MAGNETIC RESONANCE. II 393 



Since / is J/^ for the electron (as it is for the proton) the magnetic 

 moment of the free electron is: 



M. = (3^)^(6/47rmc) = 1.001146 ± 0.000012 Bohr magnetons (14) 



This is the value which is 658.2288 times the moment of the proton. 



Another case very near to the ideal is afforded by the electrons of 

 such atoms as manganese widely dispersed in a phosphorescent solid. 

 Thus, there exists a measurement of g made upon "zinc sulphide phos- 

 phor" containing manganese atoms in a concentration of 0.001 per 

 cent. The value is 2.0024 =b 0.0004. It must be said that the resonance 

 in question is complicated both by fine structure and by hyperfine struc- 

 ture, terms to be explained in following sections. It is therefore neces- 

 sary to use theory to locate, among the complex of peaks, the frequency 

 which corresponds to the appropriate value of g. 



Still another case which is close to the ideal is provided by the "F- 

 centres" in colored crystals, mention of which was made in Part I. 

 An F-centre is a cavity in a crystal lattice occupied by a free electron 

 batting around, as I said in Part I, like a wild animal in a cage. Several 

 physicists have found their resonance, present when the crystal is colored 

 and absent when the crystal is bleached. One, who produced the colora- 

 tion by neutron-bombardment, located the peak at gr = 2.00. Others 

 report 1.995 =b 0.001. 



Still another case which is close to the ideal is afforded by the con- 

 duction electrons in a metal. These are so numerous that one might 

 expect that the electron resonance that they produce must be extremely 

 prominent. Yet the first such peak to be observed has been reported 

 only as these lines are being written! The reasons for its inconspicuous 

 character are two: most of the conduction-electrons are coupled anti- 

 parallel, and the skin-effect confines the oscillating field in a conductor 

 to a very narrow region close up against the surface. The second of 

 these hindrances is overcome by using a colloidal dispersion of the metal, 

 of which the spherules are less than 10~^ cm in diameter. Data are avail- 

 able (though not yet all in print) for hthium, sodium and potassium. 

 The values of g are within a few promille of 2.000; the differences between 

 these and the ''ideal" value are small but not trivial, and in the case of 

 lithium have been explained. 



ELECTRON RESONANCE IN PARAMAGNETIC SOLIDS 



There are paramagnetic soUds that display the electron resonance. 

 A magnificent illustration is sho^vn in Fig. 2, belonging to an organic 



