458-460] Physical Phenomena 397 



the material of the latter. The significance of this fact will, perhaps, best 

 be realised by comparing it with the corresponding fact of electrostatics. 

 When an uncharged conductor is attracted by a charged body, the pheno- 

 mena in the former body which lead to this attraction are mass-phenomena : 

 currents of electricity flow through the mass of the body until its surface 

 becomes an equipotential. Thus the attraction depends solely upon the 

 shape of the body, and not upon its structure. On the other hand, the 

 phenomena which lead to the attraction of a fragment of dielectric are, as we 

 have seen, molecular phenomena. They are conditioned by the shape and 

 arrangement of the molecules, with the result that the total force depends on 

 the nature of the dielectric material. 



All magnetic phenomena occurring in material bodies must be molecular, 

 as a consequence of the fact that corresponding positive and negative poles 

 cannot be separated by more than molecular distances. Hence we should 

 naturally expect to find, as we do find, that all magnetic phenomena in 

 material bodies, and in particular the attraction of unmagnetised matter by 

 a magnet, would depend on the nature of the matter. There would be a 

 real difficulty if the attraction were found to depend only on the shape of 

 the bodies. 



460. The amount of the action due to magnetic induction varies 

 enormously more with the nature of the matter than is the case with the 

 corresponding electric action. Among common substances, the phenomenon 

 of magnetic induction is not at all well-marked, except in iron and steel. 

 These substances shew the phenomenon to a degree which appears very 

 surprising when compared with the corresponding electrostatic phenomenon. 

 After these substances, the next best for shewing the phenomena of induction 

 are nickel and cobalt, although these are very inferior to iron and steel. It 

 is worth noticing that the atomic weights of iron, nickel and cobalt, are very 

 close together*, and that the three elements hold corresponding positions in 

 the table of elements arranged according to the periodic law. 



It has recently been found that certain rare metals shew magnetic 

 induction to an extent comparable with iron, and that alloys can be formed 

 to shew great powers of induction although the elements of which these 

 alloys are formed are almost entirely non-magnetic f. 



It appears probable that all substances possess some power of magnetic 

 induction, although this is generally extremely feeble in comparison with 

 that of the substances already mentioned. In some substances, the effect is 

 of the opposite sign from that in iron, so that a fragment of such matter is 

 repelled from a magnetic pole. Substances in which the effect is of the 



. Iron = 55-5, nickel = 58-3, cobalt = 58'56. 



t For an account of the composition and properties of Heusler's alloys, see a paper by 

 J. C. McLennan, Phys. Review, Vol. 24, p. 449. 



