Which arn't that conventional.
Most lamps are inert gas filled.
Vacuum lamps are rather common.
Conventional incandescent lamps with a wattage near or over something
like 10 watts per centimeter of visibly-aparent filament length usually
get the traditional argon-nitrogen fill. The gas slows down filament
evaporation, and that allows designing for a higher filament temperature
that is better for radiating visible light.
But if the power per unit of visibly-apparent filament length (or maybe
length plus diameter) is less than something like 10 watts per centimeter,
then a vacuum is better than the usual argon-nitrogen mixture. A gas fill
in these lamps would conduct enough heat from the filament to reduce
efficiency of producing visible light more than redesigning for a higher
filament temperature (with argon-nitrogen) would increase the efficiency
of producing visible light.
As it turns out, heat conduction from a wire (or a rod) to a surrounding
gas is usually nearly enough proportional to the length of the wire, but
does not vary proportionately with the diameter. If you increase the
diameter of a hot wire, you also increase the thickness of the "boundary
layer" of hot gas around the wire, and that decreases the temperature
gradient within the boundary layer, often (not always) nearly as much as
the circumference of the wire is increased. So heat conduction by a
gas from a long thin filament is more than from a shorter, thicker or
wider (even if tightly coiled and wider) filament that has the same
exposed surface area. This explains why 15 watt 120V incandescent lamps
have a vacuum while 12V lamps of the same wattage have a gas fill.
- Don Klipstein (
[email protected])