ELECTRO-MAGNETISM. 



89 



sequence of the parts in contact moving 

 in the same direction. 



Fig. 164. 



(288.) After the removal of the cur- 

 rent which originally determined them, 

 these induced currents continue to cir- 

 culate with more or less permanence, 

 according to the degree of coercive force 

 inherent in the body. In soft iron they 

 disappear almost immediately : in steel 

 they continue to maintain themselves, 

 and constitute permanent magnets. The 

 action of heat is either to weaken or 

 destroy the currents altogether, or else 

 to derange the uniformity of their direc- 

 tion, so that they cease to act in con- 

 cert, and the steel reverts to its neutral 

 state. It is found, in conformity with the 

 theory of Ampere, that all the effects of 

 magnetic induction are produced equally 

 well by electric currents circulating 

 through spiral or heliacal conductors, as 

 by artificial magnets. 



(289.) The theory of Ampere fur- 

 nishes a key to the explanation of a 

 variety of facts attending the conversion 

 of steel bars into magnets by the ordi- 

 nary processes of magnetization, which 

 are not intelligible on any other hypothe- 

 sis. It accounts for the peculiar circum- 

 stances already noticed in the Treatise 

 on Magnetism, regarding the relative 

 advantages of the single or double touch, 

 according to the inclinations given to 

 the magnet when applied to the bar to 

 be magnetized ; and it more especially 

 explains the frequent occurrence of con- 

 secutive points when certain methods 

 are employed. Thus, let one of the 

 poles of a magnet, the north, for in- 

 stance, be placed on the middle of a 

 steel bar, at right angles to it : see fig. 

 165. The form of the steel bar will, as 

 Fig. 165. 



transverse to its length ; and the cur- 

 rents in the magnet running in this di- 

 rection are those situated on the oppo- 

 site sides of the magnet, supposed to be 

 divided by the dotted line perpendicular 

 to the length of the steel bar. But these 

 portions of currents are themselves 

 moving in contrary directions ; the cur- 

 rents they respectively induce in the 

 parts of the bar which they touch, and 

 in the neighbouring parts, must there- 

 fore, in like manner, have opposite di- 

 rections, giving rise to opposite polari- 

 ties. Thus the two ends of the bar will 

 be converted into north poles, while the 

 point immediately under the centre of 

 the magnet will be a consecutive point, 

 or south pole. 



(290.) The phenomena attending the 

 division or fracture of a magnet follow 

 very naturally from the constitution 

 assigned to it by Ampere's hypothesis ; 

 for, as the currents circulate in the 

 same direction in the divided ends while 

 they were united, they will appear to 

 circulate in opposite directions with 

 reference to the two sides of the plane 

 which divides them, and which become 

 the terminal planes of each fragment 

 when separate. The polarities of the 

 two ends must, therefore, be of opposite 

 kinds ; for the same reason that the 

 adjoining ends, B and C, fig. 163, of 

 two magnets placed in the same line, 

 with their currents having similar di- 

 rections, have opposite polarities. At 

 the poles of a horseshoe magnet, the 

 currents revolve in opposite directions 

 with respect to the two ends of the bent 

 axis ; but the directions of the adjacent 

 part of each current, as well as of the 

 remote parts, are similar. See ,/?- 166. 



Fig. 166. 



already remarked, give greater facility to 

 the induction of currents in a direction 



