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Abstract
Preliminary results of life tests on microminiature lamps and observations on dc notch formation show that longer lamp life can be achieved by improving the vacuum during exhaust. Experimental results show that early burn-out of filaments heated by dc is preceded by extensive twist and distortion of the filaments causing localized over-heating. Filament instability results from the thermal stresses developed by extensive notch formation due to differential rates of electromigration of tungsten atoms on the filament surfaces along various crystallographic planes. By improving the vacuum, notching and twisting of the filament are minimized with a subsequent seven-fold increase in filament life.