Magnetic Polarity in- Metallic Bbdies. 39 



copper, about 18 inches long and 12 inches broad. Thi-; plate 

 was placed between the poles of a horse-shoe magnet, and 

 moved in a horizontal plane. The upper surface of the plate 

 was exposed to the action of the south pole, and consequently 

 the lower surface to the action of the north pole of the magnet. 

 Nothing more will be necessary to describe the distribution 

 of the force which operates on the needle, whilst the plate is 

 in motion in the four selected directions, than merely to refer 

 to figures 21, 22, 23, and 24. The exterior arrows in each 

 figure indicate the direction in which the plate is moved; 

 and the curved systems of axTows show the distribution of the 

 force. 



In fig. 21. the distribution is similar to that shown in fig. 

 1 1. or 16 ; and the motion of that part of die metal under the 

 strongest excitation in both cases is in the same direction, i.e. 

 from left to right. The same comparison may be made be- 

 tween fig. 22. and 17, where both move from right to left 

 bel ween the poles of the exciting magnet. 



In fig. 23. the plate is introduced directly into the interior 

 between the two limbs of the magnet; and in fig. 24. it is 

 withdrawn in the same right line. The distribution of the 

 forces by these two motions are simple curves, having only one 

 direction in each. In each case, however, the curves have 

 every appearance of being continuous, running into themselves 

 between the poles of the magnet, and forming complete vor- 

 tices round a central nucleus or narrow space joining the ex- 

 citing poles. 



Now as the distributions in fig. 23. and 24. are simjile 

 vortices, they may be applied to explain the compound distri- 

 butions in the other figures. Let it be supposed that each 

 system of arrows in fig. 23. and 24. represents a complete vor- 

 tex of the force, and let an observer be supposed to be placed 

 in its centre ; tlien as the plate advances towards the poles as 

 in fig. 23, the direction of the force in every point gf the vor- 

 tex will be towards the left hand; but when the plate recedes 

 from the magnetic poles, as in fig. 24, the direction of the force 

 will be towards the right hand. These are simple elementary 

 vortices. 



Apply now each of these elementary vortices to fig. 21. and 

 22. In each figure the plate is both advancing and retiring 

 from the pole at the same time. In fig. 21. the plate is ad- 

 vancing on the left side of the magnet, and the vortex flows 

 towards the left hand of an observer placed in the centre of its 

 motion. On the right side of the magnet the plate is retiring 

 from the poles, and the vortex is flowing towards the right 

 hand, or iii the same direction as in the elementary vortex in 



