Magnetic Resonance 

 PART I — NUCLEAR MAGNETIC RESONANCE 



By KARL K. DARROW 



(Manuscript received September 3, 1952) 



Magnetic resonance is the name of a phenomenon discovered less than 

 sixteen years agOj which from the start has had a high theoretical importance 

 and is now attaining a notable practical value. Nuclear magnetic resonance 

 occurs when a substance containing magnetic nuclei is exposed to crossed 

 magnetic fields, one being steady and the other oscillating, and the strength 

 of the former field and the frequency of the latter are matched in a particular 

 way. When these are properly matched, the nuclei are turned over in the 

 steady field, and energy is absorbed from the oscillating field. Another way 

 of describing the effect is to say that resonance occurs when the applied 

 frequency is equal to the frequency of precession oj the nuclei in the steady 

 field. This phenomenon illustrates very clearly some of the fundamental 

 laws of Nature. For the purposes oj nuclear physics it is used to determine 

 the magnetic moments of nuclei and their relaxation-times in the substance 

 that contains them. It is also used for chemical analysis, fbr measurement 

 of magnetic fields, for analysis of crystal structure and for locating changes 

 of phase of the substance containing the nuclei. Magnetic resonance of 

 electrons is similar, but for a fundamental reason is confined almost ex- 

 clusively to free atoms of certain kinds, to ferromagnetic substances and to 

 certain strongly paramagnetic salts. For these last it serves to throw light on the 

 fields prevailing within the crystals. 



"Magnetic" is an ancient word in physics and so is "resonance," but 

 "magnetic resonance" is something new. It is the name of a phenomenon 

 which is sharp and clearcut and easy to evoke, which springs directly 

 from the ultimate magnetic particles of matter, which illustrates the 

 fundamental laws of these, and which has found and still is finding uses 

 of importance. There are two types of it, the nuclear and the electronic. 

 Nuclear magnetic resonance is the theme of the first part of this article: 

 it will recur from time to time in the second part (to appear in a later 

 issue of this Journal) but the main topic of that second part will be 



74 



