APPENDIX II. 



INSTRUMENTS AND METHODS USED IN RADIOMETRY. 



There are few fields of experimental investigation so beset with diffi- 

 culties as the quantitative measurement of radiant energy. This is due 

 to the fact that the radiation to be measured is generally from a surface 

 of which it is practically impossible to determine the temperature. The 

 measurement of radiant energy usually involves its transformation into 

 some other form, and the receiver used for this purpose is subject to losses 

 by heat conduction within, and by reflection, radiation, and convection 

 losses from its surface. 



As a result of inquiry into the development of the various instruments 

 and methods used in measuring radiant energy, viz, the radiometer, the ther- 

 mopile, the radiomicrometer, and the bolometer with its auxiliary galvanom- 

 eter, the writer has accumulated extensive data, part of which is included 

 here, with the hope that it may be useful to others interested in the subject. 



Various instruments have been devised, the sensibility, or the possible 

 chances for improvement, of which can be rated without further investi- 

 gation. That in many cases the sensitiveness has been overestimated, will 

 be noticed in the present paper. However, four instruments, viz, the 

 radiomicrometer, the thermopile, the bolometer, and the radiometer, have 

 been used extensively in radiation work, and in each case the inventor has 

 found qualities which seemed to him to render his instrument superior to 

 other types. But, to the writer's knowledge in no instance have all four 

 instruments been studied by any one person; and, perhaps, it should not 

 be expected. Each instrument requires a special mode of handling, and 

 has peculiarities which can be learned and controlled only after prolonged 

 use. This is particularly true of the radiometer and of the bolometer 

 with its auxiliary galvanometer. 



Having already had considerable experience with radiometers, one of 

 which was the most sensitive yet constructed, the writer has, in this exami- 

 nation, devoted most of his attention to the bolometer. The investigation 

 originated for the most part from the question whether the radiometer 

 was selective in its action, in the region of the short wave-lengths. In 

 previous work it was found 1 that the radiometer gave small deflections in 

 the violet spectrum of the arc where Snow, 2 using a bolometer, found large 

 deflections. 



1 See Investigations of Infra-red Spectra. Part II. Infra-red emission spectra. (Carnegie 

 Publication No. 35.) 



2 Snow: Physical Review, 1, p. 32, 1893. 



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