206 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



a 

 a> 

 Q 



with which the frequency of the rf generator can be measured. 

 Accuracies as high as 1 part in 10 4 to 10 5 have been obtained. 



The nuclear magnetic resonance effect is very small and the reso- 

 nance is very sharp. (The Q factor of a nuclear-resonance system is 



in the order of 10 6 ). The requirements for the 

 equipment to detect the resonance absorp- 

 tion and to measure the frequency are 

 considerable. 



A method to facilitate the detection of 

 resonance is shown in Fig. (3-1)32. The basic 

 arrangement is identical with that of Fig. 

 (3-1)29, but a second field is applied (coils D) 

 which is energized by a 25- to 60-cycle source. 

 These coils produce an a-c magnetic field of 

 approximately 5 to 30 gauss which is super- 

 imposed on the steady field and which period- 

 ically changes the resonant frequency of the 

 probe. The a-c source is also connected to the horizontal deflection 

 system of a cathode-ray oscilloscope; a signal which is proportional 

 to the absorbed power is formed in the network M and is applied to 

 the vertical deflection system. Because the a-c field periodically 

 changes the resonance frequency of the probe, a resonance curve will 



fn 



f 



Fig. (3-1)31. Nuclear mag- 

 netic resonance dispersion 

 curve. 



r f -AA/W 



generator 



30-cps 

 generator 



D 



i 



<*• 



D 



M 



N 



C 



Pz 



T- 



A^ 



Fig. (3-1)32. Nuclear magnetic resonance arrangement 

 for display of the absorption curve on the screen of a 

 cathode-ray oscillograph: P x , P 2 , poles of magnet; A T , 

 sample; C, coil; D, auxiliary coils. 



appear on the screen. The resonance frequency is read when the 

 resonance maximum appears at the center of the screen, -i.e., when 

 the instantaneous value of the a-c field is zero. The resonance curve 

 is sometimes complicated by transient phenomena. 



For detailed descriptions of this and other experimental arrangements, see 

 E. R. Andrew, "Nuclear Magnetic Resonance," chap. 3, Cambridge University 



