Light bulbs and vacuums - huh?

[Bill in SD, CA]

I always was taught that a light bulb needed a vacuum to work.

The other night during the rain one of my yard floodlights shattered and the darn thing is still working.

Totally exposed and still shining.

Explanation?



Bill

I think they are more efficient in a vacuum, but don't actually need a vacuum to function properly?

[Gomer]

The vacuum keeps the filaments from combusting.

I am guessing it is a halogen bulb within a housing and it was the housing that fractured.

[Bill in SD, CA]

No bulb - just a filament.

Bill

[OuTpaTienT]

Don't light bulbs have a certain type of gas inside? Argon or something like that. So they are not quite a vacuum.

Flood lights are going to be more sturdy plus often flood lights have a smaller bulb inside the larger encasing that you would think is the bulb but isn't.

If the outer casing is broke but the inner bulb is still intact then it'll work. If the filament is exposed to the air then I can't imagine it'll work for very much longer.

[Gait_Keeper]

What he said...........All Hail OuTpaTienT


not a true vacuum, in the vacuum sense, also the air would make the filiment burn out quick, or combust

[RedFury]

Vacuum vs. gas-filled bulbs
At first, incandescent bulbs were made with a vacuum inside them. Air oxidizes the filament at high temperatures. Later, it was discovered that filling the bulb with an inert gas such as argon or an argon-nitrogen mixture slows down evaporation of the filament. Tungsten atoms evaporating from the filament can be bounced back to the filament by gas atoms. The filament can be operated at a higher temperature with a fill gas than with a vacuum. This results in more efficient radiation of visible light. So why are some bulbs still made with a vacuum? The reason is that a fill gas conducts heat away from the filament. This conducted heat is energy that cannot be radiated by the filament and is lost, or wasted. This mechanism reduces the bulb's efficiency of producing radiation. If this is not offset by the advantage of operating the filament at a higher temperature, then the bulb is more efficient with a vacuum.
One property of thermal conduction from the filament to the gas is the strange fact that the amount of heat conducted is roughly proportional to the filament's length, but does not vary much with the filament's diameter. The reason this occurs is beyond the scope of this document.
However, this means that bulbs with thin filaments and lower currents are more efficient with a vacuum, and higher current bulbs with thicker filaments are more efficient with a fill gas. The break-even point seems to be very roughly around 6-10 watts per centimeter of filament. (This can vary with filament temperature and other factors. The break-even point may be higher in larger bulbs where convection may increase heat removal from the filament by the gas.)
Sometimes, premium fill gases such as krypton or xenon are used. These gases have larger atoms that are better at bouncing evaporated tungsten atoms back to the filament. These gases also conduct heat less than argon. Of these two gases, xenon is better, but more expensive. Either of these gases will significantly improve the life of the bulb, or result in some improvement in efficiency, or both. Often, the cost of these gases makes it uneconomical to use them.


How light bulbs burn out
Due to the high temperature that a tungsten filament is operated at, some of the tungsten evaporates during use. Furthermore, since no light bulb is perfect, the filament does not evaporate evenly. Some spots will suffer greater evaporation and become thinner than the rest of the filament.
These thin spots cause problems. Their electrical resistance is greater than that of average parts of the filament. Since the current is equal in all parts of the filament, more heat is generated where the filament is thinner. The thin parts also have less surface area to radiate heat away with. This "double whammy" causes the thin spots to have a higher temperature. Now that the thin spots are hotter, they evaporate more quickly.
It becomes apparant that as soon as a part of the filament becomes significantly thinner than the rest of it, this situation compounds itself at increasing speed until a thin part of the filament either melts or becomes weak and breaks.

Why bulbs often burn out when you turn them on
Many people wonder what goes on when you turn on a light. It is often annoying that a weak, aging light bulb will not burn out until the next time you turn it on.
The answer here is with those thin spots in the filament. Since they have less mass than the less-evaporated parts of the filament, they heat up more quickly. Part of the problem is the fact that tungsten, like most metals, has less resistance when it is cool and more resistance when it is hot. This explains the current surge that light bulbs draw when they are first turned on.
When the thin spots have reached the temperature that they would be running at, the thicker, heavier parts of the filament have not yet reached their final temperature. This means that the filament's resistance is still a bit low and excessive current is still flowing. This causes the thinner parts of the filament to get even hotter while the rest of the filament is still warming up.
This means that the thin spots, which run too hot anyway, get even hotter when the thicker parts of the filament have not yet fully warmed up. This is why weak, aging bulbs can't survive being turned on.

From HowStuffWorks.com

[Knothead]

Many hands make light work.

Just keep your hands off of the little wirey pointy part.








see, I'm helll-ping....

[Brainchild]

I have seen a light work without the bulb before.Only once and only for a minute or so.

This drunk guy came out of the bathroom holding the glass part in his hand,and yelling the light is still working
Of course being in the condition we were in,we all had to go "see".So there is 10-15 drunks in the bathroom when it went dark.Just some minor injuries,nothing seriousAnd one drunk in the bathtub

[Knothead]

Heh, funny story, Brainchild!