Magnetic Polarity in Metallic Bodies. ' 25 



the north pole be opposite the east surface, and the south pole 

 opposite the west surface of the disc, as in fig. 18, S N repre- 

 senting the upper edge of the disc. 



Let a magnetic needle be also arranged north and south,' 

 close beneath the lower edge of the plate. Rotate the plate in the 

 direction which will carry its upper edge from south to north, 

 (from S to N, fig. 18.). In this case the disc will enter be- 

 tween the poles of the exciting magnet, under precisely the 

 same circumstances as in fig. 11. and 16; the left edge in 

 either of those figures corresponding with the lower edge ia 

 fig. 1 8. The principal force which now operates on the needle 

 will be that in the lower edge of the disc; and the direction 

 of that force will be from north to south, or in the same direc- 

 tion as that in which the lower edge is in motion. (See fig. 11. 

 or 16.) The south pole of the needle is deflected towards the 

 east m precisely the same manner as it would be urged by 

 the polarizing force of an electric current running from north 

 to south through a conducting wire placed above the needle. 

 The needle S N, fig. 18, shows the position into which it is 

 carried whilst the disc is revolving over it. 



Experiment 21. Let the needle be now placed above the 

 upper edge of the disc, and its axis in the same vertical plane, 

 the rotation being continued in the same direction as before. 

 In this case the force which operates on the needle is trans- 

 mitted from north to south, the upper edge of the disc corre- 

 sponding to the right edge in fig. 11. or 16. The direction of 

 the force is tlierefore the same in this experiment as in the 

 last; but the needle is now placed above the edge, and the 

 south pole is deflected towards the west. 



If the disc be rotated in the contrary direction to that in 

 which it proceeded in the two last described experiments, the 

 thstribution of the force will be represented by fig. 17, in which 

 case its direction in the edge, both above and below the 

 njagnet (fig. IS.), will be from south to north. The south 

 ))ole of the needle, when beneath the lower edge, will be de- 

 flected towards the west; but when placed above the upper. 

 edge of the disc, the same pole will be deflected towards 

 the east; showing in a very beautiful and striking manner 

 that the forces in the edge of the disc become completely 

 reversed by simply reversing the revolving motion, and that 

 the distribution of polarity is highly imitative of that which is 

 displayed by the edges of a flag or cake of zinc, when par- 

 tially heated at one end only*; the discovery of which, as I 

 have before stated, gave me the first hint which led to the 



* Sec iiiv I'uj)cr in llic I'liil. Man. and Annab, vol. x. p. J20. 

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