[LEVI] PHOTO-ELECTRIC CONDUCTIVITY OF DIAMOND 251 



field and completing the circuit through the crystal. If the latter 

 were polarised, the galvanometer would register a current in the 

 direction opposite to that developed while the field was on. 



The effect was shown by both diamond No. 1 and No. 2, much 

 more markedly in the latter case. On the illumination of the diamonds, 

 a photo-electric current flowed in a direction opposite to that which 

 flowed when the external field was applied. The presence of 

 this current proved conclusively the existence within the crystal of 

 counter-fields of sufficient strength to develop a measurable photo- 

 electric current under the influence of light. The results obtained 

 are illustrated in Fig. 12. 



Effect of Exposure to Gamma Rays. 



Since it has been found that photo-sensitivity increased with 

 decreasing wave-length of exciting radiation, it was thought to be of 

 interest to find out the effect of radiation of very short wave-length, 

 and therefore the crystals were exposed to gamma radiation. With 

 diamond No. 1 it was found that a small photopositive current 

 resulted on exposure to the gamma rays, but with diamond No. 2 

 the rays had a repressive effect, causing a very small photo-negative 

 current. Fig. 13 and Fig. 14 illustrate the effect on the growth and 

 decay of photo-electric current of exposing the crystals simultane- 

 ously to gamma radiation and to light. The repressive effect of the 

 rays on the growth of the current in diamond No. 2 is to be noticed. 

 The currents obtained on exposure to gamma rays alone are also 

 shown in these figures. 



5. Discussion. 



(a) Chemical Composition of Photo-sensitive Substances. 



Since it has been found that not all samples of diamond are 

 sensitive to light, it is evident that photo-sensitivity is not intrinsic- 

 ally a property of a substance. The work of other observers also 

 leads to this conclusion. Over 150 minerals have been examined by 

 Case^, and in his paper he lists, among others, Wulfenite, Greenockite, 

 Sphalerite and Cinnabar as not being affected by illumination. On 

 the other hand, Gudden and Pohl' have found samples of these 

 minerals which are sensitive to a marked degree. Again, Case lists 

 Galena as photo-sensitive, while Coblentz and Kahler* were able to 



^T. W. Case, Phys. Rev. 2; p. 305, 1917. 



«Gudden and Pohl, Zeitschr. fur Physik, 2, p. 361, 1920; also 5, p. 176, 1921. 



^Coblentz and Kahler, Sci. Papers Bur. of Stands, No. 344, p. 247. 



