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

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                                4<sup>th<\sup> If a small plane circuit be placed in the field and be free to turn 
it will place its plane perpendicular to the lines of force. A small magnet 
will place itself with its axis in the direction of the lines of force. 

5<sup>th<\sup> If a long uniformly magnetized bar is placed in the field, each pole 
will be acted on by a force in the direction of the lines of force 
<s>The product of this force into<\s> The number of lines of force passing through 
unit of area is equal to the force acting on a unit pole multiplied 
by a coefficient depending on the magnetic nature of the medium 
and called the coefficient of magnetic induction. 

In fluids and isotropic solids the value of this coefficient [mu] is the same 
in whatever direction the lines of force pass through the substance 
but in crystallized, strained, and organized solids the value 
of [mu] may depend on the direction of the lines of force with 
respect to the axes of crystallization, strain or growth. 
In all bodies, [mu] is affected by temperature, and in iron 
it appears to diminish as the intensity of the magnetization 

On Magnetic Equipotential Surfaces

(51) If we explore the field with a <s>very long<\s> uniformly magnetized 
bar, so long that one of its poles is in a very weak part of 
the magnetic field, then the magnetic forces will perform work 
on the other pole as it moves about the field. 

If we start from a given point, and move this pole from 
it to any other point, the work performed will be independent 
of the path of the pole between the two points, provided 
that no electric current passes between the different paths 
pursued by the pole. 

Hence, when there are no electric currents but only magnets 
in the field, we may draw a series of surfaces such that 
the work done in passing from one to another shall be 
constant, whatever be the path pursued between them. 
Such surfaces are called Equipotential Surfaces and in ordinary 
cases are perpendicular to the Lines of magnetic force. 
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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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