Properties of Selenium Blocks. 



75 



In previous observations, the decrease in intensity of the light 

 due to the gauze was compensated for. 



With the bridges made by pressing electrodes on selenium 

 cubes, as with those made by crystallizing selenium between 

 copper and platinum electrodes, the same result was arrived 

 at: — bridge "B" was invariably more sensitive than 



bridge "A," and calculation gave an impossible value for the 

 " depth of penetration of the light." The results of expe- 

 riments with two cubes are recorded in Table II. Illumi- 

 nations of different colour were obtained by means of colour- 

 filters (obtained from Wratten and Wainwright), the energy 

 of the transmitted light being kept constant by using a 

 thermopile. 



Table II. 



1 



Illumination. 



Besistance in Direction 

 Perpendicular to that of 



Light. (Bridge " A.") 



Besistance in Direction 



Parallel to that of 

 Light. (Bridge " B.") 



iCuBE I. 



Cube II. 





Bes. in 

 Dark. 



Bes. in 



Light. 



P. c. Bes. 



Change. 



3-6 

 21 

 2-2 



Bes. in Bes. in P. c. Bes.; 

 Dark. Light. Change. 



Bed 



Green 



Blue 



733,000 

 732,000 

 727,000 



707,000 

 717,000 

 711,000 



109,400 



104,200 

 102,600 



116,200 

 113 600 



89,500 18-2 

 93,400 10-5 

 88,300 13-9 





Bed 



197,400 

 200,300 

 203,200 



193,300 

 198,300 

 200,700 



2-1 

 1-0 

 1-2 



101,400, 12-7 

 106,300 : 6-4 



99,800 S-9 



Blue 



109,600 



It is a characteristic of selenium bridges that the resistance 

 changes on reversing the current ; but it was ascertained 

 that, whichever way the current flowed through selenium 

 blocks, the relatively large sensitiveness of bridge " B " 

 showed itself. For instance, the resistance between two 

 opposite faces of a cube was 173,000 ohms, tbe change on 

 illuminating one of these faces (i. e. as a bridge of type " B") 

 being 35 per cent. On reversing the current, tbe resistance 

 changed to 210,000 ohms, the change on illuminating the 

 same surface being 29 per cent. With another bridge of 

 this type, the resistance, on reversing the current, fell from 

 40,000 ohms to 23,000 ohms, and the light-sensitiveness in 

 the two directions was 45 and 23 per cent, respectively ; 

 using the preparation as a bridge of the first type, and 

 illuminating the same surface, the resistance change never 

 exceeded 9 per cent. 



