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Re: Smiley Faces!



At 11:02 PM 9/24/98 -0400, you wrote:
|Just for fun I looked up quartz in Webster's unabridged. I turns out the
ONLY
|definition is the one for a mineral "a brilliant crystaline mineral".
There is
|no definition 2 that says "a high purity silica glass". However, a bit
further
|down we have "quartz lamp, a mercury vapor lamp having a tube made of
quartz.."

I am not sure that we should be trying to learn physics from a dictionary.

I wonder why this question is still nagging the list.

There is indeed a natural crystalline mineral called quartz, BUT, in the
context of the question about quartz-halogen lamps, it is also a clear
glass (occasionally called fused silica) Its composition is SiO2. It has
a higher softening point and lower coefficient of thermal expansion and
withstands thermal shock much better than Pyrex. It is also expensive.
This form of quartz is a glass and therefore is not crystalline.

In lamps it is needed because the lamp is operated much hotter than regular
tungsten lamps. The lamps are operated much hotter so they can be much
brighter. Because they are operated much hotter, the tungsten evaporates
much faster than for regular tungsten lamps. The metal deposits on the
inner quartz surface as expected, but then the halogen gas reacts with the
metal and forms a gaseous metal halide which in turn migrates to the hot
filament where it decomposes to the pure metal and the halogen gas. Thus
the metal is recirculated from the filament to the bulb and back to the
filament. Thus the lamp can be operated much brighter but still longer
that regular bulbs. If the chemistry were perfect the lamp would last
forever, but then the manufacturer would go out of business. (:-)


Jim Green
JMGreen@sisna.com
http://users.sisna.com/jmgreen