Physics. — “The Contributions from the Polarization and Magne- 
tization Elections to the Electric Current’. By Dr. A. D. Fokker. 
(Communicated by Prof. H. A. Lorentz). 
(Communicated at the meeting of Juve 27, 1919). 
1. An important point in the theory of electrons is how to 
evaluate the electric current proceeding from the electrons which 
in their movements are bound to the atoms of matter. We require 
it for establishing the equations of the electromagnetic field in 
ponderable matter, and we know that it is responsible for the effects 
of polarization and magnetization. 
Consider a stream of moving neutral atoms, and imagine them as 
consisting of a positive nucleus and one accompanying electron. 
The beavy nuclei will contain the centres of mass of the atoms their 
motion therefore will be identified with the motion of matter in 
bulk. The accompanying electrons will move round the nuclei or in 
their immediate neighbourhood. Now the stream of positive nuclei 
will form an electric current, and the stream of electrons of course 
will constitute another. For a great part these two currents will 
cancel one another, but not completely, as they would, if both 
motions were the same: the resulting current is clearly what arises 
from the intra-atomical motions of the bound electrons. 
Obviously we shall know this current if, given the motion of a 
stream of particles, we can find the variation effected by displacing 
them slightly from their tracks, for it is by such small displacements 
that the motion of the electrons may be found from the motions of 
the nuclei. Our problem thus presents itself as a variation problem. 
M. Born has told us*) that the idea to put it thus is due to 
Hermann Minkowski. He has developed it after Minkowskr's death 
and compared his deductions with MinkowsKrs posthumous notes. I 
venture to offer to the Academy a novel development of the same 
idea, which might claim a great simplicity and might be more 
exact in some points. In addition, a new second order contribution 
of the bound electrons is arrived at, which has been neglected until 
now, so far as I know (§§ 9 and 11). 
1) H. Minkowski—M. Born, Mine Ableitung der Grundgleichungen fiir die 
elektromagnetischen Vorgdnge in bewegten Kérpern vom Standpunkte der 
Elektronentheorie, Math. Ann. 68, p. 526, 1910. 
