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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J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’, 1864. From The Royal Society, PT/72/7
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