Sec. 3-1] 



MAGNETIC TRANSDUCERS 



207 



Press, London, 1955. For literature on the measurement of magnetic 

 fields by magnetic nuclear resonance methods, see the same book, chaps. 4.9 

 and 4.10. 



Energy can be absorbed from the electromagnetic rf field only if 

 more nuclei are in the lower energy level. The continuous absorption 

 of energy from the rf field has the effect of increasing the number of 

 nuclei in the higher level. If the population of both levels is the same, 

 the sample ceases to absorb energy from the rf field (the sample is 

 "saturated"), and no absorption effect can be noted. In order to 

 observe the nuclear-resonance effect, it is necessary, therefore, to 

 work at very low rf field strength where no saturation effect occurs. 



Fig. (3-1)33. Nuclear magnetic induction system: P lt 

 P 2 , magnetic poles; C\ and C 2 , coils, axes perpendicular 

 upon each other and upon permanent field P 1 and P 2 ; 

 A 7 , sample. 



Energy from a nucleus in a higher level can be dissipated by the 

 transfer of energy to the neighboring atoms in the lattice (and raising 

 the lattice temperature). The nuclear-lattice interaction can be 

 enhanced, and the observation of resonance absorption improved 

 through the addition of a small amount of a paramagnetic ion, such 

 as ferric nitrate, Fe(N0 3 ) 3 , or, to a lesser extent, by using H 1 nuclei 

 in highly viscous fluids. 1 



b. Nuclear Magnetic Induction. The arrangement is shown sche- 

 matically in Fig. (3-1)33. It consists of a sample N containing 

 magnetically resonant nuclei (H 1 in water or hydrocarbon, Li 7 in a 

 salt solution) in an rf energized coil C x between the poles of a magnet. 

 A pickup coil is oriented at right angles to both the direction of the 

 steady magnetic field and the axis of the rf coil. The change of 

 magnetization induces a voltage in this coil which may be amplified 

 and observed. The induced voltage is a maximum at the same fre- 



1 X. Bloembergen, E. M. Purcell, and R. V. Pound, Phys. Rev. ,73, 679 (1948). 



