Re: Season misconceptions in newspaper

[Roger Haar <haar@PHYSICS.ARIZONA.EDU>]

Hi,
        I have been skimming this thread, and so if this
has been pointed out before , please excuse.  The
trick with halogen light bulbs concerns high
temperature chemistry.  Crudely and probably
overly simplified, tungsten is constantly
evaporating from the filament.  This atomic
tungsten then interacts with the halogen (for
example Br) forming WBr.  If this molecule of WBr
contacts the very hot filament it dissociates and
deposits the tungsten atom back onto the surface
of the filament.  It is all very dynamic.  The
halogen may also react to tungsten on the quartz
envelope.  ( I read this a couple of years ago in
Scientific American's "How Does it Work" section.)

        I have wondered if running  a halogen lamp very
dim would actually shorten its life because the
dissociation on the filament might be less
effective.

Thanks
Roger Haar

******************************************************************
Brian Whatcott wrote:
> If K is the name of the game for filament life, then the next lamp fill
> should logically be radon - better thermal resistivity than Krypton
>   by a factor of two or three. But wait - would the carcinogenic
> leakage pose more health risk than the reduction in falling from
> step ladders while changing lamps?
> SNIP