(A 
The mean value of the current, in so far as it is due to the 
magnetization-electrons, is therefore 
Rot NM. 
It may be called the current equivalent to the magnetization. 
$ 7. It remains to take together the different parts of the second 
member of (1). Putting 
Oy geen et NE é (22) 
EN, (23) 
and di ROED iW - (24) 
we have 
SBL FHC H NH Rot M. 
Now, we might understand by the current in the ponderable body 
the whole of this vector. Conformly to general usage we shall however 
exclude from it the last term. We therefore define the current as 
the vector 
re ee Gh El Mt see ees (25) 
so that 
SIRE DEL a soar Ree (20) 
We may call 9 the dielectric displacement in the ponderable body, 
and & the displacement-current. As to the total current ©, the for- 
mula (25) shows that it is composed of the displacement-current, the 
conduction-current 3, the convection-current € and the fourth vector X, 
for which Porcaré has proposed the name of Rdntyen-current, because 
its electromagnetic effects have been observed in a well-known expe- 
riment of RONTGEN. 
§ 8. We shall now write down the equations that arise from 
(DV) and (1) if every term is replaced by its mean value. In order 
to obtain these formulae in a usual form, we shall put 
OPN Eh ta A A ES te (27) 
Dn MB En OS EARN 
Baya Ne EE Te en LL EN 
these quantities being the magnetic induction, the magnetic force in the 
ponderable body and the electric force in the body. 
Beginning with the equation (I), and writing @ instead of 9, for 
the (measured) density of electric charge, we find 
Dvd = 0 — Div J, 
whence 
DO yr EES 5X a Ree, Se AE Se (I') 
N 
