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

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                                diamagnetic medium in the direction of the lines of magnetic 
force produced by magnets or currents in the neighbourhood, 
the plane of polarization is caused to rotate. 

This rotation is always in the direction in which positive 
electricity must <s>circulate<\s> be carried round the diamagnetic body in 
order to produce the actual magnetization of the field. 

M. Verdet* has since discovered that if a paramagnetic 
body such as solution of perchloride of iron in ether be substituted for the diamagnetic 
body, the rotation is in the opposite direction. 

Now Professor W. Thomson+ has pointed out that no 
distribution of forces acting between the parts of a medium 
whose only <s>displacement<\s> motion is that of the luminous vibrations, 
is sufficient to account for the phenomena, but that we 
must admit the existence of a motion in the medium 
depending on the magnetization, <s>as well as<\s> in addition to 
the vibratory motion which constitutes light. 

It is true that the rotation by magnetism of the plane of 
polarization has been observed only in media of considerable 
density, but the properties of the magnetic field are <s>so little<\s> not so much 
altered by the substitution of one medium for another, or for 
a vacuum, as to allow us to suppose that the dense medium does 
anything more than merely modify the motion of the ether. 

We have therefore warrantable grounds for inquiring whether 
there may not be a motion of the ethereal medium going on 
wherever magnetic effects are observed, and we have some reason 
to suppose that this motion is one of rotation, having the direction 
of the magnetic force as its axis. 

Comptes Rendus <u>vol xliii<\s> (1856<s>)<\s> second half year p 529 and <s>xliv(<\s> 1857 first half year p 1209 
+ Proceedings of the Royal Society June 1856 & June 1861 
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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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