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

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                                (6) We may therefore receive, as a datum derived from a branch 
of science independent of that with which we have to deal, 
the existence of a pervading medium, of small but <s>measurable<\s> 
real density, capable of being set in motion, and of transmitting 
motion from one part to another with great, but not 
infinite, velocity.

Hence the parts of this medium must be so connected 
that the motion of one part depends in some way on the 
motion of the rest; and at the same time these connexions 
must be capable of a certain kind of elastic yielding, since the 
communication of motion is not instantaneous, but occupies 

The medium is therefore capable of receiving and storing 
up two kinds of energy, namely, the “actual” energy <s>consisting<\s> depending 
on the motions <s>which take place in it<\s> of its parts, and potential 
energy, consisting of the work which the medium will do in 
recovering from displacement in virtue of its elasticity. 

The propagation of undulations consists in the continual 
and alternate transformation of one of these forms of energy into 
the other, and at any instant the amount of energy in the 
whole medium is equally divided, so that half is energy of motion, 
and half is elastic resilience. 

(7) A medium having such a constitution may be capable 
of other kinds of motion and displacement than those which 
produce the phenomena of light and heat, and some of 
of these may be of such a kind that they may be evidenced 
to our senses by the phenomena they produce. 

(8) Now we know that the luminiferous medium is in 
certain cases acted on by magnetism, for Faraday* discovered 
that when a plane polarized ray traverses a transparent 

*Experimental Researches Series 19 
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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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