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

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                                greater electric displacement in solid dielectrics, such as glass 
or sulphur, than in air. 

(12) Here then, we perceive another effect of electromotive force, namely 
electric displacement, which according to our theory is a kind 
of elastic yielding to the action of the force, similar to that 
which takes place in structures and machines owing to the want 
of perfect rigidity of the connexions. 

(13) The practical investigation of the inductive capacity of 
dielectrics is rendered difficult on account of two disturbing 
phenomena. The first is the conductivity of the dielectric, which 
though in many cases exceedingly small, is not altogether insensible. 

The second is the phenomenon called electric absorption*, in 
virtue of which, when the dielectric <s>has been<\s> is exposed to electromotive 
force, the electric displacement gradually increases, and when 
the electromotive force is removed, the dielectric does not instantly 
return to its primitive state, but only discharges a portion of 
its electrification, and when left to itself gradually acquires electrifica[tion] 
on its surface, as the interior gradually becomes depolarized. 
Almost all solid dielectrics exhibit this phenomenon, which gives 
rise to the residual charge in the Leyden jar, and to several 
phenomena of electric cables described by Mr. F. Jenkin* 
(14) We have here two other kinds of yielding besides the yielding 
of the perfect dielectric, which we have compared to a perfectly elastic 
body. The yielding due to conductivity may be compared to that of 
a viscous fluid (that is to say a fluid having great internal friction) 
or a soft solid on which the smallest force produces a permanent 
alteration of figure increasing with the time during which the force acts. 
The yielding due to electric absorption may be compared to that of 
a cellular elastic body, containing a thick fluid in its cavities 
Such a body when subjected to pressure is compressed by degrees 
on account of the gradual yielding of the thick fluid, and when the 
pressure is removed it does not at once recover its figure, because 
the elasticity of the substance of the body has gradually to overcome the tenacity 
of the fluid before it can regain complete equilibrium. 
*Faraday Exp Res. 1233-1250 
+Trans British Association 1859 p248 & Report of Committee of Board of Trade on Submarine Cables 
pp 136 & 464.
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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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