concerning Volta's Contact Force. 471 



different metals, suggest, as J. Larmor points out, that the 

 speed of the negative and positive corpuscles or electrons 

 is nearly equal, but that in some metals the positive travel 

 slightly faster, in other metals the negative : the difference 

 being subordinate and secondary. 



It will turn out on this view that the " specific heat of 

 electricity " a, the coefficient of the Thomson effect, has the 

 same sign as the coefficient of magnetic curvature of current, 

 the Hall effect. For proceed with our theory of metallic 

 conduction treated as a convection of charged corpuscles 

 of both kinds, with u/v the Hittorfian migration ratio of nega- 

 tive to positive corpuscular velocity, under the action of unit 

 force; and write down as in electrolysis that the resultant 

 motion must be equal in opposite directions, 



\n dx dx / \n dx dx P 



or 



or 



dx r 



dp ne u + v, 



j u + v 7Tr 

 c(p = ne d V . 



u — v 



Then the various thermoelectric quantities get written 



exactly as above, except that the new factor must be 



everywhere introduced. u + v 



It is worth noticing that 



t . . C/A current-density .„ , ,. . 1 



ne(u + v)= = — J - S p ecl fi c conductivity =- 



nx /dx potential-gradient r J n 



wherefore 



dV f .dp 



=pmR(u-v)~ ( n T). 



This form is instructive as showing that, other things being 

 equal, highly resisting substances are likely to form the besl 

 thermoelectric materials. The metals of high conductivity 

 have, in fact, feeble thermoelectric power ; metals like bismuth 

 and antimony are poor conductors ; and the contact force 

 betweeu insulators, or still better between an insulator and a 

 conductor, may be enormous. 



But for most purposes the previous form of expression is 



