Electrical Properties of Selenium. 255 



2. The specific resistance of crystalline Se has no definite 

 value but is always very high, varying in the specimens tested 

 from about 30 to 1600 megohms. It is not in general dimi- 

 nished by prolonged heating unless the Se is in contact with 

 a metal (Exps. 1, 2). 



3. A Se cell having Pt electrodes and made with Se to 

 which about 3 per cent, of cuprous selenide has been added, 

 is, even though unannealed, greatly superior both in con- 

 ductivity and sensitiveness to a similar cell made with ordinary 

 Se and annealed for several hours. It is not, however, quite 

 equal to a cell prepared in the usual way with copper elec- 

 trodes (Exp. 3). 



4. A particular sample of commercial Se which, when 

 treated in the ordinary manner, was almost absolutely insen- 

 sitive to light, was rendered sensitive by the addition of a 

 little metallic selenide (Exps. 4-8) . 



5. The great fall of resistance exhibited by pieces of Se 

 with fused-in electrodes after the lapse of a few years, is not 

 due to any material lowering of the specific resistance of the 

 Se, but to short-circuiting by selenide formed at the expense 

 of the electrodes. The resistance of an old cell was instantly 

 raised from 10 ohms to 60,000 by connecting it with a 52-volt 

 battery and thus burning out the short circuit (Exps. 9, 10). 



6. Red Se in contact with copper or brass is quickly dark- 

 ened by the action of light, owing, it is suggested, to the 

 formation of selenide. Photographs may be printed on 

 copper or brass coated with red Se and preserved in darkness 

 for a long time (Exps. 11-13). 



7. The resistance of crystalline Se always decreases with 

 rise of temperature. The apparent increase of resistance on 

 heating generally observed when the electrodes are fused into 

 the substance, is to be explained by the fact that the heat- 

 expansion of the Se is much greater than that of the metallic 

 electrodes ; imperfect contact is consequently produced at the 

 junction (Exp. 14). 



8. Crystalline selenium is porous and absorbs moisture 

 from the air (Exps. 15, 16). 



9. The polarization of Se after being traversed by a current 

 is due to absorbed moisture (Exps. 17, 18). 



10. The moisture ordinarily present in Se renders the 

 apparent resistance much lower than the true resistance of the 

 substance (Exp. 19). 



11. The presence of moisture is not essential to sensitive- 

 ness, but appears to be in a slight degree favourable to it 

 (Exps. 19, 20). 



