J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’

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                                the communication of motion from one part of the 
medium to another, and it is by means of this force that 
the motion of one part causes motion in another part. 
When electromotive force acts on a conducting circuit, it produces 
a current, which, as it meets with resistance, occasions a 
continual transformation of electrical energy into heat, which 
is incapable of being restored again to the form of electrical 
energy by any reversal of the process. 

(11) But when electromotive force acts on a dielectric it produces 
a state of polarization of its parts similar in distribution 
to the polarity of the parts of a mass of iron under the influence 
of a magnet, and like the magnetic polarization, capable of 
being described as a state in which every particle has its opposite 
poles in opposite conditions* 

In a dielectric under the action of electromotive force we may 
conceive that the electricity in each molecule is so displaced 
that one side is rendered positively and the other negatively 
electrical, but that the electricity remains entirely connected 
with the molecule, and does not pass from one molecule to 
another. The effect of this action on the whole dielectric mass 
is to produce a general displacement of electricity in a certain 
direction. This displacement does not amount to a current 
because when it has attained to a certain value it remains 
constant, but it is the commencement of a current, and its 
variations constitute currents in the positive or the negative 
direction according as the displacement is increasing or decreasing. 
In the interior of the dielectric there is no indication of electrification 
because the electrification of the surface of any molecule is neutralized 
by the opposite electrification of the surface of the molecules in contact 
with it, but at the bounding surface of the dielectric, where the electrification 
is not neutralized, we find the phenomena which indicate<s>ing<\s> positive or 
negative electrification 
The relation between the electromotive force and the amount 
of electric displacement it produces depends on the nature of the 
dielectric, the same electromotive force producing generally a 
+Faraday Exp Res 
*Mossotti Mem della Soc. Italiana (Modena) vol XXIV part 2 (18 ) p 49 
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Manuscript details

James Clerk Maxwell
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Cite as

J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’, 1864. From The Royal Society, PT/72/7



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