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THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



one of countless compounds of hydrogen. I first take water for the 

 sample, and enumerate the other particles in water. There are the 

 nuclei of the common isotope of oxygen, oxygen 16: they are non- 

 magnetic and produce no resonance. There are the nuclei of rarer 

 isotopes of oxygen and hydrogen: they will be mentioned later. There 

 are the electrons: they are reserved for Part II of this article. We are 

 now left with the protons. 



The sample is placed between the poles of a magnet, Fig. 1, so that 

 it is in a magnetic field which should be homogeneous and is usually strong. 

 The strength of the field is denoted by H, and its direction is always that 

 of the 2-axis (and usually vertical). I should hke to call it "the steady field," 

 but usually it is modulated during the experiments, so I shall call it 

 "the big field". Actually it can be very small, but nearly always it is 

 between 8,000 and 15,000 gauss, and 10,000 gauss is a good figure to 

 keep in mind. 



The big field must not be the sole magnetic field applied to the sample. 

 There must also be an alternating or oscillating field — stationary 

 electromagnetic waves, formed in a solenoid (or sometimes in a resonant 

 cavity). Such waves comprise, as Maxwell taught us long ago, an 

 alternating electric field and an alternating magnetic field. In most of 

 the uses of electromagnetic waves it is the electric field that counts, 

 and the magnetic field is remembered only as something demanded by 

 Maxwell's equations to keep the electric field going. In this application 

 the electric field takes a back seat, and it is the magnetic field that 

 counts. This oscillating magnetic field must be at right angles to the 

 big field; we lay the x-direction along it. Its amplitude, to be denoted 



I t i i t 

 t i t t t ( 

 t i i t ^ 



SAMPLE 



■*-X 



Figr. 1 — Scheme of the apparatus for observing nuclear magnetic resonance. 

 The detecting circuits are omitted. The nuclei indicated by the arrows are of 

 "spin ^/' protons for example. 



