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

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                                Exploration of the Electromagnetic Field 

Let us now suppose the primary circuit A to be of invariable 
form and let us explore the electromagnetic field by means 
of the secondary circuit B which we shall suppose to be variable 
in form and position 

We may begin by supposing B to consist of a short straight conductor with 
its extremities sliding on two parallel conducting rails which are put 
in connexion at some distance from the sliding piece. 

Then if sliding the moveable conductor in a given direction increases 
the value of M, a negative electromotive force will act in the circuit B 
tending to produce a negative current in B during the motion of the 
sliding piece 

If a current be kept up in the circuit B, then the sliding piece will 
itself tend to move in that direction, which causes M to increase 
At every point of the field there will always be a certain direction 
such that a conductor moved in that direction does not experience 
any electromotive force in <s>any<\s> whatever direction its extremities are turned. 
A conductor carrying a current will experience no mechanical force 
urging it in that direction or the opposite 

This direction is called the direction of the line of magnetic force 
through that point. 

Motion of a conductor across such a line produces electromotive 
force in a direction perpendicular to the line and to the direction of motion 
and a conductor carrying a current is urged in a direction perpendicular 
to the line and to the direction of the current. 

(48) We may next suppose B to consist of a very small plane circuit 
capable of being placed in any position and of having its plane turned 
in any direction. The value of M will be greatest when the plane 
of the circuit is perpendicular to the line of magnetic force. Hence 
if a current is maintained in B, it will tend to set itself in this position 
and will of itself indicate, like a magnet, the direction of the magnetic 
force. 
                            
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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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