JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. VII NOVEMBER 4, 1917 No. 18 



PYROMETRY. — The -proper type of absorption glass for an 

 optical pyrometer. Paul D. Foote, F. L. Mohler, and C. 

 O. Fairchild, Bureau of Standards. 



On account of the deterioration of the filament of the stand- 

 ard lamp used in the Holborn-Kurlbaum form of optical pyrom- 

 eter the maximum temperature at which the instrument may 

 be safely operated is about 1400° or 1500°C. For the measure- 

 ment of higher temperatures the intensity of the light entering 

 the pyrometer is decreased by means of an absorption glass or 

 sectored disk. If strictly monochromatic light were employed, 

 the following relation resulting from Wien's law would apply 

 to measurements made with the diminished intensity: 



i i_xio ge r = A 



e s a 



2 



where 6 is the true absolute temperature of the black-body 

 source, S the measured a'bsolute temperature for the wave 

 length X, and T' = /(X), the transmission of the absorption 

 glass or sectored disk for the wave length X. Under such con- 

 ditions A is a constant with respect to 0. However, as one of 

 the writers 1 has shown, A is by no means in general a constant 

 when the light is not absolutely monochromatic, this condition 

 being the one experimentally obtained by the use of red glass 

 screens, and of chief interest in optical pyrometry. The object 



1 Foote. Bur. Stds. Sci. Paper No. 260. 1916. 



545 



