of Magnetic Polarity in Metallic Bodies. 327 



whole, although some rule may be observed by any one ar- 

 rangement, yet the same rule is not applicable in all cases. 

 A few experiments will show in what manner the distribution 

 of polarity in the surface of the disc may be ascertained. 



Experiment 16. — Let a copper disc of about eighteen inches 

 in diameter be placed so as to be rotated in its own plane on 

 a vertical axis ; and let a horse-shoe magnet be placed with 

 its south pole above, and its north pole below the edge of the 

 disc, reaching about two inches beyond the edge towards the 

 centre. Let a compass-needle be placed on a stage directly 

 over the centre of the disc ; and by a proper arrangement of 

 magnets, let its south pole be directed to the south pole of the 

 magnet. Turn the wheel, and the needle will move in the di- 

 rection of the plate, but will not perform a revolution. If the 

 vibrations of the needle be attended with corresponding mo- 

 tions of the plate, it may be made to sweep half a circle. 



Experiment 17. — Let now the north pole of the needle be 

 turned towards the south pole of the miignet, and again turn 

 the wheel. In this case the needle will move in the opposite 

 direction to that of the disc. 



Experiment 18. — Let the needle be placed over, and just 

 within the left edge of the disc, and not more than 90° from 

 the magnet, fig. 11. Let it also be permitted to repose with 

 its axis at right angles to the diameter over which its pivot is 

 placed. Turn the disc in the direction indicated by the large 

 exterior arrow, and the south pole will be deflected towards 

 the edge of the disc. Reverse the revolving motion of the plate, 

 and the south pole will be deflected towards the centre of it. 



Experiment 19. — Let the needle be placed below the plate, 

 and in the same vertical plane as before. The deflections 

 answering to the motions of the disc in this case will be op- 

 posite to those when the needle was above. 



In this way the needle may be placed opposite to various 

 parts of the disc, and it will be found that the deflections vary 

 in different places ; and over some places no deflection will be 

 observed by the motion of the disc in one direction, although 

 a considerable deflection will be given by the motion being 

 reversed . 



It would be very difficult to account for these extraordinary 

 pha.'nomena by any known laws of magnetics, and almost as 

 difficult to reconcile them, with our present knowledge, to the 

 laws of electro-magnetism. When these experiments were 

 first intended to be published, I had arranged them under the 

 h^ad oi Polar Ma frnetic Streams; but I have since thought that 

 the JJistril>ulio7i of Magjietic Polarity V!\\\ be a much more ap- 

 propriate term. 



It 



