COBLENTZ AND EMERSON: PHOTOELECTRIC SENSITIVITY 525 



are involved. It is suggested that a study of the relationship 

 between the intensity of light emitted and the applied potential 

 would prove of value as a spectroscopic determination of critical 

 potentials. 



PHYSICS. — The photoelectric sensitivity of various substances. 1 

 W. W. Coblentz and W. B. Emerson, Bureau of Standards. 



Some time ago an examination was made of various substances 

 to determine their electrical sensitivity to light; and in view of 

 the fact that some of the results obtained are at variance with 

 the measurements made by Case, 2 it seems desirable to publish 

 a summary of our observations. 



Two of the herein described substances were examined for 

 change in electrical conductivity caused by the action of light 

 upon them, and all of them were examined for photoelectrical 

 activity when they were charged to a negative potential in an 

 evacuated bulb and exposed to light. 



When the substances were examined for an increase in elec- 

 trical conductivity, a potential of 2 to 6 volts was connected 

 through a resistance of zero to 100,000 ohms into a circuit con- 

 taining a d'Arsonval galvanometer and the substance under 

 investigation. In most cases the substances were slightly con- 

 ducting when not exposed to light, so that the "dark current" 

 had to be annulled by joining a counter e.m.f. through a re- 

 sistance of 10,000 ohms to the terminals of the galvanometer. 

 This counter e.m.f. was obtained by shunting across a re- 

 sistance of 100 ohms which was in series with a cell of 2 volts 

 and a variable resistance of zero to 70,000 ohms. 



The source of light, when not otherwise specified, was a 

 16-c.p. carbon incandescent lamp, placed at a distance of 10 

 cm. from the substance under investigation. One disappointing 

 feature of this investigation is that no substance was found 

 which is comparable in sensitivity with the potassium photo- 

 electric cell and with the selenium cell. 



1 To be published in full in the Bulletin of the Bureau of Standards. 



2 Case. Phys. Rev., 9: 305. 1917. 



