PRESSURE ON RESISTANCE OF METALS. 



639 



TABLE XXIV. 



Comparison op Changes op Volume prodxtced by Temperature and 



Pressure. 



Riecke, Drude, and Lorentz. 

 theory is given by 



The specific resistance according to this 



w = Const = Const' ^, 



where u is the velocity of the electrons, / their mean free path, and N 

 the number per cm'. The constant has different numerical values 

 according to Drude and Lorentz. The second form of the equation is 

 obtained by putting u"^ proportional to T. 



Apart from specific heat difficulties, which it seems to me have been 

 over-emphasized, this theory has alwaj^s had difficulty in giving a 

 plausible explanation of the variation of resistance with temperature, 

 it being necessary to suppose that / decreases as the volume increases 

 with rising temperature. But since the mean free path is supposed 

 entirely determined by the positions of the atoms, these being immo- 

 bile compared with the free electrons, the hypothesis of decreasing I is 

 difficult. A suggestion as to a way out is by considering the collisions 

 among the electrons themselves; there may be a more than propor- 

 tional increase in the number of such collisions as the space between 

 the atoms becomes larger. But this possibility is removed when we 

 ■consider the changes at constant volume. It is a result of these 



