38 



MAGNETISM. 



different kinds of steel. This coercive 

 force, which exists in iron with regard 

 to the. magnetic fluids, is analogous in 

 all respects to the resistance which 

 glass, resin, and other non-conducting 

 bodies, present to the passage of the 

 electric fluids through their substance. 

 Having thus established the founda- 

 tions of the hypothesis of a double mag- 

 netic fluid adopted to the particular cir- 

 cumstances of the case, we must next en- 

 deavour to ascertain precisely the distri- 

 bution of the austral and boreal fluids in 

 magnetized bodies, conformably with 

 these principles ; and afterwards exa- 

 mine the nature of their combined ac- 

 tions upon bodies at a distance. 



(157.) For this purpose, we may first 

 take the simpler case of a cylindrical 

 needle of soft iron, of very small dia- 

 meter, and of any given length, as re- 

 presenting an elementary longitudinal 

 filament of that metal. In the natural 

 state of the needle, the two fluids it 

 contains are united in equal proportions 

 throughout its substance, so that their 

 actions, being equal and opposite at 

 all distances, totally destroy each 

 other, and no sign of magnetism is ex- 

 hibited. If we next suppose these fluids 

 to be subjected to the action of mag- 

 netizing forces, proceeding from one or 

 more centres, situated in the line of the 

 axis of the needle produced, these 

 forces will now cause the fluids to sepa- 

 rate from each other ; but each particle 

 of austral or boreal fluid can, by the hy- 

 pothesis, move only a very short distance 

 from its primitive situation; and the 

 two fluids, in their new arrangement, 

 will succeed each other alternately, 

 throughout the length of the needle, 

 which will, accordingly, be divided into 

 very small portions, composing a series, 

 each part of which will contain, as it did 

 in the neutral state, the two fluids in 

 equal quantities. The united actions of 

 every particle of decomposed fluid in 

 this series upon a particle of magnetic 

 fluid in any particular situation, compose 

 a resultant force, the intensity and di- 

 rection of which remain to be deter- 

 mined by the application of mathemati- 

 cal analysis. 



(158.) We may now proceed to con- 

 sider the more complicated case of a 

 magnetized body of indeterminate form 

 and dimensions. Attention must here be 

 paid to the lines or directions in which 

 the separation of the two fluids takes 

 place throughout its substance, and 

 in which they are arranged alternately, 



as we have just seen exemplified in the 

 case of the simple filament. These 

 lines or filaments will, in general, be 

 curved ; the nature of the curvatures 

 depending on the form of the body, and 

 on the external forces which act on the 

 two fluids. They are termed by Poisson 

 lines of magnetization, and may be con- 

 sidered as constituted by series of mag- 

 netic elements, following one another in 

 the same regular consecutive order of po- 

 lar arrangement. We have to determine, 

 then, for each point of the body which 

 is the subject of investigation, the direc- 

 tion of the line of magnetization, which 

 is also the line of polarity ; and the ac- 

 tion of the magnetic element on any 

 other point given in position, either 

 within or without the body. This action 

 is the difference of the forces exerted by 

 the two fluids contained in the element, 

 arising from the slight separation of 

 the austral and boreal particles, which 

 constitutes the state of polarity. It may 

 excite surprise that forces depending on 

 such small differential distances as 

 those of the two centres of austral and 

 boreal forces in each magnetical element, 

 should, nevertheless, be capable of pro- 

 ducing mechanical effects so conside- 

 rable as those exhibited by the magnetic 

 attractions and repulsions of bodies. By 

 applying to the subject the methods of 

 analytical investigation, Poisson arrived 

 at the conclusion, that the result of the 

 action of all the magnetical elements of 

 a magnetized body is a force equivalent 

 to the action of a very thin stratum, co- 

 vering the whole surface of the body, 

 and formed of the two fluids, the austral 

 and the boreal, occupying different parts 

 of it. We have a similar instance in the 

 case of electrical attractions and repul- 

 sions of mechanical effects, sometimes 

 very powerful, being produced by strata 

 of fluids collected at the surfaces of con- 

 ductors, and having a thickness so ex- 

 ceedingly minute as to be inappreciable 

 by any of our senses. As these observed 

 effects of the two magnetic agents result 

 only from the differences of two con- 

 trary powers, we can form no estimate 

 of the real magnitude of the forces be- 

 longing to each separate power, that is, 

 to each of the two portions of austral or 

 boreal fluid belonging to the same mag- 

 netic element ; but can only infer that 

 they are incomparably greater than the 

 resulting forces which are actually in 

 operation, and of which we witness the 

 effects. 



(159.) In the memoir on the Theory 



