JH>                the filimant to its operating temp. Fatigue from the
 JH>                expansion and contraction of many heat cycles (see #1)
 JH>                eventually causes a thin section which heats before the
 JH>                rest of the filament and melts, resulting in an arc and
The thin section forms in large part because a certain percentage of the 
Tungsten molecules vaporize from the filament, and condense in other places.  
Some condense back onto the filament, though not necessarily in the same 
spots.  Others condense on the envelope or elsewhere.  
Quartz theatrical lamps, which concentrate large amounts of energy into very 
hot filaments in order to allow focusing of leco's and acheivement of high 
color temperatures, rely on a high temperature envelope to discourage 
condensation other than back onto the filament.  The graying of an older 
incandescant bulb is metallic redeposition that has migrated from filament to 
envelope.  
Anyone for throwing in Wein's Law and Stefan's Law while we're at it?  
Terry
--- Maximus 2.01wb
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