On Wed, 6 May 1998 22:58:52 PDT, William Chops Westfield wrote: >(modern lightbulbs aren't high vacuum anymore, are they? They've got >some sort of inert gas?) True...it's due to vapor pressure, believe it or not. When the filament gets hot, some of the material (do they still use tungsten?) will vaporize. Any filament material that vaporizes and doesn't redeposit on the filament when it cools results in thinner, weaker portions of the filament, leading to reduced bulb life. Physical processes like evaporation tend to a dynamic state of equilibrium. The "vapor pressure" of atoms leaving the filament is balanced by the "partial pressure" of atoms running into the filament. In an initially evacuated bulb, with a filament at a given temperature, more atoms must leave the filament to build up a balancing pressure in the bulb than if the bulb were full of some inert gas in the first place. Also, within a gas-filled bulb, convection currents can cool the filament, meaning that a thicker, lower resistance filament must be used to to get to the same temperature and light output. Thicker filaments last longer. - Rick "Bright Ideas" Dickinson Enterprise ArchiTechs | Views expressed on topics unrelated http://www.eArchiTechs.com | to messaging are not those of my NoSpam eMail:rtd@notesguy.com | company, and may not even be mine.