178 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



harmonics (3- 13c), or it may lead to a phase shift of an a-c voltage 

 (3-13d). 



Another group of magnetic transducers is based upon the phenom- 

 enon of nuclear resonance (3-14). 



There is a further group of transducers which do not directly 

 furnish an electric output in response to a magnetic input, but will do 

 so in combination with other transducers or servosystems (3-15). The 

 direct response may be a mechanical displacement (3- 15a), a tem- 

 perature increase (3-156), or it may be of an optical character (3-15c). 



Magnetic fields have been measured in a range from 10~ 6 to about 

 10 6 oersteds (flux densities 10~ 6 to 10 6 gauss). The magnetic field of 

 the earth is in the order of 0.5 oersted; its horizontal component 

 varies between 0.15 and 0.4 oersted. Permanent magnets produce 

 fields in the order of 100 to 10,000 oersteds. The field strengths in 

 electrical machines and in particle accelerators are in the order of 

 10,000 to 20,000 oersteds. Electromagnets for laboratory use may 

 produce fields in narrow gaps up to 60,000 oersteds. Kapitza 1 ob- 

 tained field strengths in the order of 500,000 oersteds, and Wald 2 

 field strengths in the order of 450,000 oersteds. Furth and Wanick 3 

 succeeded in obtaining fields lasting from 50 to 200 fx&ec and reaching 

 values as high as 600,000 oersteds. Foner and Kolm 4 have produced 

 fields of 750,000 oersteds. 



Information about the useful range of different transducer systems 

 may be found in Table 12. The most sensitive systems are the 

 electron-beam magnetometer and the even- harmonic method. The 

 highest absolute accuracy is obtained with the nuclear-resonance 

 system . 



A system of a given sensitivity for use at low field strength may 

 well be used in fields of higher intensities if the unknown field 

 strength is compensated for by known bias field, such as the field 

 produced by a coil of well-defined dimensions through which an 

 adjustable current passes. The transducer system is then used as a 

 null detector, or it measures variations of magnetic field strength. 



For summarizing literature and references on magnetic fields and their meas- 

 urements, see L. F. Bates, "Modern Magnetism," Cambridge University Press, 

 London, 1948; R. M. Bozorth, "Ferromagnetism," Bell Laboratory Series, 

 D. Van Nostrand Company, Inc., Princeton, N.J., 1951; H. Neumann, 4rcL 

 tech. Messen, V 391-4, August, 1940; E. C. Stoner, "Magnetism and Matter," 



1 P. Kapitza, Proc. Roy. Soc. (London), (A)115, 658 (1927). 



2 T. F. Wald, J. Inst. Elec. Engrs., 64, 509 (1926). 



3 H. P. Furth and R. W. Wanick, Rev. Sci. Instr., 27, 195 (1956). 



4 S. Foner and H. H. Kolm, Rev. Sci. Instr., 27, 547 (1956). 



