ELECTRO-MAGNETISM. 



87 



arrows. 'The elementary currents of 

 those particles of the magnet which are 

 situated in the axis, that is, along the 

 line X Y, will, of course, on account of 

 the symmetry of the figure, move in 

 planes perpendicular to the axis ; as 

 also those in the medial plane M m, 



Fig. 162. 



passing through the centre of the mag- 

 net. But with regard to the currents 

 nearer to the surface, they will, by the 

 action of the interior currents, be turned 

 towards the middle of the magnet, 

 while those parts of the same currents 

 that are nearest to the axis will be 

 repelled from the middle towards the 

 adjacent extremity ; and the planes of 

 their inclination will therefore be more 

 or less inclined to the axis, as they are 

 more or less remote either from the 

 axis, and from the middle of the magnet, 

 in the manner represented by the ar- 

 rows in the figure. The total amount 

 of inclination in the lateral currents will 

 be greater in proportion to the intensity 

 of the action of the interior currents, 

 and also in proportion to their number ; 

 it will, therefore, be greater in propor- 

 tion as the thickness of the magnet is 

 greater compared with its length. We 

 may conceive this relative thickness to 

 be so excessive as that the forces tend- 

 ing to produce this inclination of the 

 currents will at length overcome the 

 coercive force, and prevent the deve- 

 lopment of magnetism. This consider- 

 ation will easily explain the difficulty 

 that is experienced in magnetizing a bai- 

 rn such a manner as that the poles may 

 be in the direction of the shorter dia- 

 meter*; a remark which leads us to the 

 subject of the induction of magnetism. 

 Let us examine with what success the 

 hypothesis of Ampere may be applied 

 to this class of phenomena. 



(283.) We have already seen that an 

 electric current communicates magnetic 

 properties by induction to such bodies 

 in the vicinity as are susceptible of 

 acquiring them. If these properties are 

 owing to electric currents circulating in 



* See Cumming's Translation of Demonferrand, 

 Manual of Electro-Dynamics, p. 167 to 1JO, and also 



p. 250. 



the particles of the magnetized body, or 

 in the magnetic elements, as they have 

 been called, (see Magnetism, 154,) 

 there are two suppositions, either of 

 which will account for this phenomenon. 

 The first is, that electric currents, which 

 did not before exist, are produced, or 

 called into action, by the influence of 

 another current in the vicinity. The 

 second hypothesis is, that the electric 

 currents pre-exist in all the particles of 

 iron, or other bodies susceptible of mag- 

 netism previous to their acquiring this 

 property, but without having any uni- 

 form direction ; under these circum- 

 stances, their actions upon any external 

 body counteract and balance one an- 

 other, so as to constitute the neutral 

 state. When, on the other hand, they 

 are under the influence of an external 

 electric current of sufficient power, they 

 are all turned by it towards the same 

 quarter, and assume a common direc- 

 tion ; they will now co-operate in their 

 action upon external bodies, and exhi- 

 bit magnetic properties. This change is 

 analogous to what takes place in the 

 rays of ordinary light when, from being 

 polarized in all possible directions, they 

 become suddenly polarized in one parti- 

 cular direction. 



(284.) It is implied in the first of 

 these hypotheses, that every electric cur- 

 "rent tends to produce currents in a simi- 

 lar direction in other bodies. Ampere 

 has proved, by the following curious 

 experiment, that a powerful voltaic cur- 

 rent possesses this power of exciting 

 currents in neighbouring bodies that are 

 not generally considered as susceptible 

 of magnetism. A copper wire of consi- 

 derable length, covered with silk thread, 

 was rolled round a cylinder, so as to 

 form a coil of some thickness. Within 

 this coil, placed in a vertical position, a 

 copper ring of smaller diameter was sus- 

 pended by a fine silk thread, passing 

 through a small glass tube, which was 

 thrust between the threads of the cop- 

 per coil. The circumference of the ring 

 was thus brought, in every part, very 

 near to the conducting coil, through 

 which a very powerful voltaic current 

 was sent. When a magnet was pre- 

 sented to the ring, under these circum- 

 stances, the latter was attracted or re- 

 pelled in the same manner as if it had 

 formed part of the same circuit as the 

 coil. Hence it was inferred, that an 

 electric current tends to induce in con- 

 ductors, placed in its immediate vicinity, 

 currents that move in similar directions. 



