The Quality of Incandescent Lamps

The new specifications for incandecent lamps determine their quality in terms of life to burnout at a specified mean efficiency. Formerly it was measured by the life to 80 per cent. of initial value. The new tests, therefore, conform more nearly to the actual practise in the use of tungsten filament lamps, as many of these lamps are above 80 per cent at time of burnout. The data on carbon lamps in the paper are based on a life exponent originally determined by Mr. Howell, one of the authors of this paper. Curves showing the relation between life, efficiency and candle power was given by him in a paper presented before the Institute on April 10, 1888. Practically the same life-candle power relation has been found to apply to the GEM, tantalum and tungsten filament lamps. Nowadays the exponent used is that applying to the life-efficiency relation and is different for each kind of filament as, they have different candle power-efficiency relations. All data are based on commercial ratings and guarantees. The quality of tungsten filament lamps has greatly improved since their commercial introduction in 1907 as is shown by the fact that the 40-watt vacuum lamp is now over eight times as good as then. There is a difference in the present relative quality of the various sizes of tungsten filament lamps. The 10-walt vacuum lamp, if operated at 10 mean lumens per watt, would live 190 hours and the 1000 watt gas-filled lamp 35,000 hours.