14 



ELECTRO-MAGNETISM. 



the unstable kind, for as soon as N is 

 removed further from the wire, the force 

 acting on S gains the advantage, and 

 turns the needle round till its revolution 

 is arrested by its coming against the 

 wire, in the position s n. 



(39.) Although, strictly speaking, the 

 tangential force exerted by an electrical 

 current upon either pole of a magnet, 

 has no tendency to cause the pole to 

 approach to, or recede from it, and, 

 therefore, does not possess the character 

 either of an attractive or of a repulsive 

 force, yet the movements of a needle, in 

 the circumstances we have just been con- 

 sidering, often resemble those of at- 

 traction and repulsion. But if viewed 

 with reference to such a cause, they 

 would appear exceedingly anomalous ; 

 and accordingly the sudden changes 

 from attraction to repulsion, which take 

 place from a slight alteration in the re- 

 lative positions of the wire and needle, 

 appeared to the earlier experimentalists 

 to be very capricious and unaccountable. 



(40.) In order fully to understand 

 these transitions, we may arrange the 

 results of the preceding investigation, 

 as they refer to any one given position 

 of the needte SN, Jig. 22, varying the 



Fig. 22. 



a 



position of the wire only, and we shall 

 then find that the lines which form the 

 boundaries between the positions of ap- 

 parent attraction and repulsion are the 

 circumference of the circle of which the 

 needle is a diameter, together with the 

 prolonged axis of the needle n s, and 

 another line crossing it, at the^centre, at 

 right angles, ef. The circumference 

 indicates the positions of the wire when 

 no apparent effect is produced on the 

 needle, or the positions of neutrality. 



The line ef is that in which an equili- 

 brium obtains. When the wire is in any 

 continuous part of the line ef, namely 

 CF and E e, the equilibrium is stable ; 

 when in any of the dotted parts E C 

 or F/, of the same line, unstable ; on 

 the line s n, the action is at the maxi- 

 mum. The letters a, a, a, a, show the 

 spaces where an apparent attraction 

 takes place between the wire and the 

 nearest pole, when the former is situated 

 in the respective spaces bounded by the 

 above lines ; and r, r, r, r> the spaces 

 where there is apparent repulsion. These 

 latter spaces are shaded for the sake of 

 distinction. Thus within the quadrant 

 SCE there is apparent attraction of the 

 pole S ; in the shaded quadrant SCF 

 there is an apparent repulsion of that 

 pole ; in the shaded quadrant EON, the 

 pole N moves from the wire; in NCF, 

 towards it. In the spaces exterior to 

 the circle the actions are exactly the 

 reverse of those in the interior ; in the 

 shaded space bounded by s S, e E, and 

 the circumference, the action in S is ap- 

 parently repulsive; in the white space 

 on the other side of S, bounded by the 

 lines S s, Yf, and the circumference, it 

 is attractive ; and the contrary obtains 

 with regard to the spaces on the other 

 side of the line ef. 



2. Movements of the Magnetic Needle 



in free space. 



(41.) In the preceding investigation 

 our attention has been exclusively di- 

 rected to the determination of the effects 

 of the electro-magnetic forces on a mag- 

 netized needle, so restricted in its motion 

 as to be capable of only turning on its 

 centre ; and we have had to consider 

 only the forces which tended to produce 

 the rotation of the needle. A part of 

 the forces, however, which act on the 

 poles is exerted in another direction, and 

 would, were the needle at liberty to obey 

 them, occasion the displacement of the 

 whole needle, that is, would produce a 

 motion of its centre. The needle being 

 confined by its pivot, the only effect 

 produced by these forces is pressure upon 

 this pivot. But if this obstacle be re- 

 moved, and the needle be allowed to 

 move freely in any direction, the action 

 of these remaining forces will become 

 manifest ; the motion of the centre of 

 the needle being determined in its quan- 

 tity and direction by the magnitude and 

 direction of the resultant force estimated 

 by referring the two component forces 

 to that point. 



