566 BURGESS AND WALTENBURG : EMISSIVITY OF METALS 



for the coefficient of total radiation which is close to the value 

 of this constant as determined by various other methods differing 

 widely in character. 



The standard of radiation, in terms of these incandescent 

 lamps, may therefore be considered fairly well established — at 

 least sufficiently to fulfill present requirements. In the lamps 

 issued to the public the intensity of the radiant energy per 

 mm.^ at a distance of 2 meters from the lamp is certified, when 

 the lamp is operated on a given current and voltage. 



For rough calibrations of radiometers in absolute measure, 

 a sperm candle or a Hefner lamp may be used. For the sperm 

 candle, burning at a mean height, the intensity of the total 

 radiation at a distance of 1 meter from the flame may be 

 taken to be 29 X 10"^ g.-cal per cm^ per second. The in- 

 tensity of the total radiation at a distance of 1 meter from the 

 Hefner lamp without a diaphragm may be taken to be 26 X 10~i'' 

 g-cal per cm"^ per second. The paper gives data also for the 

 radiation from the Hefner lamp with a diaphragm, having a 

 definite opening and situated at a definite distance in front of 

 the lamp. The flame standards are unsteady, and unsatisfactory 

 for use in refined radiation measurements. 



PHYSICS. — The emissivity of metals and oxides, II: Measure- 

 ments with the micro pyrometer. G. K. Burgess and R. G. 

 Waltenburg, Bureau of Standards.' 



The micropyrometer^ may be used conveniently for the approx- 

 imate determination of the monochromatic emissivities of metals, 

 oxides, etc., in microscopic quantities at high temperatures. It 

 is possible to determine to 1 per cent the emissivity of a mass 

 of 0.01 mgm. having an area of 0.25 mm.^ and a thickness of 

 0.005 mm. The determination of the temperature-coefficient of 

 emissivity is readily made, as well as the detection of variation 

 of emissivity with change of state, as at the melting point. 



Assuming the emissivity for solid platinum to be 0.33 for red 

 light of wave length X = 0.65^ and 0.38 for X = 0.55m, the fol- 



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



2 This Journal, 3: 7. 1912. 



