414 Dr. A. D. Fokker on the Electric Current from 

 In the second expression appear the quantities 



\ep a p c , 

 being the quadratic electric momenta of the atoms. These 

 quantities figure in recent investigations of Debye and 

 Holtsmark on the broadening of spectral lines of luminous 

 gases under increased pressure " x ". They seem to afford a 

 measure for the electrical extension of the atoms, and so it is 

 proposed to call their sum per unit of volume (provisionally) 

 the electric extension : 



K ac =ieN p a p c . 



In a form of three-dimensional vector-analysis we can con- 

 tract the three components under discussion into a vector 



k=-[( 2 K.V).w], 



where the number 2 added to the left of 2 K has to remind us 

 that 2 K is a symmetrical tensor and therefore ( 2 K.V) is a 

 differential operator wdth vector-properties f. 



This vector k is analogous to the Rontgen-vector. It 

 accounts in its curl for the second order influence of the 

 motion of polarized matter on the electric current. 



Gathering the various corrections of the polarization into 

 a single vector n, we can collect the result of the second 

 variation into the scheme for our tensor M ah : 



cm z +k z + [n.w] s — cm y — k y — [n.w] y 



-(3n„*k,-[n.w] 2 rni + k r +[n.wl 



M«*(=) 



-** -n> -a* 



whence we get by the formula 



a current : 



crotm + rotk+rot[n.w] -f n, . . . (9 a) 

 and a charge : 



— diVn {9 b) 



We notice a polarization-current k } the Rant gen- current 



* Debye, Pfcys. Zsc/w. xx. p. 160 ; Holtsmark, tf&fem, p. 162 (1919). 



f Cf. the notation of Prof. J. A. Sehouten in " Die directe Analysis 

 zur neueren Relativitatstkeorie," Transactions Kon. Akad. v. Weten- 

 schappen, Amsterdam, xii. No. 6, 1919. 



