illustrating Electrodynamic Relations. 351 



to magnetic lines of force, refer, of course, to the two cases of 

 similarly or oppositely directed lines, the positive direction of 

 a line of force being reckoned as the direction in which a 

 north-seeking pole on it would tend to move*. The mutual 

 coalescence or repulsion exerted between the lines of force of 

 unlike or like magnetic poles is familiarly employed in the 

 experimental illustration of magnetic phenomena, so well known 

 since the researches of Professor Robison and Dr. Roget on 

 the magnetic curves, and appears to have been recognized long 

 beforef . It is believed that the present is the first distinct 

 attempt to apply similar considerations to the illustration of 

 electrodynamic relations between systems of conductors car- 

 rying currents, and between conductors of currents and mag- 

 net-poles. 



The isodynamic lines, which are lines of magnetic force, 

 tend to shorten themselves. A very hasty inspection of fig. 4 

 will show that if any one of the system of lemniscates were to 

 " shorten itself," it would tend to bring the two centres nearer 

 together. Consider each isodynamic line as a ring of some 

 elastic material (as, for example, an indiarubber ring) stretched 

 around a bundle of smooth wires, the cross section of the bundle 

 having a perimeter corresponding in form to the isodynamic 

 line under consideration. The maximum shortening of such 

 an elastic ring would take place when the enclosed area was 

 made a circle. In other words, the lemniscate-form isodyna- 

 mics tend to become circles, and the two like parallel currents 

 are mutually urged towards each other. 



In figure 5 the shortening of the isodynamic lines, and their 

 approach to the truly circular form, could only be accomplished 

 by the Avider separation of the two conductors from each other. 

 Hence the mutual repulsion of two parallel conductors carry- 

 ing oppositely directed currents. 



Figures 6 and 7 show the lines of force above the parallel 

 currents when these pass horizontally below the glass. In the 

 case of like currents the lines coalesce. In the case of unlike 

 currents they repel each other, and pass between the two wires 

 in a direction vertical to the plane of the glass, where their 

 characteristic form, as lines, is lost. The observation that the 

 filings adherent to wires carrying like parallel currents are 

 mutually attractive appears to have been first made by Davy. 



Figure 8 illustrates the law of currents crossing one another 

 at a point. In the two quadrants in which the currents both 



* See Clerk-Maxwell, ' Electricity and Magnetism/ art. 489 ; L. dim- 

 ming, ' Theory of Electricity,' p. 194. 



t See Musschenbroek, Dissertatio Physica Expcrimentalis de Magneto, 

 cap. iv. exp. cxvii., and tab. iv. figs. 4 & 5. 



