MAGNETISM. 



a subject of so delicate a nature. He 

 finds that the influence of a rotating 

 body on the magnet is real, but more 

 feeble than had previously been ima- 

 gined ; and that the law of intensity of 

 action is directly as the rapidity of ro- 

 tation, and inversely as the squares of 

 the distances between the attracting 

 bodies. 



(365.) From some experiments which 

 Mr. Harris lately exhibited at the Royal 

 Institution of London, it appears, that a 

 magnetic needle, partly neutralized with 

 regard to the earth's action, and made 

 to vibrate while surrounded by a mas- 

 sive ring of copper, or other substance 

 of very weak magnetic energy, had its 

 oscillations sensibly repressed by the 

 presence of that substance, and that it 

 arrived much sooner at a state of rest 

 than when the ring was removed. The 

 magnetic influence of rotating bodies 

 was found to be intercepted by a va- 

 riety of substances interposed between 

 them. 



(366.) His last communications to 

 the Royal Society contain an account of 

 experiments tending to show that every 

 substance susceptible of magnetism by 

 induction, when interposed as a screen, 

 tends to arrest the action excited by a 

 magnet on a third substance ; and that 

 this interceptive power is directly as the 

 mass of the intervening substance, and 

 inversely as its susceptibility to receive 

 induced magnetism. A single plate of 

 iron, for instance, about the sixteenth 

 of an inch thick, is found effectually to 

 intercept the action of a revolving mag- 

 net on a disc of copper ; but the same 

 result is not obtained when the disc 

 acted upon, instead of being copper, is 

 also of iron, unless the mass of inter- 

 posed iron be very considerable. He 

 afterwards determined that this inter- 

 ceptive influence depends, not merely 

 upon the surface of the interposed iron, 

 but is in proportion to its mass. Hence 

 he was led to suspect that indications 

 might be obtained of a similar influence, 

 exerted by substances not of a ferru- 

 ginous nature, if they were interposed 

 in considerable masses ; and this con- 

 jecture was verified on trying the experi- 

 ment. He found that the action of a 

 revolving magnet upon a disc of tinned 

 iron was completely intercepted by 

 masses of about four inches in thick- 

 ness, of either copper, zinc, or silver, 

 interposed between them. 



(367.) It would appear, then, that 

 this intereeptive property is common to 



all matter, though possessed, in various 

 degrees, by different kinds of substance; 

 and that, in order to render it sensible, 

 it is only requisite to employ them in 

 masses proportionate to their respec- 

 tive magnetic susceptibilities. Lead, for 

 example, which has a weaker magnetic 

 susceptibility than copper, must be em- 

 ployed in a larger mass in order to pro- 

 duce an equal effect. By an extension 

 of this principle, it would require a 

 thickness of above thirty feet of ice, in 

 order to render its interceptive power 

 sensible. 



(368.) It is not necessary, however, 

 that the substance which exerts this 

 controlling power over the action of a 

 revolving magnet should be actually 

 interposed between the magnet and the 

 metallic disc. Mr. Harris found that, 

 in the case of iron, a mass of that 

 metal, when placed very near to that 

 surface of the magnet most distant from 

 the disc on which it was acting, had the 

 effect of neutralizing its power. With 

 regard to non-ferruginous bodies, it is 

 very difficult to render this influence 

 sensible, unless they are interposed in 

 the direct line of action. 



(369.) The temporary induction of 

 magnetism may be conceived as taking 

 place in two different ways : first, by 

 the immediate action of the magnet 

 upon each individual particle of the body 

 on which magnetism is induced ; or, 

 secondly, by the action of each particle 

 on the next adjoining to it in succession, 

 constituting a continued and successive 

 propagation of magnetism from the one 

 to the other. Although both these kinds 

 of induction may take place at the same 

 time in the same substances, yet the de- 

 grees in which they are exerted seem to 

 be in some inverse ratio the one to the 

 other : for when the absorbing or reten- 

 tive power of the substance is consider- 

 able, the power of the magnet is soon, 

 checked: because the particles of that 

 substance first acted upon begin to 

 operate as screens to the succeeding 

 particles; and the induction, after a 

 certain point, proceeds entirely by com' 

 munication from particle to particle, 

 until the whole has attained a state of 

 permanence. When, on the contrary, 

 the retentive power of the given sub- 

 stance is small, little or no interceptive 

 influence can exist among its particles ; 

 and the induction will be produced 

 solely by the direct action of the magnet 

 on all the particles. Accordingly it is 

 only by the succession or multiplication 



