Editing Incandescent light bulb (section)

===Reducing filament evaporation===
During ordinary operation, the tungsten of the filament evaporates; hotter, more-efficient filaments evaporate faster.<ref name="StdHdbk11"/> Because of this, the lifetime of a filament lamp is a trade-off between efficiency and longevity. The trade-off is typically set to provide a lifetime of 1,000 to 2,000 hours for lamps used for general illumination. Theatrical, photographic, and projection lamps may have a useful life of only a few hours, trading life expectancy for high output in a compact form. Long-life general service lamps have lower efficiency, but prior to the development of compact fluorescent and LED lamps they were useful in applications where the bulb was difficult to change.

[[Irving Langmuir]] found that an inert gas, instead of vacuum, would retard evaporation. General service incandescent light bulbs over about 25 watts in rating are now filled with a mixture of mostly [[argon]] and some [[nitrogen]],<ref name="IES1981">John Kaufman (ed.), ''IES Lighting Handbook 1981 Reference Volume'', Illuminating Engineering Society of North America, New York, 1981 {{ISBN|0-87995-007-2}} page 8-6</ref> or sometimes [[krypton]].<ref>Burgin. Lighting Research and Technology 1984 16.2 61–72</ref> While inert gas reduces filament evaporation, it also conducts heat from the filament, thereby cooling the filament and reducing efficiency. At constant pressure and temperature, the thermal conductivity of a gas depends upon the molecular weight of the gas and the cross sectional area of the gas molecules. Higher molecular weight gases have lower thermal conductivity, because both the molecular weight and cross sectional area are higher. [[Xenon]] gas improves efficiency because of its high molecular weight, but is also more expensive, so its use is limited to smaller lamps.<ref>{{cite web|url=http://donklipstein.com/xeincand.html#p|title=Premium Fill Gasses|last=Lipstein|first=Don|access-date=13 October 2011|url-status=live|archive-url=https://web.archive.org/web/20111011122615/http://donklipstein.com/xeincand.html#p|archive-date=11 October 2011}}</ref>

''Filament notching'' is due to uneven evaporation of the filament. Small variations in [[electrical resistivity|resistivity]] along the filament cause "hot spots" to form at points of higher resistivity;<ref name="Kane2001"/> a variation of diameter of only 1% will cause a 25% reduction in service life.<ref name="GETP110" /> Since filament resistance is highly temperature-dependent, spots with higher temperature will have higher resistance, causing them to dissipate more energy, making them hotter – a [[positive feedback]] loop. These hot spots evaporate faster than the rest of the filament, permanently increasing the resistance at that point. The process ends in the familiar tiny gap in an otherwise healthy-looking filament.

Lamps operated on direct current develop random stairstep irregularities on the filament surface which may cut lifespan in half compared to AC operation; different alloys of tungsten and [[rhenium]] can be used to counteract the effect.<ref>{{cite web |title=Miniature Lamps: Technical Information |publisher=Toshiba Lighting & Technology Corporation |url=https://www.tlt.co.jp/tlt/english/products/industrial_light/miniature_lamp/catalog/info.htm |access-date=25 February 2019 |archive-date=26 February 2019 |archive-url=https://web.archive.org/web/20190226112649/https://www.tlt.co.jp/tlt/english/products/industrial_light/miniature_lamp/catalog/info.htm |url-status=live }}</ref><ref>John Kaufman (ed.), ''IES Lighting Handbook 1981 Reference Volume'', Illuminating Engineering Society of North America, New York, 1981 {{ISBN|0-87995-007-2}} page 8-9</ref>

Since a filament breaking in a gas-filled bulb can form an [[electric arc]], which may spread between the terminals and draw very heavy current, intentionally thin lead-in wires or more elaborate protection devices are therefore often used as [[fuse (electrical)|fuses]] built into the light bulb.<ref>{{cite web|url=http://www.teralab.co.uk/Glass_Blowing/Lamp_Autopsy/Lamp_Autopsy_Page1.htm|title=Glass Blowing for Vacuum Devices – Lamp Autopsy|last=Hunt|first=Robert|date=2001–2006|publisher=Teralab|access-date=2 May 2007|url-status=live|archive-url=https://web.archive.org/web/20070311121008/http://www.teralab.co.uk/Glass_Blowing/Lamp_Autopsy/Lamp_Autopsy_Page1.htm|archive-date=11 March 2007}}</ref> More nitrogen is used in higher-voltage lamps to reduce the possibility of arcing.<ref name="IES1981"/>