PROFESSOR CLERK MAXWELL OX THE ELECTROMAGNETIC FIELD. 
487 
where a, /3, 7 are the components of magnetic intensity or the force on a unit magnetic 
pole, and ^a, ^7 are the components of the quantity of magnetic induction, or the 
number of lines of force in unit of area. 
In isotropic media the value of ^ is the same in all directions, and we may express 
the result more simply by saying that the intrinsic energy of any part of the magnetic 
field arising from its magnetization is 
Sir 
p 
per unit of volume, where I is the magnetic intensity. 
(72) Energy may be stored up in the field in a different way, namely, by the action 
of electromotive force in producing electric displacement. The work done by a variable 
electromotive force, P, in producing a variable displacement, f, is got by integrating 
sw 
from P = 0 to the given value of P. 
Since P =kf, equation (E), this quantity becomes 
WV=W'=W- 
Hence the intrinsic energy of any part of the field, as existing in the form of electric 
displacement, is 
42(P/+Qy+K/0<ZV. 
The total energy existing in the field is therefore 
E=2 {s l(^“+A“0+w)+W+Q?+iwOpv (i) 
The first term of this expression depends on the magnetization of the field, and is 
explained on our theory by actual motion of some kind. The second term depends on 
the electric polarization of the field, and is explained on our theory by strain of some 
kind in an elastic medium. 
(73) I have on a former occasion * attempted to describe a particular kind of motion 
and a particular kind of strain, so arranged as to account for the phenomena. In the 
present paper I avoid any hypothesis of this kind ; and in using such words as electric 
momentum and electric elasticity in reference to the known phenomena of the induc- 
tion of currents and the polarization of dielectrics, I wish merely to direct the mind of 
the reader to mechanical phenomena which will assist him in understanding the elec- 
trical ones. All such phrases in the present paper are to be considered as illustrative, 
not as explanatory. 
(74) In speaking of the Energy of the field, however, I wish to be understood literally. 
All energy is the same as mechanical energy, whether it exists in the form of motion or 
in that of elasticity, or in any other form. The energy in electromagnetic phenomena is 
mechanical energy. The only question is, Where does it reside 1 On the old theories 
* “ On Physical Lines of Force,” Philosophical Magazine, 1861-62. 
