364 BELL SYSTEM TECHNL '... /OURNAL 



Everyone now agrees with the idea, proposed more than a century 

 ago by Ampere, that atoms are magnets because of the circulating 

 charges which they contain. The estimates of atomic moments 

 deduced from Hne-spectra are based on this assumption, and the 

 verified correctness of these estimates sustains it. Now, if a magnetic 

 atom is a whirl of electricity, it possesses angular momentum as well 

 as magnetic moment. If so, the process of magnetizing an iron wire 

 involves the bringing-into-parallelism of myriads of spinning-tops, 

 of which the angular momenta when all aligned combine into a 

 respectable sum. If this goes on inside a wire during magnetization, 

 there should be a "recoil" somewhere, comparable to the recoil of 

 a gun when a shell is fired — the suspension of the wire should receive 

 an opposite angular momentum, experience a torque. Conversely, 

 the process of twisting an unmagnetized wire should impress a lateral 

 torque upon myriads of spinning-tops of which the axes point in 

 directions scattered at random; each of these should be urged to 

 set itself more nearly parallel to the axis of the twist, which is the 

 axis of the wire; and the twisting should therefore magnetize the wire. 



Both of these effects, which jointly are called the " gyromagnetic 

 efifect," have been detected and measured. From the measurements 

 (thus far performed upon iron, nickel, cobalt, magnetite and a Heusler 

 alloy), it results that the ratio of the angular momentum P to the 

 magnetic moment M of an elementary magnet conforms to the 

 equation : 



P/M = mc/e, 



in which m stands for the mass of the electron and e/c for its charge 

 measured in electromagnetic units. This is the value which would be 

 expected for the ratio, if the elementary magnet is an electron spinning 

 upon itself. 



Now there are weighty reasons for supposing that the conception of 

 a "spinning electron," possessing a fixed characteristic angular 

 momentum and a permanent magnetic moment elmc times as great, 

 may be what is required to complete the theory of line-spectra of 

 free atoms which Bohr began. The gyromagnetic effect of the 

 ferromagnetic solids therefore indicates that the elementary magnets 

 scattered through these are the same as the elementary magnets 

 located in free atoms — they are electrons, or groups of electrons 

 suitably linked together. The test cannot be made upon para- 

 magnetics, for they cannot be (or at least have not yet been) strongly 

 enough magnetized. Ferromagnetic substances are the only ones 

 which in a feasible field acquire so great a magnetization that the 



