Rob, etal,
The buildup on the inside of the glass envelope is residue from the
filament, caused by the electrons "boiling" off of it and carrying some of
the metal from the filament away. If you look at very old radio vacuum tubes
(valves to our English listeners) and see a similar plating on the glass, it
was purposely caused by a separate element installed in the tube to get the
remaining air out of it after the manufacturing process: this element worked
like a flashbulb, and it was called a "getter". Vacuum tube
technology...ain't life grand!
Jim Leach, PTC Seattle
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