[LEVI] PHOTO-ELECTRIC CONDUCTIVITY OF DIAMOND 255 



primary current, and between the photo-negative reaction and the 

 secondary current. In the work with the diamond the effects obtained 

 could be accounted for by the hypothesis of a counter electromotive 

 force, the existence of which is demonstrated by the polarization 

 currents in Fig. 15. 



6. CONCLUSION 



From the above considerations it is evident that there is no simple 

 explanation for the phenomenon of photo-electric conductivity. It 

 may be that the explanation lies in the crystal structure of the sub- 

 stance.' Some weight is given to this idea by the fact that hammering 

 a piece of photo-sensitive acanthite^^ rendered it entirely insensitive 

 to light, while merely rubbing the surface of a piece of light sensitive 

 molybdenite with a toothpick^* decreased its sensitivity. In order to 

 approach the problem from this angle an investigation of electrical 

 conductivity in crystals, together with a sound knowledge of crystal 

 structure, is necessary. 



7. SUMMARY 



1. Photo-electrical conductivity of two diamonds has been in- 

 vestigated. It has been found that there is considerable lag both in 

 growth and decay of the photo-electric currents, and that the photo- 

 electric sensitivity increases with decreasing wave-length of exciting 

 light. 



2. It has been found that the photo-sensitive diamonds exhibit 

 the phenomenon of unidirectional conductivity, and the characteristics 

 of the dark currents have been investigated. 



3. It has been shown that on application of an electric field the 

 diamonds become polarised, and that counter fields develop within 

 the crystal. 



4. On exposure to gamma rays, a photo-positive reaction was 

 obtained with one diamond, and a photo-negative reaction with the 

 other. 



5. From considerations of the various properties of the crystals 

 it has been shown that no simple explanation of the phenomenon has 

 been reached, and it is suggested that the problem may be solved by 

 consideration of crystal structure and conductivity in crystals. 



"Coblentz and Kahler, Sci. Papers Bur. of Stands, No. 344, p. 243, etc. 

 "Coblentz and Kahler, Sci. Papers Bur. of Stands, No. 338, pp. 152-153. 



