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-watt vacuum lamp, if operated at 10 mean lumens per watt, would live 190 hours and the 1000 watt gas-filled lamp 35,000 hours. The lives of other sizes of lamps at this efficiency is between these two extremes. There has been an enormous improvement since Edison's first commercial carbon lamp of 1880. It is estimated that if the present 40-watt tungsten filament lamp were made for the same mean efficiency as the 1880 lamp, the tungsten filament lamp should have a life of over a hundred and fifty thousand years. And this does not include the enormous improvement in the larger sizes of lamps due to Dr. Langmuir's invention of the gas-filled lamp. Owing to these enormous differences, the other term of quality measurement, indicating the mean efficiency for a given life, is therefore used to show the improvements since 1880. A table gives these data in chronological order. It is estimated that in 1880 about 50 lumen-hours of light were obtained for one cent, covering the cost of current and lamp renewals. The amount now obtainable is very much greater, due to lamp improvements, reduction in their prices and reduction in rates for current. The public has utilized these by using more light. With the present 40-watt tungsten filament lamps and with current at the present approximate general average rate of 4 1/2 cents kw-hr., 1700 lumen-hours can be had for one cent. It is impossible to show what part of this actual increase is due to lamp improvements except to indicate what would have been obtainable with one factor without the other. Thus had there been no rate reduction, 432 lumen-hours would now be had due to actual lamp improvements. Without the lamp improvements 190 lumen-hours would now be had due to the reduction in the general average rate. To indicate the gain due to Dr. Langmuir's invention of the gas-filled lamp, in which the high wattage sizes are the most efficient, 3820 lumen-hours can be had with the 1000 watt lamp at 4 1/2 cents per kw-hr. About half a billion dollars were spent in the United States in 1922 for current used for lighting. If the same amount of light were produced by the original 1880 bamboo carbon lamp, the cost would have been increased 3 1/2 billion dollars requiring about fifty billion extra tons of coal, equal to about ten per cent of the total coal production in the United States.