871 
magnet moving at right angles to its magnetization. We shall follow 
the distinction of “internal” and “external” field at the end of § 14. 
The effect of this electric polarization n, called into existence by 
the motion of magnetized matter, is to produce an internal electric 
field (18.41): 
P= hh 
This could be expected to act on free electrons, present in the 
magnet, and cause a conduction current. But these electrons are 
carried along with matter and therefore are moving with velocity 
w through the internal magnetic field where the induction vector 
is (see § 18.42): 
cB — cm + k + [n.w] 
and, where the external field may be neglected *), they consequently 
are subjected to the Newtonian force 
(E+ = [w . B]). 
This expression vanishes according to the formulae of $$ 16.2 and 
19, so that the free electrons moving along with the magnet are 
not driven sideways. 
Therefore it is impossible with sliding contacts at the magnet’s 
sides to get a current from it, and the experiment with the long 
magnet drawn across a circular spring is explained. (§ 12). 
On the other hand, if we cut the magnet at right angles to the 
magnetization, and take out an infinitely thin lamella, so that a 
thin wire might be kept in the same place while the magnet is 
drawn across, then the “external” field in this split will simply be 
the continuation of the internal field. and the free electrons in the 
wire, not sharing the motion of the magnet, will be subjected to 
the electric force E only, so that an induction current will be set 
up in the wire. 
Thus we see that it is the polarization of moving magnetism that 
accounts for the inductive force, when a magnetic pole moves 
across a wire, in a case where the magnetic-field is homogeneous and 
stationary. 
Conclusive Remarks. 
21. In conclusion we may remark that the result of the first 
variation is wholly incorporated in the polarization tensor. The 
') Suppose the magnetization as being homogeneous, and the free poles of the 
magnet as being at infinite distance. 
