
Newsgroups: sci.space
Subject: Re: The great light bulb debate
Message-ID: <1990Nov13.172642.10333@cbnewsl.att.com>
Date: 13 Nov 90 17:26:42 GMT
References: <9011092213.AA05755@cmr.ncsl.nist.gov> <1990Nov13.035815.10203@zoo.toronto.edu>
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Reply-To: feg@moss.ATT.COM (Forrest Gehrke)
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In article <1990Nov13.035815.10203@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:
>In article <9011092213.AA05755@cmr.ncsl.nist.gov> roberts@CMR.NCSL.NIST.GOV (John Roberts) writes:
>>The "inert" gas usually cited in the literature is nitrogen. Why would 
>>krypton be better than argon? 
>
>Nitrogen is not really inert when temperatures get high.  For example,
>if you burn titanium in air -- 80% nitrogen, 20% oxygen, roughly -- the
>ash is about 80% titanium nitride and 20% titanium oxide.  For applications
>involving incandescent metals, you want something that is really inert.
>(If you're being really picky, the noble gases [the preferred modern term]
>are not really inert either, but under these conditions they qualify.)
>
>Krypton is better than argon for the same reason that argon is better than
>vacuum:  the denser gas slows the evaporation of the tungsten filament.


And because of the slower evaporation there is another advantage:
The tungsten filament can be operated at a higher temperature; these 
bulbs are significantly brighter.  

Halogens (usually pure iodine) are still better (although a different 
process is taking place).  But now, due to the high temperatures being 
reached, quartz glass must be used for the envelop.  This is expensive 
and their applications become more specialized, such as automobile 
headlights, photography, etc.

Forrest Gehrke feg@moss.att.com