Closing Years cf the^ Nineteenth Century. 46 T 



an examination of the circumstances of a collision, in the manner 

 of the kinetic theory of gases, shows that (b - a) must have the 

 form - ur^ (r)/l, where I denotes a constant which is closely 

 related to the mean free path of the electrons. In the terms 

 on the right-hand side of the equation, on the other hand, 

 Maxwell's term gives a result different from zero; and in 

 comparison with this we may neglect the terms which arise 

 from u\ (r). Thus we have 



urv(r) leE d 8\ N --, 



/ \m 



or 



lu --, fieNE d (N\ 2M* da) . 



and thus the law of distribution of velocities is determined. 

 The electric current i is determined by the equation 



i = e Jj'J uf (it, v, w) du dv dw, 



where the integration is extended over all possible values of the 

 components of velocity of the electrons. The Maxwellian term 

 in f (u, v, w) furnishes no contribution to this integral, so we 



have 



i = e JJJ v? x (r) du dv dw. 



When the integration is performed, this formula becomes 





mu dx ' dxf 



or 



STT^W a . m /a 2 dN da\ 



'~&N l + 2~e(N~fa'* a dx)' 



The coefficient of i in this equation must evidently represent 

 the ohmic specific resistance of the metal ; so if y denote the 

 specific conductivity, we have 



4/r N 



Let the equation be next applied to the case of two metals 

 A and B in contact at the . same . temperature T, forming an 



