NUCLEAR MAGNETIC RESONANCE 75 



electronic resonance. It is fitting that they should be treated in this 

 order, for the nuclear type of resonance is less distorted by complexities 

 than is the other. Perhaps it is not premature to say that while nuclear 

 magnetic resonance always goes by that name, the electronic type is 

 usually called "paramagnetic resonance" or "ferromagnetic resonance." 



Nuclear magnetic resonance was realized in 1937, in molecular-beam 

 experiments. The war distracted physicists, and the next great step 

 was not made until after — but very soon after — the armistices. In 

 the winter months of 1945-46 the phenomenon was produced in liquids 

 and in solids. The news burst upon the world from the pages of The 

 Physical Review in the early weeks of 1946, causing among physicists 

 an immediate and an immense sensation. Of some discoveries one 

 wonders how they came to be made at all, of others one wonders after- 

 ward why they were not made earlier. Nuclear magnetic resonance is of 

 the latter class. But this is a discovery that could not have been made 

 much sooner than it was, for it required the apparatus and techniques of 

 short-wave radio and microwaves, and these are recent. 



The work of 1945 was done by two independent groups three thousand 

 miles apart, using somewhat different experimental methods and ex- 

 pounding the theory in somewhat different ways. The differences are 

 really superficial, and in the course of time will probably be minimized; 

 but the two streams of later work that rose from those two sources are 

 still distinguishable. The methods are called the nuclear resonance 

 absorption method and the nuclear induction method: I treat them in 

 this order. A sketchy account of the molecular-beam method will 

 follow upon these, and then several of the applications — which of these 

 are major and which are minor must be left for history to decide. 



On the first few pages, and on many thereafter, the talk will be of 

 protons. Protons are the commonest material particles in Nature, 

 electrons excepted (neutrons are also an exception but not an important 

 one here, as they are seldom found free). Protons also have the happy 

 attribute called "spin J^" soon to be explained, which simplifies the 

 exposition greatly. This is one of the rare fields of physics in which the 

 simplest case, the commonest case, and the most useful case, are all 

 three of them one and the same. 



PROTON RESONANCE ABSORPTION 



To begin with, there must be a sample containing hydrogen, protons 

 being the nuclei of ordinary (as distinguished from heavy) hydrogen 

 atoms. It may be pure hydrogen in gaseous, liquid or solid form, or any 



