386 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



sample; it is vertical and its strength is denoted by H. This is the field 

 wdth respect to which the protons are oriented. These magnetic particles 

 are represented by small arrows within the rectangle, the point of each 

 arrow corresponding to the north pole of the corresponding proton. 

 Slightly more than half of them are pointed in what I call the "up" 

 orientation, which is that of lesser energy. The rest are pointed in the 

 "down" orientation, that of greater energy. Magnetic resonance absorp- 

 tion of protons is the turning of ''up" protons into the ''down" direction. 



The field which does the turning is an oscillating magnetic field with 

 frequency (denoted by v) in the radio-frequency range. It is horizontal, 

 thus at right angles to the big field. It is produced either in a solenoid 

 (the usual method for nuclear resonance) or in a resonant cavity (the 

 usual scheme for electronic resonance) which encloses the sample but 

 in Fig. 1 is left to the imagination of the reader. 



Magnetic resonance occurs when the quantum-energy hv of the oscil- 

 lating field is equal to the work required to turn the proton from the up 

 orientation to the down one: 



hv = work of turning (1) 



h standing for Planck's constant. In Part I it was shown that the "work 

 of turning" or energy-difference between the two orientations is equal 

 to 2upH: here /Xp stands for the magnetic moment of the proton, soon to 

 be more carefully defined. Thus : 



hv = 2n^H (2) 



When V and H are related by this equation one finds proton resonance 

 absorption, which manifests itself by a splendid peak in the curve of 

 absorption versus H for constant v or the curve of absorption versus v 

 for constant H. For the frequency 42.6 megacycles the peak is found at 

 H = 10,000 gauss. 



To arrive at the basic formula for electron resonance we simply take 

 (2) and substitute into it /x« , the magnetic moment of the electron, for 



hv = 2m^ (3) 



The magnetic moment of the electron is about 660 times that of the 

 proton. Therefore if one works with such a field strength as brings the 

 proton resonance into the radio frequency range, the electron resonance 

 is to be sought in the microwave range. One might think that now I 

 have said all that there is to be said about electron resonance ; but this is 

 only the beginning. 



