Mr. W. Sturgeon on the Distribution of Magnetic Polarity. 3 1 



to the Government to have thrown aside the rule of Lieutenant 

 Drummond, founded on true principles, and taken in its stead 

 Dr. M'Intyre's, which satisfies no mathematical principle 

 whatever. ^- V. 



IX. On the Distribution of Magnetic Polarity in Metallic 

 Bodies. By W. Sturgeon, Lecturer on Experimental Phi- 

 losophy at the Honourable East India Company's Military 

 Academy, Addiscombe. 



[With a Plate.] 



IN my last communication (Phil. Mag. and Annals, N.S. 

 vol. xi. pp. 270, 324,) I described the instrument (fig. 9. 

 Plate III. vol. xi.) by means of which the experiments were first 

 made, which indicated an extraordinary and novel distribution 

 of magnetic polarity on the surfaces of copper and other non- 

 ferruginous metallic discs. I also pointed out, though briefly, 

 the method by which I detected the curiously winding force 

 which actuates the needle on those surfaces when rotated be- 

 tween the poles of a horse-shoe magnet. In that communi- 

 cation, however, I described the distribution of that force no 

 further than as it is developed by one condition of motion 

 given to the disc ; i. e. whilst it rotates in the direction of the 

 large exterior arrow, fig. 11. Plate III. In continuation, 

 therefore, I now proceed to show in what manner that force 

 (still supposing it to be the electric) becomes distributed over 

 the surface of the copper disc, when the rotation is carried on 

 in the reverse order; the magnet still remaining in the same 

 position as in fig. 11. Plate III. It may be necessary, how- 

 ever, in the first place, to make some further observations as 

 to the manner by which I have been enabled to trace the 

 curious windings which this force appears to take whilst in 

 operation on the magnetic needle, or the mode by which I 

 obtained the data necessary to the formation of the conclusions 

 at which I have arrived concerning it. 



In Experiments 16. and 17. it is shown that when the needle 

 is placed over the centre of the disc, and its axis in the same 

 vertical plane as that joining the poles of the exciting magnet, 

 it is a matter of no consequence in which of the directions the 

 poles of the needle be placed ; the deflections will depend upon 

 the direction in which the disc is caused to rotate. For, 

 although the needle will in some cases follow the direction of 

 the rotating disc, and in others travel the contrary way, ac- 

 cording to the character of the pole which is directed towards 

 the pole of the exciting magnet, still it will have a dependence 

 upon the direction of motion given to the plate; so that if the 



