The
efficiency of an incandescent lamp design centers about attaining high
filament temperatures without degradation and loss of heat. Edison’s early
selection of carbon, the highest melting temperature element, with a melting
point of 3,599 oC or 6510 oF seemed the obvious choice.
The problem with carbon is that at high operating temperatures it evaporates,
or sublimes, relatively quickly at 0.1 torr at 2,675 oC, resulting
in short filament life.
The
early solution to this dilemma was to operate the filament at lower temperatures
to attain reasonable life. However, the incandescent brightness of the
bulb was sacrificed in the process.
Other
light bulb inventors tried two new filament materials to improve bulb brightness.
In 1898, Karl Auer used osmium, which has a melting point of 2,700 oC
/ 4,890
oF. Then in 1903, Siemens and Halske worked with tantalum,
which melts at 2,996 oC / 5,425 oF. These elements
drew attention because they could operate at higher temperatures with longer
life and less evaporation. |
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