238 



Mr. Shelford Bidwell on the 



crystallized. That in which there was no Cu 2 Se was annealed 

 for six hours. For comparison, the table is headed with 

 particulars of a good ordinary copper-wire cell of similar 

 dimensions. 



Table I. 



Composition of Cell. 



Annealed or 

 Unannealed. 



Percentage 

 of added 



CugSe. 



Resistance 

 in dark of 

 1 sq. cna. 



Megohms. 



Percentage 



decrease in 



light. 



Cu Wire and Se 



Well annealed 



» >» 

 Unannealed. 



>? 



77 

 71 

 97 











0-5 



1-5 



3 



4 



5 



07 



139 



12-0 



147 



36 



090 



0-25 



45 to 50 

 33 

 32 



57 

 47 



38 

 28 



Pt Wire and Se 



Pt wire and Se-f-Cu 2 Se 



r> • H 

 ?! yy 

 t> r> 



In the cell containing 1*5 per cent, of Cu 2 Se, the thickness 

 of the Se coaling was accidentally made much less than the 

 diameter of the wires, instead of equal to it, as in the other 

 cells : hence its unduly high apparent resistance and its 

 correspondingly great sensitiveness (for its exposed surface 

 was disproportionately great). In other cases the results are 

 as regular as could be expected. They demonstrate clearly 

 enough that a cell of fairly good sensitiveness and conductivity 

 may be formed without any annealing, if a suitable quantity 

 of metallic selenide be first added to the Se. They also 

 render very evident the fact that an excessive quantity of 

 selenide, while it reduces the resistance to a comparatively 

 low point, only does so at the cost of the sensitiveness. This 

 agrees with the well-known fact that an ordinary Se cell 

 whose resistance has been unduly lowered by too long heating 

 is only moderately sensitive to light. Exactly what part is 

 played by the free selenium remains yet to be determined ; 

 it is certain that cuprous selenide by itself is quite insensible. 



§ 6. Sensitizing Insensitive Selenium. 



During the last fifteen years 1 have worked with a great 

 many different samples of Se, and have observed that they 



