SuperCow said:
The area within the lamp is filled with hydrogen, rather than air because hydrogen expands less under the influence of heat. However, this is not to say that it does not expand. So, when one portion of the quartz was weakened, all of the pressure exerted by the expanded hydrogen was exacted on that spot, causing it to burst, in the manner depicted in the picture.
It's not hydrogen, it's a
halogen gas. I'm not sure which one, but my guess would be florine.
Are you sure about the expanding less under the influence of heat thing, or was that just an educated guess? It's quite different than the explanation that I heard for why the lamp is filled with
halogen.
A standard incandesant light
bulb consists of a tungeston
filament surrounded by an inert gas (usually nitrogen or one of the noble gasses) and encased in a glass
envelope. As electricity passes through the tungeston, some of it escapes and is deposited on the
envelope. You can see the
effect of this on some incandescant light bulbs that are near the ends of their lives, especially on the higher-wattage ones, in the form of black deposits on the inside of the glass. The result of this is shorter lamp life and reduced light output near the end of the lamp life.
The solution is to fill the lamp with a
halogen gas, which is very reactive. The
halogen reacts with the tungeston as it is escaping from the
filament and then deposits it back on the
filament, instead of on the
envelope. This lengthens the life of the lamp and also helps keep a more constant light output throughout the life of the lamp.
As far as the need for quartz instead of glass, I don't know if it is something about the reaction between the tungeston and
halogen gas that generates so much heat, or if it's just the fact that the lamp is burning at more than 500 Watts. My guess would be the former, because before they invented tungeston-halogen lamps, they used regular incandescants in the theater, and they were made of glass and large and
spherical to dissapate the heat.
Another advantage using quartz instead of glass is that because quartz is so much more heat-resistant than glass, you can put it closer to the
filament, making the lamp smaller. This makes it easier to put the
filament of the lamp precisely in the
focal point of whatever type of
reflector you're using, and results in more efficiant use of the light.
Oh, and incidentially, I have a lamp at my school that has bulged out on one side but not ruptured, and the
filament is bent into the bulge, but the lamp still works. We're saving it for when we need a lamp really badly just as a substitute until we can get another one. Maybe I'll take a picture of it next week and post it.