( 258 ) oe 
4. In the second place, we shall consider particles having in one 
part of their volume or surface a positive and in another part an 
equal negative charge. In this case, for which a pair of equal and 
opposite electrons would be the most simple example, the surrounding 
field is due to (4) and (5). We shall say that a particle of this kind is 
electrically polarized and, denoting by r the veetor drawn from the 
origin towards the element of volume dr, we shall call the vector 
fertr=y Et ei eee RN 
the electric moment of the partiele. In virtue of the supposition 
fesr=o, 
this vector is independent of the position of the origin of coordinates. 
From (7) we may infer immediately 
| ox PE Pax , etc. : fe dr pe . ete. 
In all dielectrics, and perhaps in conductors as well, we must ad- 
mit the existence of particles that may be electrically polarized. We 
shall refer to their charges by the name of polarization-electrons. d 
c. Finally, let there be a class of particles whose field is solely | 
due to the expressions (6), the integrals (3), (4), (5) being all 0. If 
we suppose the values of : 
' 
fexar, fessvar, feszde, etc. 
= a . 
not to vary in the course of time, we can express all the integrals 
(6) by means of the vector : 
- 
ee fe F 
= ae fe ie) a nk RA ieee 
— e a 
ay ; : 
Le. of the vector whose components are [ 
4 
log: 
Me = 5 Oo (y vz — 284) dr, ete., 
Indeed, we shall have 
Je WX as. | 
bs e 
The field produced by a particle satisfying the above conditions 
» 
0 Tr ea | er 2dr + m, ete. (9) 
may be shown to be identical to the field due to a small magnet 
Whose moment is yw. For this reason, we shall speak of a magnetized 
particle and we shall call m its magnetic moment. 
a 
Ea; 
TN va 
