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XLII. The E/fect of Temperature upon the Electrical Resist- 

 ance of Selenium. By Shelford Bidwell, M.A.* 



T is well known that the electrical resistance of crystalline 

 selenium is greatly affected by changes of temperature ; 

 but authorities differ in a remarkable manner as to whether a 

 rise in temperature produces an increase or decrease of resist- 

 ance. Some experiments which I have lately made may per- 

 haps account for this discrepancy. 



The temperature of the room being 14° Centigrade, a sele- 

 nium-cell, made as described in ' Nature,' vol. xxiii. p. 58, 

 was immersed in turpentine at 8° C. There was a great 

 and sudden fall in the resistance. The temperature was then 

 gradually raised. In passing from 8° to 24° the resistance 

 steadily increased ; from 24° upwards it rapidly diminished. 

 For this cell, therefore, the resistance is greatest at 24° 0. 

 Five other cells were afterwards submitted to the same opera- 

 tion ; and their resistance was found to be greatest at tempe- 

 ratures of 23°, 14°, 30°, 25°, and 22° respectively. 



It should be noticed with regard to the cell whose tempe- 

 rature of maximum resistance is 14° (8° lower than any other), 

 that it was prepared by very slowly cooling melted selenium 

 from a temperature of about 250° 0. Its surface is coarsely 

 granular ; its resistance is comparatively low; and it is not very 

 sensitive to light. All my other cells were crystallized by 

 gradually heating the vitreous selenium up to nearly 217° 0. 

 Their structure is very much finer ; and though their resistance 

 is higher, they are exceedingly sensitive to light. 



The above results were obtained by observing galvanometer- 

 deflections. 



In order to form some idea of the actual amount of varia- 

 tion in resistance caused by extremes of heat or cold, an ex- 

 periment was made with a cell whose resistance at ordinary 

 temperatures in a dim light was about 110,000 ohms. When 

 immersed in turpentine at — 6° C. it was reduced to 49,000 

 ohms; and in paraffin at 115° C. it measured only 18,000 

 ohms. 



All the experiments were made in a uniform light, which 

 was only just strong enough to enable me to read the ther- 

 mometer. 



* Communicated by the Author. 



