200 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



Because of the large residual field deformation caused by the probe, 

 the method lends itself primarily to relative measurements. 



b. Time Variation of Magnetic Permeability. A variation of the 

 above method is illustrated in Fig. (3-1)24. The transducer system 

 contains a fine wire M of Mumetal 1 (a ferromagnetic alloy with a 

 permeability which changes strongly with the magnetizing field). 

 The wire is connected in a loop and arranged within a coil L. An a-c 



current of audiofrequency passes through 

 the wire M and produces a circular field in 

 and around the wire. This field changes 

 the permeability of the Mumetal wire, but 

 because of its direction it will not, by itself, 

 induce a voltage in the coil L. If now an 

 external field H is applied in the direction 

 of the coil axis, it will produce a magnetic 

 induction B z = juH . Since ft is a time vary- 

 ing magnitude, B z varies with time and 



Fig. (3-1)24. Magnetic 

 transducer based upon the 

 variation with time of a 

 Mumetal wire M; L, pickup 

 coil. 



causes a voltage to be induced in the coil L. 



H n 



The frequency of this voltage contains pri- 

 marily the even harmonics of the applied 

 audiofrequency. Experimental calibration 

 is required. If the instrument is to be used 

 as a zero device, a d-c current can be sent through the coil L which 

 compensates the external field H . Palmer 2 describes an instru- 

 ment based upon this principle, which is capable of measuring field 

 strengths from 2 x 10~ 3 (10~ 5 with 

 great care) to 50 oersteds. The 

 accuracy is better than . 1 per cent . 

 The probe is about 4 cm long and 

 0.5 cm in diameter. A similar de- 

 vice with two coils is described by 

 Gregg. 3 The permalloy wire is 0.3 

 cm long and has a diameter of 0.25 

 mm. The finished probe is 0.3 cm 

 long and has a diameter of 0.2 cm. 

 c. "Even-harmonic'''' Method. 

 Higher sensitivity and higher 

 accuracy than can be achieved with the methods described above can 

 be obtained with the system shown schematically in Figs. (3-1)25 



1 Mumetal, trade name, Allegheny Ludlum Steel Corporation. 



2 T. M. Palmer, Proc. Inst. Elec.^Engrs. (London), 100, (II) 545 (1953). 



3 E. C. Gregg, Rev. Set. Instr., 18, 77 (1947). 



Fig. (3-1)25. Magnetic transducer based 

 upon the "even-harmonic" method: C l 

 and C 2 primary, D t and D 2 secondary 

 windings of a transformer. 



