<P> The bulb is filled with an inert gas, to reduce evaporation of the filament and prevent its oxidation at a pressure of about 70 kPa (0.7 atm). </P> <P> The role of the gas is to prevent evaporation of the filament, without introducing significant heat losses . For these properties, chemical inertness and high atomic or molecular weight is desirable . The presence of gas molecules knocks the liberated tungsten atoms back to the filament, reducing its evaporation and allowing it to be operated at higher temperature without reducing its life (or, for operating at the same temperature, prolongs the filament life). It however introduces heat losses (and therefore efficiency loss) from the filament, by heat conduction and heat convection . </P> <P> Early lamps, and some small modern lamps used only a vacuum to protect the filament from oxygen . This however increases evaporation of the filament, albeit it eliminates the heat losses . </P> <P> The most common fills are: </P>

Why does a light bulb need a vacuum