JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. IV DECEMBER 4, 1914 No. 20 



PHYSICS. — Measurements on standards of radiation in absolute 

 value. ^ W. W. Coblentz, Bureau of Standards. 



One of the chief needs in the measurement of radiant energy 

 is a convenient standard against which the radiometer may be 

 cahbrated. The great utihty of a seasoned carbon incandescent 

 lamp as a photometric standard is well recognized, and such a 

 lamp has every desideratum of a standard of radiation, when 

 calibrated against a black body as the primary standard of radi- 

 ation. The object of the present investigation is to establish 

 and maintain a standard of radiation in the form of seasoned 

 incandescent lamps, and to issue to experimenters similar stand- 

 ards of radiation, whereby may be obtained an accurate cali- 

 bration of their radiometers in absolute value. 



The incandescent lamps were standardized by two independent 

 methods which are in agreement within 0.5 per cent. The first 

 method consisted in comparing the radiation from a black body 

 with the radiation from the incandescent lamp, assuming a value 

 for the coefficient (the "Stefan-Boltzmann constant") of total 

 radiation of the black body. For this purpose a thermopile 

 was directed towards the black body, then towards the incan- 

 descent lamp. The second method consisted in a direct measure- 

 ment, in absolute value, of the energy of the incandescent lamp 

 with a modified form of Angstrom pyrheliometer. This p}T- 

 hehometer gave a value {a = 5.61 X 10" ^^ watt cm-^ deg-") 



1 Detailed paper to appear in the Bulletin of the Bureau of Standards. 



565 



