Re: [Phys-l] 50 year incandescent bulb plus 56 more years

[Joseph Bellina <jbellina@saintmarys.edu>]

see below
Joseph J. Bellina, Jr. Ph.D.
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556

On May 2, 2007, at 10:53 AM, Paul Lulai wrote:

> I would guess the 100+ year bulb is low wattage.  This would allow  
> it to
> glow dimly w/o worrying about evaporation.  Thanks to Woolf, this
> probably means it has an increased resistance (skinnier or longer).
>
>
>
> To any h.s. teachers out there, the Line of Resistance activities are
> pretty darn good.
>
>
>
> Joe Bellina knows and mentions other alloys.  I don't know the variety
> of options that exist. I believe thorium is one common option (maybe a
> thorium oxide?).  I think the migration of the alloy to the surface
> would tweak the work function of the filament.  Would this have any
> noticeable effect on the function of the light bulb?

You are right, the material migrating to the surface has a lower work  
function which increases the emission of electrons, but as I recall  
has little effect on the emissivity and hence the emission of light.   
W filaments have been coated with thorium, barium, and strontium  
oxides to lower their work function, but I suspect the alloy would be  
with the metal not the oxide.
I just checked the web and it seems that lamp wire is doped with  
potassium, not thorium.  Not sure about the surface segregation of K,  
but once on the surface I would expect to to readily evaporate, or  
desorb as we call it.


> Paul Lulai
>
>
>
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