92 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



tion-method ; a small capsule of gaseous hydrogen at a pressure of only 

 one atmosphere will show the resonance of protons. The second isotope 

 of hydrogen is normally present in that substance in an abundance of 

 only 1.5 parts in ten thousand, the second isotope of oxygen exists only 

 in an abundance of four parts in ten thousand; neither was discovered 

 for more than a decade after the search for isotopes was well under 

 way; but both of them have been detected by nuclear resonance. 



Another application is to crystallography. In the crystal called gyp- 

 sum, each proton is exposed to a magnetic field of the order of ten gauss 

 from its neighboring protons. The resonance-peak is split into two or 

 three or even four, depending on the inclination of the big field to the 

 crystal axes. It would take many pages to describe this effect in detail, 

 but it is so intelligible that one may deduce from it the positions of the 

 protons in the crystal lattice. Nuclear resonance in fact seemed called 

 to play a great role in crystallography, since the principal tool of the 

 crystallographer has been the diffraction of X-rays, and this will not 

 disclose the presence nor a fortiori the locations of protons in a crystal 

 lattice. However this promising child of resonance has apparently been 

 throttled in its cradle, for the still newer art of neutron-diffraction has 

 proved itself adequate for finding the protons in a lattice. 



Another application is to the measurement of magnetic field strengths. 

 One sees that if proton-resonance is produced at a measured frequency in 

 a steady field of which the magnitude H is unknown, H may be determined 

 by equation (3) with an accuracy contingent on the accuracy with which 

 fjLp is kno\vn, and this is pretty high. This has become a common method 

 of measuring magnetic field strengths. 



RELAXATION 



If anyone were asked to guess the most important use of nuclear 

 magnetic resonance, he would have two good reasons for choosing the 

 study of relaxation. More pages of the scientific journals have been 

 devoted to it than to any other application. Moreover, the discoverers 

 spoke of it almost as soon as they spoke of the discovery; one has the 

 feeling that they were so confident of the discovery, that as soon as it 

 was made they considered it much less important for its own sake than 

 as a tool. 



"Relaxation" is a word that entered long ago into physics. Its general 

 meaning is the gradual self-adjustment of a system to a sudden change 

 in conditions. In the immediate instance the system is our sample in the 

 big magnetic field; the sudden change in conditions is the starting or 



