Off Topic Introduction
More than half of the introduction for incandescent light bulbs is talking about switching from them to CFLs. This seems completely bizzare and out-of-place. 69.141.68.106 01:21, 23 May 2007 (UTC)
Light bulbs are blackbodies...
...not spontaneous emitters. The following:
"As the electrons that travel through the filament bump into the atoms, some of the electrons in the atom may become excited. This means they temporarily boost its energy level and raise to higher orbit. When they fall back, energy is released as photons, a photon being the particle form of light."
is true, but the vast majority of the light comes from blackbody radiation, which isn't even mentioned. I guess this is what happens when you copy from howstuffworks.com... 168.230.48.248 03:25, 5 July 2006 (UTC) (Andrew)
I agree - as a physics graduate, incandescent bulb spectrums are NOT shaped primarily by electron transitions in **atomic** tungsten. This is somewhat misleading and is wrong if you're trying to explain how an incandescent bulb gets its light spectrum. Black body radiation accounts for most of the spectrum's (continuous) shape and is also the reason why incandescent bulbs tend to look very natural to the human eye compared to other types of lighting - the sun's light is mostly shaped a lot by black body radiation too! Though electron transitions facilitate blackbody radiation, it's not the atomic structure of tungsten that provides the spectrum. I've reworded this. Physicists please check that I've given a correct explanation. 18.250.0.177 15:02, 8 November 2006 (UTC)
Material from external website?
In revision 139201 an anonymous user inserted a large block of slightly modified text from external websites (, , , ). The contributor that did this seems to have copied passsages of text from several different wesites and pasted them together with minor to moderate rewording of some sentences. This is not covered under fair use. Fortunately, others have since copyedited the text and have obscured much of the obvious parentage. More is needed however. Please keep this in mind while editing. --mav
Vacuum versus gas filling
Hey, I'm all kinds of new here, but is there *any* substantiation to this "modern lightbulbs filled with inert gas" buisness? I know we have neon lights, but the article seems to imply that modern incandescent bulbs are not evacuated, but any good (although not safe, and not reccomended) smashing of lightbulbs proves that they are at least nearly evacuated. -j
Be careful using a shatter test on 'neon' lamps. If you get a cut it can take a long time to heal due to the chemicals used to coat the inside of the glass. At least so I have heard.
Plus, drops of liquid mercury and other nasty chemicals in fluorescent lamps likely turn to minute droplets and go airborne when the tubes are smashed.
Davy's work
The whole paragraph about Davy's work could stand to be rewritten, preferably by someone who has consulted contemporary sources.
Edison 21:22, 20 June 2007 (UTC)
incandescant platinum
The article Light bulb says in section History of the light bulb
I assume (not much of chemist, i) that oxidation of Pt is unlikely, and that this misinterprets Lights by Edward Willett, or something similar, which refers to
- Davy's "fine platinum wire" and
- to other incandescants where "the filament burned away very quickly".
I say "oxidation ... unlikely" in the belief that Davy chose Pt bcz that (and maybe Au) were the only things he could find that didn't oxidize when incandescant (and maybe bcz Au melts a lot earlier).
Here are some #s from WP:
Parameter ---------- W ----- Pt
Melting point --- 3695 K 2041.4 K
Boiling point --- 5828 K 4098 K
ρ -1 (10 6 /m ohm) -- 18.9 09.66 (or should i be suspicious of the leading zero?)
At. wt. (amu) --- 183.84 195.078
What i understand to be the main failure mechanism in ductile-tungsten filaments is the positive feedback at the hottest points, of
- increased thermal migration of W,
- thinning,
- increase of resistance-per-unit-length,
- increased share of delivered power, and
- increased T,
culminating in local melting across the whole cross-section, of enuf length that sag (or just instability?) lets the liquid portion flow away from its center. (Haven't noticed if that's in the article; it should be IMO.)
