From: Jason Alferness
Sent: Tue 5/1/2007 8:59 PM
Subject: Re: [Phys-l] 50 year incandescent bulb
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..
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A thicker filament, with the same length, would have lower resistance. Since bulbs operate at constant voltage, the power dissipated would be higher (P=V^2/R), so the filament would be hotter. Therefore the vaporization rate of the filament would increase, as would the light output.
If you made the metal used for the filament "dirtier," it would likely lose ductility and not be able to be drawn into fine wire and formed into a coiled coil. Also, tungsten is the most refractory of the ductile metals.
Surfaces of lamp housings do darken due to the tungsten vapor condensing on the relatively cool glass housing.