90 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



means that the angular momentum and the magnetic moment are 

 parallel. A few are negative: for these (one of which is the neutron) the 

 angular momentum and the magnetic moment are anti-parallel. 



These values of /x (for, I repeat, gl is ju expressed in terms of a par- 

 ticular unit) are useful as challenges and as aids to the nuclear theorists. 

 They are challenges, because the /x-value of a given nucleus is something 

 to be explained ; they may be aids, because a theory may be fortified by 

 giving the right value of /z or confuted by leading to a wrong one. Now, 

 nuclear theory is difficult, and by and large it is not so far advanced 

 that it can demand experimental values accurate to let us say, the 

 legendary "sixth place of decimals." This is a piece of temporary good 

 fortune, for two reasons. 



First, the strength of the big field H may not be known with adequate 

 accuracy at the place where the nuclei are. It can however be ignored 

 if one is concerned only to measure the ratio of the (m//) values of two 

 nuclei. The experimenter has then to put into his apparatus successively 

 samples containing the two kinds of nuclei, or a single sample containing 

 them both: the ratio of the frequencies at which the resonance-peaks 

 appear is the ratio of the {ixjl) values, and H vanishes in the division. 

 Often the comparison-nucleus is the proton, so that many published 

 values of gl come ultimately from ratios in which gl for the proton 

 stands in the denominator. Such ratios are frequently adequate for the 

 testing of theories, and their accuracies may be very good indeed, even 

 attaining the sixth significant figure. (The basic determination of /x for the 

 proton itself will be mentioned in Part XL) 



Second, the true field which the nuclei experience may be slightly 

 different from the big field H, because of local fields within the sub- 

 stance. This is of course an admission that our fundamental equation, 

 (5) or (9) in this article, can be wrong. So it can be, and this is a de- 

 velopment that may be thought distressing. But such developments are 

 almost the rule in physics, whenever the art of measurement is bettered ; 

 and in the present case the errors in equation (9) must be regarded as 

 felicitous, for they lead to some of the most fascinating applications of 

 nuclear resonance. 



Thus when ammonium nitrate, NH4NO3, is put into the apparatus, 

 there are two peaks of nitrogen instead of one. They are not far apart — 

 if for the frequency in use one is at i/ = 10,000 gauss the other is at 

 9,997. The formula NH4NO3 suggests, and the diagram of the molecule 

 would confirm if we had it here, that the two nitrogen nuclei are dif- 

 ferently placed in the molecule: one may say that they have different 

 atomic surroundings. Thus the position of either of the peaks is dis- 



