162 



RADIOMETRY. 



tiveness by as much as 50 per cent. Porter's radiometer was the most 



sensitive of the instruments having a period of 90 seconds. But he gained 



little, on the whole, for the vanes were so light that he could work only 



during the quiet hours at night. On the other hand, the writer, 1 after 



trying light vanes, adopted the heavy ones, and was thus able to work at 



all hours, even with a large air-compressor in operation, in an adjoining 



room. 



Table VI. Sensitiveness of Various Radiometers. 



Observer. 



E. F. Nichols: 



Phys. Rev., 4, p. 297, 1897 



Astrophys. Jour., 13, p. 101, 1901 



Stewart: 



Phys. Rev., 13., p. 257, 1901 



Drew: (Phys. Rev., 17, p. 321, 1903) 



Porter (Astrophys. Jour., 22, p. 229, 1905) 



Coblentz 



Abs. Spectra 



Phys. Rev., 16, 20, and 22. (Vac. tube).. 



Another vane 



(Ibid.) 



Full period. 



mm. sec. 

 O 12 



O 



I 



5 



1 

 1 

 1 



1 

 2 



1 



11 



20 



30 

 3 

 30 



5 

 40 



3 

 10 



Area of vanes. 



sq. mm. 

 2X15 



3-i 



3 



7 



3-6 



15(10X1.5) 

 12 

 11 (nXi) 



4-5 



4-5 



Deflections per 

 square mm . area 

 of exposed vane; 

 candle and scale 

 each at 1 m.' 



5<?) 



12.5 

 4.9 



17 

 17.1 



27-5 

 8 to 10 

 1 o to 12 

 52 

 7i 

 35 



Sensitiveness Compared with Wave-Length of Exciting Source. 



In his investigations of emission spectra of the alkali metals, using a 

 bolometer, and a prism and lenses of quartz, Snow 2 found that the vapor 

 of the carbon arc had the larger portion of its energy concentrated in one 

 large band in the violet. The writer, 3 using a radiometer, a rock-salt 

 prism, and a mirror spectrometer for investigating infra-red emission 

 spectra, found that the radiometer gave but small if any deflections in the 

 violet. The violet band is far enough from the reflection minimum of 

 silver not to be weakened by it, hence it appeared that the radiometer 

 might be selective in its behavior to radiant energy. 



To test this point the following experiment was tried. The total 

 radiation from an aluminum spark (with glass plate condenser) on a 

 10,000 volt transformer was measured with a very sensitive radiometer 

 (period 65 to 70 seconds; sensitiveness 35 cm. per square millimeter) just 

 described, and with a bolometer, to be described subsequently. The win- 

 dow of the radiometer was of white fluorite 2 mm. thick, hence transparent 

 to the ultra-violet, but opaque beyond 10 ft. A large point of the energy 

 of the aluminum spark lies in the ultra-violet. The maximum energy of 

 the warm electrodes occurs at about 8 ft. The radiation from the spark 



1 "Investigations of Infra-red Spectra," Carnegie Publication No. 35, 1905. 



2 Snow: Phys. Rev., 1, pp. 28 and 221, 1893. 



3 Carnegie Publication No. 35, Part II, 1905. 



