350 Prof. S. P. Thompson on Magnetic Figures 



mic line, just as no work is done in moving a magnet-pole 

 along in an equipotential surface. The isodynamic lines 

 occupy, therefore, exactly the same relation to the element of 

 the circuit, as do the equipotential surfaces to a magnet-pole 

 or to an electrified point. 



Figure 2 represents the field above a horizontal wire carry- 

 ing a current, and separated from the filings by the thickness 

 of the glass (about 1*7 millim.). The lines cross the wire at 

 right angles, and are really the projections of a series of such 

 circles as exist in figure 1. 



Figure 3 exhibits the form assumed by the filings when the 

 wire beneath the plate was coiled into a simple loop, a small 

 piece of mica being inserted to prevent contact where it re- 

 crossed its path. The lines of the field within the loop run 

 longitudinally ; and their projections on the surface are mere 

 points, as the filings show. 



In figures 4 and 5, two wires pass vertically through the 

 plane of the figures, carrying parallel currrents, which in 

 figure 4 are in the same direction, in figure 5 in opposite 

 directions. Ampere's well-known law of the attraction in the 

 former case, and of the repulsion in the latter, is well illus- 

 trated by the forms of the magnetic curves. In the former, 

 where the parallel currents attract, the outer isodynamic lines 

 are closed curves embracing both centres, the inner are dis- 

 torted ovals about each centre — the whole forming a system of 

 lemniscates, as would necessarily be the case, since the at- 

 traction at any point in the plate varies inversely as the square 

 of the distance from each current*. 



In figure 5, where the parallel currents repel each other, 

 the lines of force due to either current in no case enter or 

 coalesce with those of the other current. They form two series 

 of ovals of a peculiar form, flattened on the sides presented 

 towards the opposing series. 



The conception of Faraday, " that the lines of magnetic 

 force tend to shorten themselves, and that they repel each other 

 when placed side by side," has been shown by Clerk-Maxwell, 

 who thus concisely states it, to be perfectly consistent with the 

 theory that explains electromagnetic force as the result of a 

 state of stress in the medium filling the surrounding space f. 

 Faraday also observes that u unlike magnetic lines, when end 

 on, repel each other, as when similar poles are face to face," 

 and that " like magnetic lines of force," when end on to each 

 other, coalesce. The terms " like " and " unlike," as applied 



* See Thomson and Tait, ' Natural Philosophy/ art. 508, vol. i. p. 382. 

 \ Oler^-Maxwell, * Electricity and Magnetism/ vol. ii. art. 64-5 ; Fara- 

 day, ' Experimental Researches/ 326G, 3267, 3268. 



