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Abstract
The “nucleation” of a localized hot spot which causes burnout of an incandescent lamp filament is considered. It is proposed that the bubbles filled with potassium vapor in doped, non-sag tungsten filaments migrate unidirectionally in a temperature gradient and coalesce to form large bubbles sufficient to develop localized hot spots responsible for burnout. The effect of increasing the fill pressure in a gas-filled lamp on reducing the equilibrium size of a bubble is calculated by incorporating the virial coefficients of potassium vapor within a bubble to account for its non-ideal behavior at high pressures and temperatures. The results suggest that increasing the fill pressure is more effective in extending filament life for lamps of high wattage than those of low wattage. It is further proposed that the increase in filament life of quartz-halogen lamps over that of the standard incandescent lamps owes its origin primarily to the pressure effect on the equilibrium size of bubbles within a hot spot to minimize localized overheating.