852 Prof. S. P. Thompson on Magnetic Figures 



run to or from the central point, the lines of force tend to 

 coalesce. In the alternate quadrants they mutually repel each 

 other ; and the angle of these quadrants tends to increase. 



In figure 9 one current is carried horizontally below the glass, 

 the other traverses the plane of the figure normally. The dis- 

 symmetry of the resultant distribution of the lines reveals the 

 dissymmetrical nature of the force which tends to bring the 

 currents into parallelism. Any shortening of the isodynamic 

 lines would tend to move the vertical current from that qua- 

 drant over which the lines are unbroken. Figures have also 

 been obtained with two currents crossing the plate in a vertical 

 plane of incidence, but each at 45° to the normal. With some 

 distortion, these figures bear a general resemblance to figs. 4 

 and 5 in the two cases of the currents passing through the 

 plate in similar or in opposed directions. In the former case 

 their angular separation tends to diminish, in the latter to in- 

 crease. ' 



Figures 10, 11, and 12 introduce the action of a vertical cur- 

 rent upon a small magnetic needle lying on the glass plate. 

 In the first case the needle lies in stable equilibrium almost 

 tangentially to the isodynamic lines ; in the third its position 

 is reversed and unstable ; in the second case it is set at right 

 angles to the directive action of the current. The dissymme- 

 trical action of the forces on its poles produces a couple tend- 

 ing to turn it about its centre, as would be inferred from an 

 inspection of the lines of force of the figure. 



Figures 13 and 14 illustrate a deduction from the theory of 

 the magnetic shell. A conductor carrying a current is acted 

 upon by a force urging it forward so as to make the number 

 of like lines of force included within it a maximum* ; that 

 is to say, a north-seeking pole is attracted into a circuit 

 on the side from which the positive current appears to circu- 

 late in a right-handed cyclical order (or in the same direction 

 as the hands of a clock). Similarly the circuit is urged back- 

 wards from a contrary pole, and tends to make the number of 

 unlike lines of force included within it a minimum. In the 

 figures a current ascends through the plane of the figure on 

 the left, and descends through it on the right, in a right-handed 

 cyclical order as seen from the magnet. Hence a north- 

 seeking pole is attracted, and a south-seeking pole repelled. 



Figure 15 results from the action of two magnet-poles upon a 

 vertical conductor, which in this case is attracted between the 

 poles. 



Figure 16 illustrates the mutual tendency to rotation between 

 a magnet-pole and a conductor carrying a current parallel to 

 * Clerk-Maxwell, ' Electricity and Magnetism,' art. 490. 



