Tuesday, July 7, 2009

Vacuum applications: the Nernst lamp

Nernst lamps were an early form of electrically powered incandescent lamps. Nernst lamps did not use a glowing tungsten filament. Instead, they used a ceramic rod that was heated to incandescence. Because the rod (unlike tungsten wire) would not further oxidize when exposed to air, there was no need to enclose it within a vacuum or noble gas environment; the burners in Nernst lamps could operate exposed to the air and were only enclosed in glass to isolate the hot incandescent emitter from its environment.
Developed by Walther Nernst in 1897 at Goettingen University, these lamps were about twice as efficient as carbon filament lamps and they emitted a more "natural" light (more similar in spectrum to daylight). The lamps were quite successfully marketed for a time, although they eventually lost out to the more-efficient tungsten filament incandescent light bulb. One disadvantage of the Nernst design was that the ceramic rod was not electrically conductive at room temperature so the lamps needed a separate heater filament to heat the ceramic hot enough to begin conducting electricity on its own.
In the U.S., Nernst sold the patent to George Westinghouse who founded the Nernst Lamp Company at Pittsburgh in 1901.
After Nernst lamps fell into obsolescence the term "Nernst glower" went on to be used to describe the infrared-emitting source used in IR spectroscopy devices. (Recently, even this term has become obsolete as Nernst glowers have been largely replaced for this purpose by silicon carbide glow bars which are conductive even at room temperature and therefore need no preheating.).
Glower or Glowbar or even Globar, is indeed a term indicating a silicon carbide rod of 5 to 10 mm width and 20 to 50 mm length which is electrically heated up to 1000 to 1650 °C (1800 to 3000 °F). When combined with a downstream variable interference filter, it emits radiation from 4 to 15 micrometres wavelength