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

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                                Part II On ElectromagneticInduction. 
Electromagnetic Momentum of a Current.

(22) We may begin by considering the state of the field in the 
neighbourhood of an electric current. We know that magnetic 
forces are excited in the field, their direction and magnitude 
depending according to known laws upon the form of the 
conductor carrying the current. When the strength of the current 
is increased, all the magnetic effects are increased in the same 
proportion. Now, if the magnetic state of the field depends on 
motions of the medium, a certain force must be exerted in 
order to increase or diminish these motions, and when the 
motions are excited they continue, so that the effect of the connexion 
between the current and the electromagnetic field surrounding 
it is to endow the current with a kind of momentum, 
just as the connexion between the driving point of a machine and a flywheel 
endows the driving-point with an additional momentum which 
may be called the momentum of the fly wheel reduced to 
the driving-point. The unbalanced force acting on the 
driving point increases this momentum, and is measured by 
the rate of its increase. 
In the case of electric currents the resistance to sudden increase 
or diminution of strength produces effects exactly like those 
of momentum but the amount of this momentum depends 
on the shape of the conductor and the relative position of 
its different parts. 
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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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