ELECTRO-MAGNETISM. 



91 



lakes the place of the attraction before 

 observed, notwithstanding the similarity 

 of the currents in the two edges at S 

 and s that are nearest each other. The 

 reason is, that the attraction of the ad- 



Fig. 168. 





jacent sides is now much weakened both 

 by the greater distances of their remoter 

 portions at N and ?i, and also on ac- 

 count of the great obliquity of that 

 action. The repulsions, on the contrary, 

 exerted between the adjacent side of the 

 one and the remote side of the other 

 magnet, become very powerful, both 

 from their increased proximity and more 

 direct action ; and they predominate 

 accordingly. A similar account may be 

 given of the attraction which takes 

 place between dissimilar poles placed in 

 a similar situation. The reasoning in 

 both instances being analogous to what 

 was stated ( 236) with respect to the 

 action of a helix upon an elementary 

 portion of current placed in different 

 situations with respect to the helix. 



(296.) In the case of magnets that 

 are not of a prismatic or cylindric 

 shape, nor terminated by plane surfaces 

 perpendicular to their axes, the estima- 

 tion of the resultant force becomes much 

 more complicated. All that we have 

 now said on this subject, indeed, can 

 only afford approximations to the solu- 

 tion of the problem of finding this re- 

 sultant. The rigorous investigation of 

 this problem would involve mathemati- 

 cal considerations of too great an extent 

 for a treatise of this kind. The reader 

 who may wish to prosecute the inquiry 

 is referred more especially to the works 

 of Ampere, in which the subject is 

 treated with a masterly hand. 



(297.) The magnetic influence of the 

 earth being so perfectly analogous to 

 that of other magnetic bodies, the theory 

 of Ampere with respect to the constitu- 

 tion of such bodies must, if founded in 

 truth, apply also to terrestrial magnet- 

 ism, which must, according to that 

 theory.be derived from electric currents 

 circulating in the globe from east to 

 west in planes parallel to the magnetic 

 equator. The united effect of such 

 currents would be to produce a southern 

 polarity on the northern side of these 



planes, and a northern' polarity on the 

 southern side. It is scarcely necessaryfl r ^ 

 to point out how exactly thu. phenomena f ' 

 described in Chapter IX. ($ f48't 147,-) 

 accord with the consequences of siidh-iV ***,, 

 an hypothesis. The magnetic axis of 

 the earth, according to this view of the 

 subject, is merely an imaginary line, per- 

 pendicular to the planes of the electric 

 currents circulating in the earlh, and 

 passing through the centres of the cir- 

 cles described by those currents ; and 

 the directive power of the globe which 

 acts on iron and on magnets on its sur- 

 face, is the result, not of any real influ- 

 ence proceeding from those portions of 

 the earth to which their poles point, but 

 of the electro-dynamic action of cur- 

 rents circulating in the plane of the 

 magnetic equator, in obedience to which 

 the corresponding currents which cir- 

 culate in the magnet place themselves, 

 so as to approach to parallelism with the 

 former; that is, to attain the position of 

 equilibrium between the forces in opera- 

 tion. This position is that of a plane 

 perpendicular to the line of magnetic 

 direction, or the line of dip : and ac- 

 cordingly, since the currents in the mag- 

 net are themselves perpendicular to its 

 axis, they will tend to bring that axis in 

 that very line. Hence the phenomena 

 of the clipping-needle, and hence the 

 position assumed by the compass-needle, 

 in the plane of the magnetic meridian, 

 as being the nearest approach which its 

 mode of suspension will allow it to 

 make to the line of dip. 



(298.) All the effects of terrestrial 

 magnetism may be imitated by distri- 

 buting wires round the surface of an 

 artificial globe, so as to direct a galvanic 

 current through them. Mr. Barlow, in 

 a paper lately read at the Royal Society, 

 describes the following experiment which 

 he made with this view. A hollow 

 wooden globe, sixteen inches in diame- 

 ter, was furnished with copper wires 

 passing in grooves along each parallel 

 of latitude for every tenth degree. \Vhen 

 an electric current was made to pass 

 through these wires, in the same direc- 

 tion in each, it was found that a mag- 

 netic needle, properly neutralized with 

 regard to the earth's action, and sus- 

 pended in different situations near the 

 surface of the artificial globe, arranged 

 itself in positions perfectly analogous 

 to those actually assumed by the dip- 

 ping-needle in corresponding regions of 

 the earth. It is probable that if we 

 could indefinitely multiply these electric 



