Closing Years of the Nineteenth Century. 427 



is proportional to t7, and the fictitious charge on the sur- 

 faces of the dielectric is proportional to + (a - 1) U. It is evident 

 from this that if a plane condenser is charged to a given 

 difference of potential, and is rotated in its own plane, the 

 magnetic field produced is proportional to * if (as in Kowland's 

 experiment*) the coatings are rotated while the dielectric 

 remains at rest, but is in the opposite direction, and is propor- 

 tional to (c - 1) if (as in Kontgen's experiment) the dielectric is 

 rotated while the coatings remain at rest. If the coatings and 

 dielectric are rotated together, the magnetic action (being the 

 sum of these) should be independent of f a conclusion which 

 was verified later by Eichenwald.f 



Hitherto we have taken no account of the possible mag- 

 netization of the ponderable body. This would modify the 

 equations in the usual manner,:}: so that they finally take the 

 form 



div D = p, (I) 



div B = 0, (II) 



curl H = 47rS, (III) 



-curl.E = B, (IV), 



where S denotes the total current formed of the displacement - 

 current, the convection-current, the conduction-current, and the 

 current of dielectric convection. Moreover, since 



S =pv + d'/47rc 2 , 

 we have 



div S = div pv + (l/4;rc 2 ) div (ad/80 



= div v 



*Cf. p. 339. 



t Ann. d. Piiys. xi (1903), p. 421 ; xiii (1904), p. 919. Eichenwald performed 

 other experiments of a similar character, e.g. he observed the magnetic field due 

 to the changes of polarization in a dielectric which was moved in a non- 

 homogeneous electric field. 



J It is possible to construct a purely electronic theory of magnetization, a 

 magnetic molecule being supposed to contain electrons in orbital revolution. It 

 then appears that the vector which represents the average value of h. is not H, 

 but B. 



