ABSTRACTS 



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RADIOMETRY. — The photo-electric cell and other selective radiometers. 

 W. W. CoBLENTz. Bur. stand. Sci. Paper., No. 319. Pp. 29. 

 1918. 



This paper deals with the apphcation of special physical and chemi- 

 cal properties of matter, as a means of quantitatively measuring radi- 

 ant energy. 



Certain substances have the property of decreasing in electrical 

 resistance when exposed to radiant energy of short wave-lengths, 

 especially the visible and ultra-violet rays. Crystalline selenium 

 belongs to this class of substances. The sensitivity of the selenium 

 cell varies not only .with the wave-length but also with the intensity 

 of the light stimulus; and it recovers but slowly from the effects of 

 the light stimulus. It therefore fails to meet the requirements of a 

 radiometer, except that of high sensitivity. 



The application of the photo-chemical action upon a photographic 

 plate, as a means of making quantitative radiometric measurements is 

 considered. While this method of radiometry has been used success- 

 fully, its applications seem to be rather limited. 



The alkali metals, and especially their hydrides, are very sensitive 

 to light stimuli. Photo-electric cells made of these substances can be 

 constructed and operated so that the response (photo-electric current 

 released) is proportional to the intensity of the stimulus. This meets 

 one of the principal requirements of a satisfactory radiometer. Details 

 of constructions, operation, and characteristics of the photo-electric 

 cell are given, and a satisfactory, high-resistance, iron-clad Thomson 

 galvanometer is described, which may be used successfully with the 

 photo-electric cell. 



The advantages of the photo-electric cell over the thermopile are 

 considered and the application of the former is advocated for measure- 

 ments of radiant energy (especially ratios of intensities) in the violet 

 and ultra-violet parts of the spectrum, where the photo-electric cell 

 greatly exceeds the thermopile and the bolometer in sensitivity 



w. w. c. 



536 



