The present status of the standards of thermal radiation maintained by the Bureau of Standards

The basis of the lamp standards of radiation, established and maintained by the Bureau of Standards since 1913, is the black body using the Stefan-Boltzmann constant of total radiation <7= 5.70X10 watt/cm/deg. 4 In this paper the present status of the value of the constant of total radiation is reviewed, and it is concluded that, for the present, the basis of the lamp calibration remains unchanged. These carbon-filament lamp standards of thermal radiation maintained by the Bureau of Standards (S227) were intercompared after a lapse of 12 years and found in good agreement with the original standards which were established in 1913 by direct comparison with a black body. Two reproductions of these standards which, according to published reports, showed rapid deterioration in radiant flux, were recalibrated. No deterioration, to 2 parts in 840, could be measured in the radiation emitted by one of these lamps which had been in use 8 years. In the other lamp an increase in radiant flux, amounting to about 1 percent, was found. This was caused by a hot spot in the filament, probably resulting from usage or perhaps by injury in shipment. In a life test, a standard of radiation was operated for 245 hours before an increase in radiant flux became appreciable, and at the expiration of 300 hours this increase amounted to only 0.8 percent. One section is devoted to the technique of operation, and specific directions are given to insure high precision.