March 9, 1922] 



NA TURE 



321 



Ewing's Theory of Magnetic Induction. 



AT the Royal Society of Edinburgh on February 

 -^^^ 20, Sir J. Alfred Ewing read a paper on 

 " Models of Ferromagnetic Induction," giving a 

 detailed account of his most recent work in magnetism. 

 In this paper Sir Alfred Ewing develops the theory 

 of magnetic induction put forward by him in 1890, 

 and discusses the reasons which have led him to 

 modify the theory in an important particular. The 

 theory was based on Weber's conception that a 



ubstance capable of strong magnetisation, such as 

 iron, owes its magnetic quality to the presence 

 within it of ultimate magnetic particles capable of 

 being turned, and that the process of magnetising 

 consists in compelling these particles to face more or 

 less completely in one direction. When all the Weber 

 particles are facing one way the iron is magnetically 

 saturated. 



What the author showed in 1890 was that the 

 control under which the Weber particles turned was 

 a magnetic control, and that in turning they fell over 

 from one position of stable equihbrium to another, 

 through an unstable phase, thereby producing the 

 phenomena of magnetic hysteresis. This funda- 

 mental feature of the theory is retained but the 



/ 



Fig. I. 



author has now abandoned his further idea that the 

 control of the particles was due simply to their mutual 

 magnetic forces, acting from atom to atom, because 

 I quantitative examination of the forces produced 

 in that way has convinced him that other forces are 

 also involved. These other forces are those which 

 exist within each individual atom, between the 

 Weber particle and the rest of the atom. We now 

 know the atom to be a very complex whole, compris- 

 ing many moving electrons. In a substance such as 

 iron each atom contains a Weber particle — a thing 

 that turns under the influence of an external magnetis- 

 ing force. It is not the atom as a whole that turns, 

 but only a part of it. According to the author's 

 view there is magnetic control exerted between the 

 part that turns and an outer shell which is held fixed 

 in relation to neighbouring atoms. He now shows 

 that all the characteristics of the magnetising process 

 can be accounted for on this basis, and may be re- 

 produced by means of illustrative models. 



The first part of the paper is a study of the equili- 

 brium of pivoted magnets, undertaken with reference 

 to the author's model of 1890, in which the Weber 

 particles were represented as rows of little magnets 

 controlUng one another by their mutual forces only. 

 It is shown that this model fails quantitatively 

 because when the magnets are placed near enough 

 together to give the correct form to the curve of 

 magnetisation, in its several stages, the deflecting 

 force which is required to break up the row is 

 enormously greater than that which suffices to 

 produce strong magnetisation in iron. Iron acquires 



only about one per cent, of its magnetism of saturation 

 during the first or quasi-elastic stage in the deflection 

 of the Weber particles, before irreversible turning sets 

 in. This means that the magnets of the old model 

 had to be set with so small a clearance between them 

 that the stability of the row was far too great. In 

 the new model the stability can be reduced to any 



desired extent, for it depends on the balance of 

 attracting and repelling forces due to the action of 

 opposite portions of the outer shell of the atom on the 

 Weber particle within. 



Several forms of the new model were exhibited, 

 some with pivoted magnets to represent the Weber 

 particles and fixed magnets to represent the con- 

 trolling portions of the atomic shell. Thus in the 



Fig. 3. 



model of Fig. i a pivoted magnet in the centre turns 

 between four fixed magnets all of which present 

 towards it poles of the same name. In the model 

 which is shown in Fig. 2, the Weber particle is a 

 group of eight magnetic poles, turning as a whole 

 within a group of eight fixed magnets. The arrange- 

 ment is a cubically symmetrical one appropriate to 

 a metal such as iron, in the crystals of which the 

 space-lattice is known to be the centred cube. In 

 another model (Fig. 3) the Rutherford-Bohr con- 

 ception of an atom with large electron orbits is 

 realised. The orbits are represented by elliptically 

 shaped coils with the nucleus of the atom at their 



NO. 2732, VOL. TO9] 



