182 Mr. S. Bidwell on the Sensitiveness 



disclosed the presence of the metals iron, lead, and arsenic*, 

 all of which would form conducting selenides. Nevertheless 

 I thought it would be worth while to ascertain roughly the 

 specific resistance of a piece of selenium which, since it has 

 come into my possession, has never been in contact with metal. 

 The selenium (which was supplied by Messrs. Hopkin and 

 Williams) was melted in a mould built up of slips of glass, 

 crystallized and " annealed " in the usual way ; but, contrary 

 to the general practice, it was not fitted with metallic electrodes 

 before annealing. A plate of crystalline selenium was thus 

 formed, having a thickness of about 2 millim. and a superficial 

 area of 1 square centim. The two opposite surfaces were 

 rendered smooth and clean by rubbing them upon a flat board 

 covered with fine glass-paper, and the plate was placed between 

 two layers of thick tinfoil which were pressed into good con- 

 tact with it by a weight of 500 grammes. When this arrange- 

 ment was connected in circuit with 6 Leclanche cells and a 

 reflecting galvanometer, a deflection was produced indicating 

 a current of about ^q micro-ampere. Assuming the electro- 

 motive force of the battery to have been 10 volts, the resistance 

 of the plate would be 500 megohms; and therefore the resistance 

 of a cubic centimetre of the selenium between opposite faces 

 (i. e. its specific resistance) would be 2500 megohms. From 

 the dimensions and resistance of a good selenium cell with 

 copper electrodes, which I have in my possession, I calculated 

 that the specific resistance of the selenium contained in it was 

 about # 9 megohm. Thus, so far as the result of a single rough 

 experiment can be trusted, it appears that the conductivity of 

 selenium which has been annealed in contact with copper is 

 nearly 3000 times greater than that of selenium which has 

 undergone similar treatment without the presence of a metal. 

 Whether selenium, when perfectly pure, is altogether a non- 

 conductor, would be an interesting question for an expert 

 chemist to determine |» It is sufficient for the theory which 

 I am at present advocating that its specific resistance should 

 be very high. 



By assuming the admixture with the selenium of metallic 

 selenides, an explanation is afforded of the following facts : — 



(1) The diminished resistance produced by annealing. 



(2) The fact, first pointed out by Graham Bell, that the 

 resistance of selenium appears to depend greatly upon the 

 nature of the metals of which the electrodes are formed. For 

 obtaining low resistance he recommends the use of brass in 



* Paper read before the National Academy of Sciences, April 21, 1881 . 

 f On more mature consideration I am inclined to think that it is not. 



