Properties of Selenium Blocks. 



373 



temperature — was taken, and from it was cut a rectangular 

 block of dimensions 10 mm. x 10 mm. X 8 mm. (as shown in 

 fig. 1). To the sides of the plate thus cut, a copper 

 plate B and the glass plate C carrying the thin platinum 

 film were clamped (fig. 2). Selenium contracts considerably 



Fio;. 1. 





on changing from the liquid amorphous state to the cry- 

 stalline^ and on putting a mould containing amorphous 

 selenium into an oven at the temperature necessary to 

 crystallize, large cavities are formed in the selenium on 

 solidifying. To obviate this as much as possible, in crystal- 

 lizing the blocks, the following method was adopted : — Into 

 the rectangular mould prepared as above, molten selenium 

 was poured to a depth of about 1'5 mm. ; the mould was 

 then kept in a hot oven until the selenium crystallized. The 

 surface of the crystallized selenium was then melted by 

 means of a hot steel spatula, and then molten selenium 

 poured in to about the same depth as before and crystallized. 

 This was repeated until the mould was full ; the whole was 

 then kept at a temperature of about 200° C. for several days 

 to ensure complete crystallization. 



The selenium in the bridge thus prepared could be first 

 illuminated through the platinum film (as a bridge of 

 type B), and then the direction of the light could be changed, 

 and the surface of the selenium exposed to the air illuminated 

 (bridge A), the resistance in both cases being measured from 

 the copper plate B to the platinum film C. 



