# What makes an incandescent light bulb run out?

What is or are, the most likely and common ways - the weak links in the chain (including I assume poor electricity wiring / configured housing) - that cause a traditional incandescent light bulb to blow?

And how much does manufacturing / materials quality of the bulb make a difference in any of those factors? (In other words, should an expensively, state-of-the-art constructed bulb in terms of materials purity and construction robustness, generally last a lot longer than the average bulb on the market?)

I understand it is a fairly simple lighting technology, so what complications cause them to not last longer than they generally do? Is 'blowing' the only way that an incandescent can ever expire?

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Manufacture is almost always the cause. –  Ignacio Vazquez-Abrams Jul 3 at 4:19
I had an answer from a couple of years ago with the 100 year light bulb in it (carbon filament, low power), but I can't seem to find it right now. –  pjc50 Jul 3 at 6:22
@pjc50 The Centennial Bulb in the fire station in Livermore, CA? –  David Richerby Jul 3 at 10:42

Most light bulbs have a number of service hours designed in. This is achieved fairly accurately and on purpose during manufacturing. Consumer grade light bulbs burn out faster and the awful truth is that in that way the manufacturer ensures it can keep producing the light bulbs and make money. The main cause is the tungsten filament slowly evaporating until it gets too thin to carry the current. The trick during manufacturing is to etch the filament to a carefully designed thickness so the lifetime is reduced programmed.

The origins of programmed life light bulbs began with the Phoebus Cartel in 1924.

There are also light bulbs with a special stronger filament (basically they skip the 'etch-cycle'), these are for use in high reliable applications and last longer. They are commonly used in places that are hard to reach and are more expensive. Not sure about the proper name of these light bulbs. Because of the extended life time these bulbs are more expensive as a manufacturer has only limited room for producing spares. (What need is there for spares if the bulb doesn't fail?)

Where Australia and EU banned the consumer grade bulbs, the strengthened filament is not banned because of its specialized application. The latter type is just not available in your regular shop around the corner.

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I can also add, that a relative just told me that his mother had an incandescent that was used as the outside house light (so even exposed to the elements quite a bit!), that lasted for 15 years, and I assume it was used quite a lot if it was an outside light too! And furthermore, I'll have to Google now, but I am reminded now of reading once of a famous incandescent bulb that apparently lasted 100 years (of, IIRC, CONTINUOUS use), and now it sounds like it's actually a credible story based on what we know. 'They don't make em like they used to', has never applied more. Disgusting –  foregon Jul 3 at 6:06
@jippie: While I'm sure you answer is credible, it would be nice to know where you got this information from. –  nijoakim Jul 3 at 8:50
Isn't the original Edison bulb still burning? –  Scott Seidman Jul 3 at 11:06
I'm downvoting even though the basic explanation (filament evaporation) is correct. The "programmed lifetime" is just silly. For a given voltage the thinner the filament the hotter and more efficient it gets, and the shorter its lifetime (faster evaporation). The extended-lifetime bulbs you describe (and they do have thicker filaments) do exist. The reason they last is that they don't get as hot as regular bulbs. This also means they produce less light for a given power. Some of the old bulbs are the same - they last but (because) they don't put out as much light. –  WhatRoughBeast Jul 3 at 12:29

I'd say that an incandescent bulb is quite forgiving about power quality. They generally blow up because tungsten, that is, the material the filament is made of, slowly evaporates until the filament breaks. So what really comes into play is the filament manufacturing, how it's held in place, if its thickness is regular or not, if the tungsten is pure enough, and so on.

A filament breaks because it eventually becomes a little thinner at a point. That point heats more, causing more tungsten to evaporate, making it even thinner... That's good old nasty positive reaction. Thermal shock can do the job too, that's why you tell your children to stop disco-switching the lights.

Interestingly enough since the filament usually just breaks in one point, it's possible to try to give a broken lamp some afterlife time. If you have the broken bulb plugged in$^1$ and shake it, it's possible for the two filament ends to touch: the point heats a lot and the filament might solder itself and give you ten or so more hours of life.

$^{(1)}$ Please, just don't try that if you are not sure it's safe.

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+1 "stop disco-switching the lights" –  Blup1980 Jul 3 at 6:04