IMO, the presence or absence of air doesn't make a lot of difference in that kind of failure, bcz even if convection is more efficient for the first few inches than radiation alone, incandescence is required, and the operating temperatures are going to be essentially the same.
With Pt in the same geometry such failure just happens sooner; i imagine much sooner: i base that on the principle that chemical reaction rates go up very rapidly with temperature bcz they are controlled by the population in excess of the activation energy, which increases much faster than mean energy, and on my belief that thermal migration should similarly limited by an activation energy representing a surface atom breaking all but two (or if not, all but one) of its bonds, so it can swing to its next position.
The higher conductance of W (which surprises me, but whadda i know) does mean the Pt filament has twice the cross-section area for the same length, voltage and current, but that may not be what determines the geometry: with modern W, it may be economical for filaments to be made thicker and longer (for ruggedness and reduced thermal-migration failure, but with Pt the cost of filament material has to limit the feasible increase of dimensions. So i think the lower conductance is completely swamped by the melting point and perhaps dimension issues, and i am convinced the same non-oxidative failure mechanism can explain Davy's results. --Jerzy '03 Dec 15
Sublimation
Thx! On reflection i agree. I may have been searching for my keys under the street lamp, esp since i despair of having any hope of estimating sublimation rates, either relative between materials or relative to the solid-state thermal surface migration -- other than to note that activation energy for sublimation has to be higher than for surface diffusion. Maybe i should have taken another thermo course or four.
I think sublimation can also participate in the feedback loop, but at least initially it promotes a graceful degradation, with the bulb getting cooler (and dimmer) as the filament thins and delivered power drops.
(My first thot on reading the sublimation contrib was "Ah, then the bulb maintains a vapor pressure of the filament material, and makes a difference other than via restricting convection to a remote heat-flow mechanism." No, fool, the bulb just makes it obvious that sublimation is a second thermal-migration mechanism that you didn't even consider: thermal migration via gas phase to the sub-incancesent bulb instead of via solid phase within the filament. 'Scuse me, gotta go off and cry somewhere.) --Jerzy 19:41, 2003 Dec 16 (UTC)
Don't be too hard on yourself, Jerzy. You were right to dispute the "oxidized" claim (which I just changed).
Looking into this further, I think sublimation may be a less graceful degradation mechanism than you say, but I'm not sure. For analytical purposes, I imagined a filament divided into 100 segments in series. For a 240 V 100 W bulb (common here in Britain), I calculate the filament resistance when hot to be nominally 576 R. In my scheme that's 100 5.76 R segments in series. If, due to evaporation, one segment doubles in resistance to 11.52 R, then the total filament resistance will increase by 1% to 581.76 R. With the same supply voltage, the power will drop by nearly 1% to 99.01 W. As you said, this will cool the bulb and help to slow down the evaporation. On the other hand, think what happens to the thinned segment. The current through the filament has now dropped by 1% (from 0.4167 A to 0.4125 A), but the resistance of the unfortunate segment has doubled, so the power in that one segment has now increased by 96% (from 1 W to 1.9606 W), making it evaporate much faster! It seems that the increase in local power dissipation will strongly dominate the slight decrease in global power dissipation, causing a runaway failure, but I haven't proved that this is true for all cases.
If you have been, thanks for reading this far! -- Heron
A big issue i keep coming back to, tho, is that we'd like to find phenomena that work when the non-uniformities are infinitesimal. I think your example points out the fact that power goes up faster than segment resistance, because P = VR 2 , which enhances the feedback, at least once the uniformities get large. But what i'm starting to worry about is that unless we're talking about irregularities that are there from the factory machining, they are likely to be swamped by the thermal noise. IMO there are some things that work against the feedback: i think infinitesimal surface roughnesses are subject to negative feedback: atoms at convex surface points are bound to fewer atoms and therefore (i think) less strongly bound; atoms at concave surface points by the same token more strongly bound. We therefore expect the background surface diffusion (the component not driven by temp differences) to favor moving atoms from convexities to concavities, and smooth out the surface. Gets more complicated as w
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