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| Authored by: jesse on Sunday, June 17 2012 @ 09:40 AM EDT |
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| Authored by: Anonymous on Sunday, June 17 2012 @ 12:04 PM EDT |
Wow, almost everything except the first sentence in that post is
wrong!
The light bulbs of the time were vacuum based.
As is
the standard incandescent light bulb to this day.
This prevented heat
loss other
than through radiation.
But you want to minimize heat loss -
The point of an incandescent bulb is to heat the filament so it glows (almost-)
white hot. About 3000 C. If you cool it then it doesn't glow.
It also
causes the metal to evaporate into the vacuum.It will evaporate regardless
of the atmosphere (or lack thereof), only dependent on the filament temperature.
(Subject to redeposition, see below)
That causes the core of the
wire to melt.
There's no connection between evaporation and melting of
the wire.
This was also detected by the "Edison Effect" as some of
the metal
would be deposited on the inside of the glass. Though the effect was
on the
electrons flowing, the discoloration is from the metal.
The
Edison effect is the emission of electrons, not metal atoms, from a hot metal.
It is used to good effect in vacuum tube (valve) electronics. Basically the hot
electrons have enough energy to fly free of the metal. But yes, the
discoloration in an old incandescent bulb is metal evaporated from the filament
and deposited on the glass.
The current tungsten filaments are in a
nitrogen athmosphere which provides convection cooling.
Um, no. Perhaps
you're thinking of tungsten halogen bulbs. As I said, cooling the filament is
contrary to the entire operating principle of the incandescent bulb, which is to
heat the filament hot enough to glow. But, in a tungsten halogen bulb the
envelope is filled with a halogen gas (iodine, etc). The clever idea here is
that when tungsten atoms evaporate from the filament they combine with the
halogen. This forms a stable compound that floats around in the envelope rather
than the metal being deposited on the glass. The really clever part is that when
the tungsten halide compound comes in contact with the hot filament the compound
decomposes and deposits the metal back on the filament, and releases the
gas to continue the cycle. This allows the tungsten halogen (AKA quartz) bulbs
to actually run the filament hotter, without shorting the life as would
happen otherwise.
SB
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