208 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



quency/ at which resonance absorption occurs. The physical proc- 

 ess is similar to that of the absorption phenomenon. 1 



All nuclear-resonance methods require a good field homogeneity 

 (gradient not more than several gauss per centimeter throughout the 

 volume of the probe), since otherwise the resonance curve is too 

 broad for convenient measurement. Small probes containing not 

 more than 0.03 cm 3 of water have been used successfully for measur- 

 ing field gradients up to 300 to 400 gauss/cm. The value of the 

 nuclear magnetic resonance method lies primarily in its high absolute 

 accuracy. The experimental requirements are considerable. 



3-15. Indirect Systems 



The following systems do not furnish an electric signal in response 

 to a magnetic input; they produce a mechanical force, a displace- 

 ment, a thermal, or an optical output which is converted into an 

 electric signal by additional means. 



a. Mechanical Methods, freely suspended magnets. In prin- 

 ciple, any magnetometer, i.e., a system of freely suspended magnets, 



can be used as a magnetoelectric trans- 

 ducer, if means are provided to con- 

 vert the angular displacements of the 

 magnets into electric signals. A combi- 

 nation of this kind has been described 

 by Gollian and Reilly 2 and is shown 

 schematically in Fig. (3-1)34. A mag- 

 net needle N is mounted in an enclosed 

 capsule F so as to turn freely in a hori- 

 zontal plane around the pivot A. The 

 capsule is filled with a semiconducting 

 liquid (commercial ethyl alcohol) ; four 

 contacts B, C, D x , and D 2 are pro- 

 vided. The arrangement is connected 

 in a bridge circuit. Any deviation of the needle from the symmetry 

 position causes a bridge unbalance and produces a voltage e at the 

 output terminals. The output signal changes as an approximately 

 parabolic function with the angular needle displacement, and varies 

 from 0.3 to 1 volt for a variation of position from zero to 10°. The 

 resistance between the electrodes B and D for symmetry position of 

 the needle is in the order of 50,000 ohms. 



1 See Andrew, op. cit., chap. .3.8, and F. Bloch, W. W. Hansen, and M. E. 

 Packard, Phys. Rev., 69, 127 (1946), and Phys. Rev., 70, 474 (1946). 



2 S. E. Gollian and E. G. Reilly, Rev. Sci. Instr., 22, 753 (1951). 



Fig. (3-1)34. Mechanoelectric mag- 

 netic-field transducer: N, magnet 

 needle; A, pivot; F, capsule filled 

 with semiconducting fluid; B, C, 

 Dj. and D 2 , contacts. 



