362 BELL SYSTEM TECHNICAL JOURNAL 



those which lengthen and those which shorten when magnetization 

 commences. The alloy which is most rapidly magnetized when the 

 field is gradually increased from zero, and which dissipates the smallest 

 amount of energy when the field is varied in cyclic fashion, is also 

 precisely the one which suffers the least deformation. From this 

 McKeehan drew the inference, that were it not for the deformation 

 inseparable from the act of magnetizing, the initial curve for every 

 metal would rise swiftly from the origin to saturation, and the sides of 

 the hysteresis-loop would fall together. 



D. The Atomic Magnets 



Had I announced at the beginning of this article that some sixty 

 pages would be spent over the data of ferromagnetism and the theories 

 of the influence of elementary or atomic magnets on one another, 

 and only a few closing paragraphs over the atoms which are supposedly 

 responsible for the whole afifair, the plan might have seemed most 

 ill-adjusted to the relative interest of these divisions. Now, I hope, 

 it will seem less perverse. The truth is, that we do not understand 

 ferromagnetism well enough to draw from it any reliable conclusions 

 concerning the atomic magnets. For these, we must consult the 

 behavior of paramagnetic substances, and line-spectra, and the 

 observations of Gerlach and Stern and their followers upon streams 

 of atoms flying through magnetic fields. 



In the apparatus of Gerlach and Stern, the atoms are probably as 

 nearly free from mutual forces as atoms in the laboratory can ever be; 

 having issued from a small hole in the wall of a furnace full of hot 

 vapor, they rush swiftly across a high vacuum while they are being 

 examined. In the mapping of absorption-spectra, the atoms are those 

 of a rarefied gas, and are "free" in the sense in which atoms of gases 

 are free — that is to say, they are influenced only by those agencies 

 which establish and maintain thermal equilibrium, agencies which we 

 commonly conceive as short, sharp collisions between atom and atom. 

 Some paramagnetic gases behave toward an applied magnetic field as 

 though their molecules, some salt-solutions behave as though their 

 ions, were magnets of fixed permanent moment on which the field 

 can act, but otherwise were free in the foregoing sense. Other gases 

 and salt-solutions behave as though their molecules or ions were 

 permanent magnets, influenced by the applied magnetic field and by 

 an extra field proportional to the magnetization of the assemblage, 

 and otherwise free except for the agencies which establish thermal 

 equilibrium and maintain it. 



In all the foregoing cases of atoms or molecules or ions enjoying 



