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
 Author
 James Clerk Maxwell
 Reference
 PT/72/7
 Series
 PT
 Date
 1864
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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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