

Electrical Properties of Selenium. 

 Table IT. 



243 



Temperature. 

 Degrees C. 



Eesistance between 



Electrodes fused 



into ends. 



Megohms. 



Resistance between 



Electrodes pressed 



against surface. 



Megohms. 



-2 

 

 2 

 4 

 6 

 8 

 10 

 12 



45-6 



48-7 



52-0 



523 



53-3* 



52-5 



49-6 



46-7 



385 

 36-8 

 340 

 302 

 26-6 

 23-9 



* Maximum. 



Thus with rising temperature the resistance of the same 

 piece of Se appears to reach a maximum when measured 

 between fused-in wires, and to decrease ah initio when 

 measured between electrodes which are pressed upon its 

 surface. 



Beyond doubt the resistance of the Se really falls in both 

 cases. The apparent preliminary rise when the electrodes 

 are fused in, is due to the unequal heat expansion of the Se 

 and of the metal. The coefficient of linear expansion of Se is 

 twice as great as that of copper and three times as great as 

 that of platinum. The effect of heating is to loosen the tight 

 grip of the Se upon the metal and so to increase the resistance 

 of the junction. The rate at which this junction-resistance 

 increases is at first greater than that at which the specific 

 resistance of the Se diminishes : soon, however, the latter 

 effect predominates, and thus the resistance of the combina- 

 tion of Se and a metal passes a maximum. 



§ 10. Porosity of Crystalline Selenium. 



The consideration of certain phenomena, which will be 

 referred to later, led me to believe that crystalline Se, not- 

 withstanding its metallic appearance, must be a sensibly 

 porous substance. Experiments were made with the view of 

 ascertaining whether this was the fact. 



