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HI. On the connexion between Electric Current and the Electric and Magnetic 
Inductions in the surrounding field* 
By J. H. Poynting, M.A., late Fellow of Trinity College, Cambridge, 
Professor of Physics, Mason College, Birmingham. 
Communicated by Lord Rayleigh, M.A., D.C.L., F. It S. 
Received January 31,—Read February 12, 1885. 
In a paper published in the Philosophical Transactions for 1884 (Part II., pp. 343- 
361), I have deduced from Maxwell’s equations for the electromagnetic field the 
mode in which the energy moves in the field. The result there obtained is that the 
energy moves at any point perpendicularly to the plane containing the directions of 
the electric and magnetic intensities, and in the direction in which a right-handed 
screw would move if turned round from the positive direction of the electric intensity 
to the positive direction of the magnetic intensity. The quantity crossing the plane 
per unit area per second is equal to the product of the two intensities multiplied by 
the sine of the included angle divided by 47 r.t 
Hence it follows that the energy moves along the intersections of the two sets of 
level surfaces, electric and magnetic, where they both exist, their intersections giving, 
as it were, the lines of flow. In the particular case of a steady current in a wire 
[Added July 15.—Since the reading of the paper I have found a remarkable passage in Faraday’s 
Experimental Researches,’ vol. 1, p. 529, § 1659, which I give below. The words I have put in italics 
might be regarded as the starting point of the views which I have attempted to develop in this paper. 
§ 1659. According to the beautiful theory of Ampere, the transverse force of a current may be repre¬ 
sented by its attraction for a similar current and its repulsion of a contrary current. May not then the 
equivalent transverse force of static electricity be represented by that lateral tension or repulsion which 
the lines of inductive action appear to possess (1304) ? Then, again, when current or discharge occurs 
between two bodies, 'previously under inductrical relations to each other, the lines of inductive force will 
iveahen and Jade away, and, as their lateral repulsive tension diminishes, will contract and ultimately dis¬ 
appear in the line of discharge. May not this be an effect identical with the attractions of similar 
cunents, i.e., may not the passage of static electricity into current electricity, and that of the lateral 
tension of the lines of inductive force into the lateral attraction of lines of similar discharge, have the 
same relation and dependence, and run parallel to each other ? ”] 
T I heie adopt the simpler term “ Electric Intensity,” denoted by E.I., instead of “ Electromotive 
Intensity, for the force w r hich would act on a small body charged with unit of positive electrification. 
lhe magnetic intensity, i.e., the force which would act on a unit north-seeking Pole, will be denoted 
by M.I. 
MDCCCLXXXV. o t) 
