NUCLEAR MAGNETIC RESONANCE 89 



now been used in both these senses, and there are variants within each 

 sense, depending on the unit that is preferred by the user. The ap- 

 pearance of either ''gyromagnetic ratio" or "magneto-mechanical ratio" 

 in a paper is a red light warning the reader to make sure just what the 

 author means by it. In this paper both of these terms are discarded 

 with regret. 



An experimenter may give his value of (ju//) directly, or may give 

 his value of g, which is (fi/I) expressed in terms of a peculiar unit. The 

 peculiar unit is eh/4:TrmpC, in which nip stands for the rest-mass of the 

 proton and the other symbols have their normal meanings. This unit 

 got into the picture because there was a doctrine that (fi/I) for the 

 proton ought to be just two of it. This was based on an analogy with 

 the electron which, to the consternation of theorists and the complica- 

 tion of Nature, proved to be fallacious. Reported values of g range 

 from nearly 6 to 0.143; the proton has one of the two highest values, 

 the triton or nucleus of hydrogen 3 has the other. Since all these values 

 may be described without much extravagance as being "of the order of 

 2," the use of g remains convenient.* 



Many people say that they have measured fx. Formally this is all 

 wrong, but practically it is usually all right, for in most if not all cases 

 / is known from experiments of other kinds. Most of these people give 

 the value of n in "nuclear magnetons." This means that they are giving 

 the value of gl, as is seen from the following equation which resumes in 

 notation what I just said in words, and provides the definition of g: 



II = gl(eh/^irmpc) (10) 



The quantity in brackets is called "nuclear magneton." 



Now that this tiresome but necessary passage is behind us, we can 

 review the results. 



Values of gl — or of some other of the quantities catalogued above 

 — have been published for about forty nuclei. The values of gl available 

 toward the end of 1950 were gathered together and published in an 

 article to which I give the reference in a footnote, f The largest is about 

 twenty-five times the smallest: this is a wide range of variation, yet 

 not so wide as that of the nuclear charges or the nuclear masses. Isotopes 

 of one another may have values nearly the same or considerably dif- 

 ferent; the same is true of isobars. Most of the values are positive: this 



* It is perhaps not premature to mention that in optical spectroscopy and m 

 electronic magnetic resonance, the symbol g is used with a similar but not an 

 identical meaning. _ 



t Pake, G. E., American Journal of Physics, 18, pp. 438^52, pp. 473-86, 1950. 

 The table is on p. 440 of the October issue. 



