Magnetic Polarity in Metallic Bodies. 33 



arranged, first a few degrees from the one, and tlien a few 

 degrees from the other of its former positions, still keeping 

 the south pole towards the right. The disc is to be put in 

 motion (in that direction only indicated by the large exterior 

 arrows, fig. 14. and 15.), whilst the needle is in each of the 

 positions last given to it; and if the south pole now travels in 

 the same direction as it did from both its former positions, it 

 is plain that the excited forces still urge it towards some point, 

 the situation of which is between those in which it was last 

 placed. The needle is therefore again to be drawn still nearer 

 to its destination indicated by the last trials, and the disc 

 again put in motion in the same direction as before ; the 

 deflections are again to be observed, and the line, to which 

 they indicate a tendency, to be still nearer approached by the 

 position of the needle, for the next trials. In this manner the 

 line to which the excited forces of the disc urge the needle is 

 to be gradually approached, and its true position at length 

 correctly ascertained. 



The deflections will gradually diminish, becoming smaller 

 and smaller in proportion to the advances of the needle towards 

 this neutyal line : and when it is placed directly in the position 

 of this line, the deflections will cease to be exhibited by the 

 direction of rotation selected for this illustration; for the needle 

 will now have a position of stability, or a position which the 

 forces excited in the disc alone tend to preserve it in. If it 

 be drawn only two or three degrees out of this line on either 

 side, the slightest motion of the disc will urge it towards that 

 line again; and if the needle be made completely indifferent 

 to the influence of any other forces than those excited in the 

 disc, a deviation even of one degree on either side of the 

 neutral line may be detected by a tendency which will be in- 

 dicated to resume the position of that line again whenever the 

 plate is rotated in the proper direction*. 



The process of experimenting is exceedingly tedidus, but 

 it is the only method by which the true position, to which the 

 forces excited in the disc tend to urge the needle, can possibly 

 be ascertained. And if those forces be electric, and endued 

 with the same magnetic polarity as that exhibited by the forces 

 of a galvanic conducting wire, then the directions of the electric 



* I have been particular during this description in adhering to the effects 

 of those forces vvhicli become excited by the disc revolving in one direction 

 only; because it so h.-ippens that the two neutral lines indicated by the needle 

 whilst placed over the centre of the disc are not coincident, but intersect 

 each other at some consideral)le ansle. Hence, althonph a position may 

 be given to the needle from which it will not deviate whiltt the plate re- 

 volver, ill one direction, a considerable deflection may be given by reversing 

 the rotatory motion. 



Third Series. Vol. 1. No. 1. Jidi/ 1832. F 



