Sec. 3-1] 



MAGNETIC TRANSDUCERS 



205 



Fig. (3-1)2!). Nuclear magnetic reso- 

 nance system, basic diagram: C, coil; 

 N, sample; P t and P 2 poles of magnet. 



1654.61 ^0.1 cps/gauss. Measurement of magnetic field strength 

 can be accomplished, therefore, by the determination of the fre- 

 quency at which a sample containing hydrogen or lithium absorbs 

 energy. Using these substances and flux densities of 10 2 to 10 4 gauss, 

 the nuclear-resonance frequency is 

 in the range between 10 5 and 10 8 

 cps. 



A schematic diagram of a trans- 

 ducer system based upon this 

 nuclear-resonance phenomenon is 

 shown in Fig. (3-1)29. 



A sample N of a salt or a solution 

 of a substance containing H 1 , for 

 instance, or Li 7 , is mounted within 

 a coil C and is brought into the 



magnetic field between the poles P x and P 2 of a magnet, so that the 

 axis of C is perpendicular to the magnetic field. The output from an 

 rf oscillator is applied to the coil. The frequency of the oscillator is 



varied continuously through resonance, and 

 the absorption of energy is observed with 

 the measuring system M . The frequency at 

 which a maximum of absorption occurs is 

 measured, and the unknown field strength 

 is computed from Eq. (1). 



Two different systems M for the detection 

 of the nuclear magnetic resonance frequency 

 can be used. The measuring system M can 

 be constructed so that its output indicates 

 the change in the resistive component of the 

 complex coil impedance. In this case a curve 

 with a maximum is found as shown in Fig. (3-1)30 (nuclear-resonance 

 absorption). Or the measuring system M furnishes an output which 

 is proportional to the reactive component of the resonant circuit, so 

 that a curve is found as shown in Fig. (3-1)31 (nuclear-resonance 

 dispersion). 



The method is practical for field strengths from about 300 to 20,000 

 gauss (for proton probes mostly from 300 to 8,000 gauss, for lithium 

 probes from 2,000 to 20,000 gauss) corresponding to a frequency 

 range from 1 to about 30 Mc. Field strengths have been measured 

 down to the order of the earth's magnetic field. 1 The accuracy of the 

 method is very high and is primarily determined by the accuracy 

 X M. E. Packard and R. Varian, Phys. Rev., 93, 941 (1954). 



/o 



f 



Fig. (3-1)30. Nuclear mag- 

 netic absorption curve. 



