CHAP. Ill] HYSTERESIS AND EDDY CURRENTS 35 



small intensities of the external field, the molecules of iron return 

 into their original stable positions as soon as the external m.in.f. 

 is removed; when, however, the external magnetic intensity 

 becomes considerable some of the molecules turn violently and 

 assume new groupings of stable equilibrium. Therefore, when 

 the external m.m.f. is removed, there is some intrinsic magneti- 

 zation left, and we have the phenomenon of residual mag- 

 netism. 



With an ever-increasing external m.m.f., more and more of the 

 molecules are oriented so that their m.m.fs. are in the same direc- 

 tion as the external field, the iron then approaching saturation. 

 Any further increase in the flux density is then mainly due to the 

 flux between the molecules, the same as in any non-magnetic 

 medium. 



According to the foregoing theory, an external m.m.f. turns 

 the internal m.m.fs. into more or less the same direction; these 

 m.m.fs. then help to establish the flux in the intermolecular spaces 

 which are much greater than the molecules themselves. There- 

 fore, the higher flux density in iron is not due to a greater permea- 

 bility of the iron itself, but to an increased m.m.f. It is never- 

 theless permissible, for practical purposes, to speak of a higher 

 permeability of -the iron, disregarding the internal m.m.fs., ami 

 considering the permeability, according to eq. (16), as the ratio 

 of the flux density to the externally applied magnetic intensity. 



The foregoing theory explains also the general character of 

 the permeability curve of iron. With very small values of H 

 the molecules of a piece of iron are oriented but very little, but 

 are rapidly oriented more and more as H is increased. There- 

 fore, for small values of //, /< must be expected to increase with //. 

 On the other hand, when the saturation is very hijrh. an increase 

 in // changes B but little, because practically all of the available 

 internal m.ni.fs. have been ntili/.e.l. Therefore, for lar^e values 

 of //, n decreases with increasing //. Consequently, there is a 

 value of //for which //isa maximum. This is the actual shape of 

 permeability curves (see for instance th< re to the Standard 



Handbook given in Art. 12 abo 



The phenomenon of magneti/atinn j sible because the 



Changes from one Stable urom molecules to the next are 



HioMen. Kach molecule, in ch. o a new grouping, acquires 



kinetir energy, and oscillates about its new (> uilil.- 



