14 Dec
2004
14 Dec
'04
9:29 a.m.
>>>> "Vintage" == Vintage
Computer Festival <vcf(a)siconic.com> writes:
Vintage> On Mon, 13 Dec 2004, Paul Koning wrote:
> Interesting! That suggests that DC should be no
big problem for
> halogen lamps (incandescents with iodine vapor fill).
Vintage> How do you figure? There's still evaporation in halogens
Vintage> also, just not as much as in incandescents, right?
Halogen lamps have a negative feedback system: the tungsten that
evaporates is carried back to the hot spots of the filament (which are
the thin spots). That's why they last so much longer.
The Edison effect says that the positive end gets hotter so it wears
faster -- but the iodine says that the tungsten will be brought back
predominantly to that end. So that (largely) cancels out.
The other aspect, as Dwight Elvey pointed out, is that modern light
bulbs are not vacuum. I don't know how halogen bulb pressures compare
with "regular" ones -- they may well be higher.
paul