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Re: [Phys-l] 50 year incandescent bulb



I was wondering if that was the case...

But wouldn't a beefier filament be an option? Or is that also a resistivity compromise for ohmic heating? If so, how bout makign some dirtier (presumably more resistive) W or perhaps some other metal with a lower evaporation at the temp of interest? (perhaps not, as W seems to be the metal of choice)

I'm sure at some point, the inner surface of the bulb would be pretty opaque, but that wouldn't seem to be a probelm for awhile since I don't recall any noticable film on any bulb I've ever seen... but maybe I'm just not lookign close enough...


At 08:35 PM 5/1/2007, you wrote:
At 10:21 AM -0700 on 5/1/07, Jason Alferness wrote
>But then, I'm still a little confused why they can't design a 50 year incandescent bulb...

Chuck Britton replied:
My (limited) understanding is that a 50 incandescent bulb could be
designed - but it would not give off as much light as you would
expect.
I notice that (some?) 'long life' incandescent bulbs are 'rated' at
130 volts rather than the more common 120 volts.
I.e. the filament is cooler when it is used at 120 volts and is
putting out less than the rated wattage (wattage rated at 130 volts
=> longer life due to 120 volt use)
============================================

Bulb lifetimes are limited by evaporation of the tungsten filament. At lower filament temperatures, the vaporization rate of the tungsten filament is reduced, but so is the light output. So the filament temperature is a compromise between light output and lifetime.

Larry Woolf
General Atomics
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Jason Alferness
University of Washington
Room B256B, Physics & Astronomy Building
Campus Box 351560
Seattle, Washington. 98195-1560

Phone: (206) 221-2974
FAX : (206) 685-0635
email: alf@phys.washington.edu