'Resistance of Selenium ivitJi Temperature. 467 



the total change of resistance from 130° C. to 170° 0. 

 is about 1/30 of that from 0° C. to 130° C. It is also 

 remarkable that beyond 130° G. all time lags, as well as the 

 light and voltage effects, disappear. 



Several experiments were made to find out the resistance 

 between 170° C. to 217° 0. (molting point), but no definite 

 conclusions could be arrived from them. Beyond 170° C, 

 selenium begins to sublime, so that the results obtained 

 cannot be taken as trustworthy; but in almost all the 

 cases the cells showed a decrease in conductivity. Whether 

 this is due to a partial transformation into the amorphous 

 variety or a true increase in resistance, could not be defi- 

 nitely ascertained *. It is, however, striking that the 

 calculated values also increase beyond 170° 0. 



4. Discussion of Results. 



The results of the present experiment are thus in serious 

 contradiction to the results obtained by previous observers ; 

 for S. Bidwe'l t> who made a similar set of experiments, has 

 found that " when the temperature of the cell reaches a 

 point which is in general a few degrees higher than the 

 average temperature of the air, a maximum resistance is 

 obtained ; and if the heating is continued, the resistance 

 begins to decrease." As is well shown by the curve, not 

 the slightest tendency is noticed of any increase in resistance 

 between 0° C. and 170° C. It is, however, not clear from his 

 paper whether he experienced the influence of " time " pre- 

 viously described and took the necessary precautions. 



S. Bid well used his results in an attempt to refute 

 Dr. Moser's view J that the light sensitiveness is perhaps 

 due to the heating caused by light. The result of the 

 present experiment therefore reopens the question. 



A separate experiment was therefore made to test how far 

 the heating of the selenium due to light is responsible for the 

 light sensitiveness. One of those cells whose temperature 

 coefficient had been previously determined was therefore 

 enclosed in a light-tight box provided with a photographic 

 shutter, the whole being placed in a Dewar's flask, which 

 forms the simplest heat insulator. First of all, the selenium 

 circuit was made for a few seconds, with the selenium in the 

 dark, in order to determine the dark current. The shutter 



* It may be a transformation into the (3 modification of Ries {toe. cit.), 

 which shows an increase of resistance with temperature, 

 t S. Bidwell, loc. cit. 

 % Moser, Phil. Mag. (5) vol. xii. p. 212, 



