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Q & A: halogen bulbs

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Most recent answer: 10/22/2007
I have a question about quartz halogen bulbs that redeposit the vaporized tungsten on the filament. After the tungsten in vaporized, it would have a tendency to redeposit itself on the coolest part of the filament, which would be the ends of the filament after it has begun to cool. Would this mean that the center of the filament would not recieve tungsten like the rest of the filament?
- Jake Elder (age 17)
Sheldon High School, Eugene, Oregon
That's a really thoughtful question, but our initial answer at least will be somewhat guesswork.

There are a couple competing effects in determining where most of the tungsten gets redeposited. One is as you say that molecules (of the tungsten halide gas) are more likely to stick longer to the cooler spot. However, their decomposition to redeposited metal and halide gas is driven by heat, so it may go faster at the hotter spots. I'm not sure where the deposition ends up being fastest.
Since these bulbs do ultimately burn out, effects like these, which cause the filament to become non-uniform, could be involved. If the redeposition goes fastest at the hottest spots, that would help prolong filament life since those will tend to be the thinnest spots in the filament.

Mike W.

(published on 10/22/2007)

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