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After long periods of use a grey spot develops on the inside of the bulb. Why is it so?
- Frank (age 20)
Frank- I'm pretty sure that the cause is evaporation of the tungsten
from the hot filament. Some of the tungsten atoms fly off the hot
filament and recondense on a nearby part of the cooler bulb, forming a
coating which I believe is the grey stuff you see. When too much
tungsten has evaporated, the filament gets thin and breaks.
There's an interesting way to reverse this process. Tungsten forms
a gaseous compound with certain halogens (e.g. bromine) at room
temperature. At high temperature, that compound breaks down. So when a
molecule of it happens to hit the hot filament, it decomposes and
redeposits the tungsten on the filament. That allows the bulb to run
with a very hot filament for a long time. That's how halogen bulbs are
made. The bulb has to be kept small, so the tungsten has a good chance
of getting back to the filament. In a small bulb, the walls need to be
made of material that doesn't absorb much light and can withstand a lot
of heat. Hence we have quartz halogen bulbs, rather than glass halogen
(republished on 07/28/06)
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