Sec. 3-1] MAGNETIC Til. 1 XShl Y 'KRS L99 



and Thermomagnet ic Effects in Metals," in "Solid State Physics," pp. Iff and 

 sec. 10, p. 30, F. Seitz and D. Turnbull (eds.), Academic Press, Inc., New 

 York, 1957. 



3-13. Transducers Based on Permeability Variation 



The magnetic permeability fi(=~ dB/dH) of ferromagnetic ma- 

 terials varies with the magnetization and is, therefore, a function of 

 the external magnetic field to which a ferromagnetic body is ex- 

 posed. Various transducer systems are based upon this phenome- 

 non ; they differ by the means by which the change in permeability 

 is detected. 



a. Inductance Variation. Boucke 1 describes a System consisting of 

 a coil with a ferromagnetic core; the inductance of the coil is a func- 

 tion of the permeability of the core, and it changes with the external 

 magnetic field. A ferromagnetic core of high permeability would 

 cause a field deformation and introduce a considerable error in the 

 measurements. This error is reduced to about 10 to 20 per cent by 

 the use of a core of compressed powdered magnetic material (50 to 70 

 per cent iron powder: grain size, several 

 microns, with a resinous binder). The 

 most suitable core material shows an 

 initial permeability between 10 and 12, 



which diminishes to about 2 to 3 in 



, . n i i k a i • Fig. (3-1)23. Inductance-varia- 



strong magnetic fields. A fiat washer is .. ... A 



& & tion magnetic transducer. 



formed of this material, and a toroidal 



coil is wound around it, as shown in Fig. (3-1)23. The final coil is 

 about 1 mm thick and about 6 mm in diameter; it is electrostatically 

 shielded and has an inductance of several microhenrys. 



The transfer characteristic, i.e., the fractional change of induc- 

 tance (AL/L) as a function of the externally applied field, is non- 

 linear; in the range from zero to 12 oersteds the value of A.L/L 

 increases slightly; beyond this range it decreases in the form of an S 

 curve. The fractional change of inductance is about 10 per cent for a 

 field variation from zero to 24,000 oersteds. The sensitivity changes 

 with the direction of the magnetic field with respect to the coil. The 

 core material does not show any hysteresis. 



Using the most sensitive method for the measurement of the 

 inductance variation AL (rf beat-frequency method), a magnetic- 

 field variation of 0.01 per cent can be observed in the range be- 

 tween 300 and 2,000 oersteds. The smallest absolute field strength 

 to be measured is about 1 oersted. Empirical calibration is required. 



1 H. Boucke, Arch. tech. Messen, V 391-1, June, 1937. 



