MAGNETIC RESONANCE. II 



385 



subatomic particles: nuclei and electrons. These, apart from the nuclei 

 that are non-magnetic, may be visualized as minuscule barmagnets. 

 The laws of resonance are determined by the fact that in a steady mag- 

 netic field, the magnetic moments of these particles may not point in 

 any and every direction: instead, they are constrained to a finite and 

 small number of what are called "permitted orientations." To each of 

 these corresponds a special value of the energy of the little magnet in 

 the field: thus the energy also is constrained to a finite and small num- 

 ber of ''permitted" values. These are often called ''Zeeman levels" or 

 just "levels"; and the word "level" should be well known to those who 

 are going to delve into the literature. 



H,-^ 



i t I i t 

 t I t t t I 



I ▼ ▼ T SAMPLE 



^X 



Fig. 1. — Scheme of the apparatus for observing magnetic resonance. The high- 

 frequency circuits are omitted. The arrows within the sample maj^ be taken as 

 portraying the magnetic moments of either protons or electrons: their orientations 

 are as given by the old quantum-theory. 



Consider two orientations or levels of different energy-values. It will 

 take work to turn the tiny magnet from the one of lesser energy to the one 

 of greater energy. Magnetic resonance — • and now I ought perhaps to 

 speak specifically of magnetic resonance absorption — is such a turning. 

 The agent of the turning and the source of the work is an alternating or 

 oscillating magnetic field. The simplest cases are those in which the 

 particle in question has only two permitted orientations. Many nuclei, 

 among them the proton, belong to this class, and the electron belongs 

 to it also. It is the analogy between proton and electron which I mil 

 develop. 



Fig. 1 of this part is also Fig. 1 of Part I. The central rectangle depicts 

 the sample, which for the study of proton resonance must be hydrogen 

 or a compound thereof. The big arrow on the left represents a big mag- 

 netic field, of the order of several thousand gauss, which pervades the 



