( 264 ) 
/ 
fe Ml te ok 
4 dt 
and for a physically infinitely small space, partaking of the visible 
motion 
/ “ad = 
ODIE = = Pa 
hi dt 
On account of (20) this is equal to 
/ 
(SW). 
dt 
so that 
fe 1 éi 1 
y= — dt—=- 5D 
oO „Je oat 5 En Dj 
In performing the differentiation we must attend to the change of Pin 
a point that moves with the velocity Ww. If relates to a fixed point 
of space, we have 
= a oy 
— == fy aire ny 
and, since 
ds ' 
a Lie Ie, 
dt 
5 i oy op , 
ov = P + W‚ Ste Wy, Te +- We aa = Y Div w. 
ae 
~ 
Combining this with ie we get for the mean value of the cur- 
rent corresponding to the motion of the polarization-electrons 
PH Rot [P. rw]. 
c.  Maynetization-clectrons. If the body contains magnetized particles 
(§ 4, c), we have nothing to add to 9 and gw. There will however be 
a new part of ov. We can calculate it by applying (18), because the 
quantities (5) vanish for every particle. 
Let us first replace, in the formulae of § 5d, q by ev,. We 
then find 
qn 10; dy = — Wz, qz = + m,, 
and, if we denote by M the magnetic moment for unit volume or 
the magnetization, a vector that is to be defined in a similar way as p; 
DE Dn Me, =d 9 
Finally, by (18), 
My. 
os OM, ON, 
with similar expressions for ov, and ove. 
