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

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                                The case of a condenser composed of parallel layers of substances 
of different electric resistances and inductive capacities is next examined 
and it is shown that the phenomenon called electric absorption 
will generally occur, that is, the condenser, when suddenly discharged, will 
after a short time show signs of a <u>residual<\u>  charge. 

(20) The general equations are next applied to the case of a magnetic 
disturbance propagated through a non conducting field, and it is 
shown that the only disturbances which can be so propagated 
are those which are transverse to the direction of propagation, 
and that the velocity of propagation is the velocity v, found from experiments 
such as those of Weber which expresses the number of electrostatic units 
of electricity which are contained in one electromagnetic unit. 
This velocity is so nearly that of light, that it seems we have strong 
reason to conclude that light itself (including radiant heat, and other radia 
radiations if any) is an electromagnetic disturbance in the form of waves 
propagated through the electromagnetic field according to electromagnetic 
laws. If so, the agreement between the elasticity of the medium 
as calculated from the rapid alternations of <s>a<\s> luminous vibrations 
and as found by the slow processes of electrical experiments, shows 
how perfect and regular the elastic properties of the medium must be 
when not encumbered with any matter grosser than air. If the same 
character of the elasticity is retained in dense transparent bodies, it appears 
that the square of the index of refraction is equal to the product 
of the specific dielectric capacity and the specific magnetic capacity. 
Conducting media are shown to absorb such radiations rapidly 
and therefore to be generally opaque. 

The conception of the propagation of transverse magnetic disturbances 
to the exclusion of normal ones is distinctly set forth by Professor 
Faraday* in his “Thoughts on Ray Vibrations” The electromagnetic 
theory of light as proposed by him is the same in substance as that 
which I have begun to develope in this paper except that in 1846 
there were no data to calculate the velocity of <s>[text?]<\s> propagation. 
*Phil. Mag. May 1846 or Exp Res III p 447
                            
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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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