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LIII. On the Variation of Resistance of Selenium toith 

 Temperature. By Snehamoy Datta, M.Sc, D.I.C., 

 Research Student, Imperial College of Science and 

 Technology *. 



1. Introductory. 



r¥l HOUGH the light sensitiveness of selenium has been 

 I very well studied by a large number of workers, it 

 seems rather surprising that the corresponding heat sensi- 

 tiveness has been studied by only two observers |, whose 

 results appear to differ from one another considerably. It 

 was therefore thought worth while to measure the tempera- 

 ture coefficient of electrical resistance of selenium, with a 

 view to decide the more important question — viz., how far 

 the special properties of: selenium are attributable to the 

 changes in conductivity caused by changes in temperature. 

 The results of the experiments have given rise to some 

 suggestion?, which have also been discussed here. 



2. Experiments and Observations. 



Several cells were prepared by pressing melted amorphous 

 selenium between two glass plates about 1 sq. in. in size and 

 having a thickness of about *025 in. Two copper electrodes 

 and one junction of a copper-constantan thermo-couple were 

 imbedded in selenium before it was annealed. The process 

 of annealing was carried out with the cell in a light-tight 

 enclosure, and usually consisted in keeping the cell at about 

 200° C. for nearly five minutes and subsequently lowering- 

 its temperature to about 170° (J. It was kept at this tem- 

 perature until the cell reached its maximum conductivity. 

 This Avas determined by putting the cell in series with a 

 battery and a galvanometer while annealing. At first no 

 deflexion was observed — presumably the selenium was in 

 the non-conducting state. In about half an hour slight 

 conductivity was noticed ; from then onward, deflexions of 

 the galvanometer were observed every five minutes, showing 

 a gradual increase in conductivity, at first slowly and then 

 rapidly. After about three hours the rate of increase in 

 conductivity appeared to be very slow, and subsequently a 

 very slow decrease was noticed. In most cases at this stage 

 the annealed selenium was allowed to cool suddenly. On 

 two occasions, however, annealing was further continued, 



* Communicated by Prof. A. 0. Rankine, D.Sc. 



t S. Bidwell, Phil. Mag. vol. xv. p. 31 ; C. Ries, TJu/s. Zeit. vol. ix. 

 p. 228 (1908). 



