32 



Mr. S. Bid well on the Electrical 



higher than the average temperature of the air a maximum 

 resistance is attained ; and if the heating is continued, the 

 resistance begins to decrease. 



I gave a short account of this phenomenon in the ( Philoso- 

 phical Magazine ' of April 1881. Since this was published, I 

 have made further and very careful experiments, the results 

 of which are shown in the curves, figs. 1, 2, and 3. 



Ohms. 



100,000 

 140,000 

 120,000 

 100,000 

 80,000 



































60,000 

 40,000 

 20,000 



































40° 6( 



Degrees Centigrade. 



100° 



A selenium cell was placed in an air-bath in absolute dark- 



