a. Conduction-electrons. The mean value of 9, in so far as it depends 
on these, may be called the (measurable) density of electric charge; 
we shall denote it by @,. 
The mean value © of ow may be represented by 
¢ — 0, Ww. 
This is the convection-current, and the vector 
= pe: 
taken for the conduction-electrons, may fitly be called the conduction- 
current. 
bh. Polarization-electrons. Let the body contain innumerable particles 
electrically polarized, each having an electric moment ». The vector 
defined by the equation 
1 
Dh Nt a! lew acre a ett) oP (0 
JN ge (20) 
where the sign + is to be understood in the same sense as in the 
formula (19), is the electric moment for unit volume or the electric 
polarization of the body. Replacing q by 9 in the formulae of $ 5, d, 
and taking into account (7); we find for the part of @ that is due to 
the polarization-eleetrons, 
One Div W. 
We may next remark that the visible velocity w is practically the 
same in all points of a particle. Since, for the space occupied by it, 
fe desi 
we have likewise 
fe Ww, dt = fe W, be => ko Wd ==.0, 
a 
so that the values of gw, @W,, @ w. may be found by means of 
(18). The result is 
EW = — Div (W, ), etc. ntt hd RER) 
We have finally to determine gv. Now, the quantities ev,, ory, 
ov. are of the kind considered at the end of $ 5, ¢. However, there 
are cases, especially if the velocities ¥,, ¥,, Do and the dimensions of 
the particles are sufficiently small, in which the parts of or, Ov, ov. 
corresponding to g, of $ 5, e, may be neglected. Confining ourselves 
to such cases, we shall determine g> without taking into consideration 
the particles intersected by the surface o. 
For a single particle we may write 
