164 proceedings: philosophical society 



Discussion. The paper was discussed by Messrs. Abbot, Bauer, 

 and Weed. Mr. Sosman inquired as to the nature of the effect of 

 adding carbon. Mr. Clime stated that the increase in response seemed 

 out of all proportion to the increase in absorption due to blackening 

 the surface. Mr. Briggs asked about a lack of symmetry in the il- 

 luminated system, and Mr. Swann spoke of observing small electro- 

 motive forces in very thin films of platinum illuminated by a beam of 

 light not perpendicular to the surface. The chair expressed the thanks 

 of the society to Mr. Clime for his interesting communication. 



Mr. W. W. Coblentz then gave a paper on Characteristics and com- 

 parative sensitiveness of various types of radiometers. 



The various types of radiometers were divided into three groups. 



Group I includes radiometers which are nonselective in their re- 

 sponse to stimuli of radiant energy of all wave lengths. In these instru- 

 ments the radiant energy is absorbed and converted into heat. They 

 include the Nichols radiometer, the thermocouple, the Boys radio- 

 micrometer, and the Langley bolometer. In all these instruments 

 the response is proportional to the stimulus, and they are well adapted 

 for quantitative work. 



Group II includes substances which have the property of decreasing 

 in electrical resistance when exposed to radiant energy of short wave- 

 lengths, especially visible and ultraviolet rays. The phenomenon de- 

 pends entirely upon the wave-length of the light stimulus. Many 

 substances are "light-sensitive," e.g., copper oxide, sulphides of an- 

 timony and silver, and crystalline selenium. The latter has been the 

 most extensively investigated. Its sensitivity depends upon heat treat- 

 ment, and varies with the intensity and wave-length of the light stim- 

 ulus. After exposure to light for a few seconds, it requires a long 

 time to recover its normal ("dark") resistance. It is not adapted to 

 quantitative work. 



Group III includes the photo-electric cells, substances which, when 

 charged to a negative potential, lose their charge when exposed to 

 light; especially violet and ultra-violet rays. This emission of elec- 

 trons is a surface phenomenon, easily disturbed by oxidation of the 

 surface. The photo-electric cells investigated consisted of potassium 

 and rubidium. .It was found that the response (galvanometer deflec- 

 tion) was not proportional to the intensity of the stimulus, but, dif- 

 fering from the selenium cell, this lack of proportionality of response 

 does not appear to depend upon the wave-length of the exciting light. 

 In 1907 Angstrom described a new method for studying solar radia- 

 tion. By covering his pyrheliometer with a blue glass he was able 

 to study atmospheric diffusion of solar radiation independently of the 

 water vapor present. He obtained some very interesting data on the 

 temporary variations in the solar constant. In view of the fact that 

 the potassium, photoelectric cell has its maximum sensitivity in the 

 violet, the speaker tried it as a pyrheliometer. It was found entirely 

 too sensitive (on only 2 volts) to be used with an insensitive d'Arson- 

 nal galvanometer. Using a higher voltage and a milliammeter, the 



