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

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                                Several solid bodies in which no such structure as we have 
supposed can be found, seem to possess a mechanical property 
of this kind*, and it seems probable that the same substances <s>when<\s> if 
dielectrics may possess the analogous electrical property, and 
if magnetic, may have corresponding properties relating to the 
acquisition retention, and loss of magnetic polarity. 

(15) It appears therefore that certain phenomena in electricity 
and magnetism lead to the same conclusion as those of optics 
namely that there is an aetherial medium pervading all bodies 
and modified only in degree <s>altered modified<\s> by their presence; that 
the parts of this medium are capable of being set in motion 
by electric currents and magnets; that this motion is 
communicated from one part of the medium to another 
by forces arising from the connexions of those parts; 
that under the action of these forces there is a certain 
yielding, depending on the elasticity of these connexions 
and that therefore energy in two different forms may exist in 
the medium, the one form being the actual energy of motion 
of its parts, and the other being the potential energy stored up 
in the connexions, in virtue of their elasticity. 

(16) Thus then we are led to the conception of a complicated 
mechanism, capable of a vast variety of motion, but at 
the same time so connected that the motion of one part 
depends, according to definite relations, on the motion of 
other parts, these motions being communicated by forces arising 
from the relative displacement of the connected parts, in virtue 
of their elasticity. Such a mechanism must be subject to 
the general laws of Dynamics, and we ought to be able to 
work out all the consequences of its motion, provided we 
know the form of the relation between the motions of the 
parts. 

*As for instance the composition of glue treacle &c of which small plastic figures are 
made, which after being distorted gradually recover their shape.
                            
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Manuscript details

Author
James Clerk Maxwell
Reference
PT/72/7
Series
PT
Date
1864
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J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’, 1864. From The Royal Society, PT/72/7

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