PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



PHILOSOPHICAL SOCIETY 



83 2ND MEETING 



The 832nd meeting was held at the Cosmos Club, March 13, 1920. 

 President Sosman presiding and 35 persons present. 



W. W. CoblEnTz : Some characteristics of spectro-photoelectrical sen- 

 sitivity in solids. 



In introducing the subject the speaker said that numerous solid 

 substances, which have a low electrical conductivity in the dark, have 

 a high electrical conductivity when exposed to light, selenium being 

 the most widely known example. This photo-sensitive property is a 

 function of the temperature of the substance; also of the intensity 

 and of the wave length of the thermal radiation stimulus. It is found 

 that the photo-electrical response of substances in general is fairly 

 regular throughout the visible spectrum, terminating in a band or 

 bands of high sensitivity in the extreme red or near infra-red spectrum. 

 On decreasing the temperature of the substance, its intrinsic photo- 

 electrical sensitivity is greatly increased throughout the whole spec- 

 trum and the point of maximum sensitivity is shifted toward the short 

 wave lengths. 



On a few rare occasions experimenters have observed that on ex- 

 posure of selenium to light from an incandescent lamp the galvanometer 

 gave a negative deflection, indicating an apparent rise in resistance 

 which is higher than that which obtains in the dark, whence the name 

 "photo-negative response." In one case, using light dispersed into a 

 spectrum, it was observed in a sample of stibnite (Sb2S3) that this 

 photo-negative property is caused by yellow and green light (wave 

 lengths less than o.Gsn), whereas the red and infra-red rays produced 

 a photo-positive reaction. 



Molybdenite is a natural sulfide of molybdenum, some samples 

 of which mineral are photoelectrically sensitive. In addition to the 

 photo-positive properties just described, the speaker presented ob- 

 servations on a photo -negative reaction which was found localized 

 in a small spot, perhaps i mm. in diameter, in the crystal. This photo- 

 negative property appears to depend upon the magnitude of the po- 

 tential applied to the cr^'^stal, and is produced by wave lengths less than 

 0.65/i. Furthermore, this photo-negative property seems to depend 

 not only upon the magnitude of the potential applied but also upon the 

 direction of the current through the crystal. The photo-positive 

 action is the same as a resistance decrease caused by a rise in tem- 

 perature of the molybdenite. The photo-negative reaction is similar 

 to the building up of a counter e. m. f. 



524 



