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If an inert gas fill
is used, it is usually a mixture of
argon and nitrogen. Argon is an
inexpensive inert gas that conducts
heat less than nitrogen does.
Destructive electric arcs form
easily in pure argon, so nitrogen is
added
to keep arcs from
forming easily. A gas fill slows
down evaporation of the filament
compared to a vacuum
because gas atoms can
"bounce" atoms of evaporated
tungsten back onto the filament. So
with a gas fill,
the filament can be
operated at a higher temperature
more favorable to radiating visible
light. A gas fill has
the disadvantage of
conducting heat from the filament.
Heat conducted from the filament is
energy that is lost with no chance
of producing light. But the
advantages of a gas fill usually
outweigh the disadvantages if the
wattage is near or over 25 watts per
inch or 10 watts per centimeter of
visible apparent filament length.
Most 120 volt
lightbulbs of 60 watts or more have
a gas fill. Most 120 volt lightbulbs
of 15 watts or less have
a vacuum. Lightbulbs
of 25 to 40 watts usually have a
vacuum if the visibly apparent
filament length is much
over 1 inch (2.5 cm),
whether or not the filament is
"compacted" into a looping or
multiple-angle shape with filament
supports. A few lightbulbs have
premium gas fills using krypton or
xenon, which conduct heat less
than argon does and
reduce filament evaporation more
than argon does. |
