Then
the invention of ductile tungsten, a much improved filament material, sparked
the development of the modern tungsten filament incandescent light bulb
by the General Electric Company and William Coolidge in 1906-10. This is
the light bulb we know today. Ductile tungsten has many favorable properties
such as
-
a high
melting point: 3,410 oC / 6,170 oF
-
low
evaporation rate at high temperatures: 10-4 torr at 2,757 oC
/ 4,995 oF
-
tensile
strength greater than steel
Because
of its strength, ductility and workability, tungsten can readily be formed
into the filament coils, used to enhance performance in modern incandescent
bulbs. Due to its high melting point, tungsten can be heated to 3000oC
/ 5,432 oF, where it glows white hot providing very good brightness.
However, the early tungsten filaments still sublimed too quickly at such
high temperatures. As they sublimed, they also coated the bulbs with
a thin black tungsten film, reducing their light output.
Inert
gases such as nitrogen and argon were later added to bulbs to reduce tungsten
evaporation, or sublimation. While these gases reduced evaporation and
increased filament life, they also carried heat away from the filament,
reducing its temperature and brightness. Winding the wires into fine coils,
as used in modern incandescent filaments, reduced convective heat loss,
allowing the filament to operate at the desired temperatures.
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