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BELL SYSTEM TECHNICAL JOURNAL 



electron spin only is operative; the ratio would be one if change in 

 orbit orientation were the only efifect. The apparent slight difference 

 of most of the values from two, indicates that there is some small but 

 definite change in orbit-orientation in ferromagnetic materials when 

 they are magnetized. In the weakly ferromagnetic pyrrhotite (FeS) 

 the experimental value 0.63 is in harmony with the theoretical value, 

 0.67, for a possible state of the iron atom {s = — Xjl, I = 2, j = 3/2) 

 in which orbital moment is of importance. 



Gyromagnetic ratios for paramagnetic materials have been deter- 

 mined by Sucksmith ^^ and are given in Table IV. The departures 



TABLE IV 

 Values of g for Some Paramagnetic Substances (Sucksmith) 



from the values 1 and 2 show that changes in both spin and orbital 

 moments occur during magnetization. In the last column are added 

 theoretical values deduced from spectroscopic data. 



Summary 



In this paper the author has discussed some of the difficulties en- 

 countered in the interpretation of the fundamental phenomena of 

 ferromagnetism, and some of the successes that have been attained by 

 applying our recent knowledge of the structure of atoms in solids. 

 The difficulties are large because the atomic forces controlling the 

 magnetism are small compared to those that hold the atoms together 

 in a solid. The successes have come largely as a result of the quantum 

 theory which has explained, mainly in a qualitative way, many of the 

 phenomena previously correlated by the empirical Weiss theory of the 

 molecular field. 



In some ways magnetic studies have aided materially in clarifying 

 our picture of the atom; this has been brought out in a discussion of 



'^W. Sucksmith, Proc. Roy. Soc, USA, 179-188 (1931); USA, 276-281 (1932); 

 Helv. Phys. Acta, 8, 205-210 (1935). 



