380 The Properties of Selenium Blocks. 



succeeded in obtaining continuous crystalline films by- 

 covering the amorphous film with a varnish before heating. 

 The author attempted to crystallize the films by depositing 

 the selenium directly on a plate placed opposite the cathode 

 and heated by an electric current to a temperature just 

 below the melting-point of metallic selenium. Since at that 

 temperature amorphous selenium cannot exist, it was hoped 

 that the particles on being deposited would be changed at 

 once to the metallic state, and that the resulting film would 

 be continuous and light-sensitive. It was difficult to obtain 

 the required temperature and keep it constant, since heat 

 was supplied to the plate from the hot cathode as well as 

 from the heating wire; the pressure in the tube, the 

 "cathode fall/' and the strength of the heating current 

 would therefore have an effect on the temperature of the 

 plate. By suitably choosing the conditions, grey films were 

 obtained which appeared crystalline and continuous ; on 

 being tested they were found to be insensitive to light, but 

 after heating at a temperature of 180° C. for an hour, 

 the films became sensitive to light but " light-negative " 

 (the resistance increased on illumination). Since these 

 results were obtained Brown * has published work showing- 

 that light-negative selenium is produced by the action of 

 mercury on selenium, and the property of the films obtained 

 as above was evidently due to the presence of mercury 

 vapour from the pump and pressure-gauge connected to the 

 ct discharge-tube." The above was repeated excluding 

 mercury. The films obtained had a high specific resistance, 

 and either showed a slight sensitiveness to light in a positive 

 direction, or were practically insensitive : they became, 

 however, much more sensitive on heating for a short time in 

 air. The method can be applied to the construction of 

 selenium bridges. 



Summary. 



(1) When a selenium block is illuminated, the resistance 

 change in the direction of the light is appreciable even with 

 thick blocks. 



(2) This fact is utilised to prepare a selenium block the 

 conductivity of which can be measured (a) in a direction 

 parallel to the illuminated face (" bridge " of the first type), 

 and (b) in a direction perpendicular to this face ( ;i bridge 9> 

 of second type). 



The properties of the two types are compared. 



* Phys. Rev. vol. i. 1913, p. 237. 



