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

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                                Coefficients of Induction for Two Circuits 17. 
(26) In the electromagnetic field the values of L M N depend 
on the distribution of the magnetic effects due to the two circuits 
and this distribution depends only on the form and relative 
position of the circuits. Hence L M N are 
quantities depending on the form and relative position of the circuits 
and are subject to variation <s>when<\s> with the motion of the conductors 
It will be presently seen that L, M, N are geometrical quantities 
of the nature of lines, that is, of one dimension in space 
L depends on the form of the first conductor which we shall call A 
N on that of the second, which we shall call B, and M on 
the relative position of A & B 

(27) Let [xi] be the electromotive force acting on A <s>and<\s>  x the 
strength of the current and R the resistance, then Rx will 
be the resisting force. In steady currents the electromotive force 
just balances the resisting force, but in variable currents 
the resultant force [xi] - Rx  is expended in increasing 
the “electromagnetic momentum” using the word momentum 
merely to express that which is generated by a force acting during 
a certain time, that is, a velocity existing in a body. 

In the case of electric currents, the force in action is not 
ordinary mechanical force, at least we are not as yet able to 
measure it as common force, but we call it electromotive force 
and the body moved is not merely the electricity in the conductor 
but something outside the conductor capable of being affected by 
other conductors in the neighbourhood carrying currents. In this 
it resembles rather the reduced momentum of a driving point 
of a machine as influenced by its mechanical connexions, than 
that of a simple moving body like a cannon ball or water 
in a tube. 
                            
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Manuscript details

Author
James Clerk Maxwell
Reference
PT/72/7
Series
PT
Date
1864
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J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’, 1864. From The Royal Society, PT/72/7

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