Sec. 3-1] 



MAC IX E TIC T /.'. 1 NSDUGE RS 



197 



temperature changes if the metal is exposed to a magnetic field. The 

 effect is noticeable in some metals at room temperature and is 

 strongest in bismuth; the resistivity of bismuth in strong magnetic 

 fields (of the order of 20,000 oersteds) can rise to twice that measured 

 at zero field strength. This phenomenon forms the basis of the 

 magneto-resistive transducer system shown in Fig. (3-1)21 . 



A thin wire of bismuth is wound in the form of a flat bifilar spiral of 

 about \ to 3 cm diameter. The spiral is cemented between two thin 



Mica 



Fig .(3-1)21. Magnetoresist ive trans- 

 ducer (bismuth spiral). 



5,000 10,000 15,000 



B t gauss 



Fig. (3-1)22. Calibration curve of a 

 magnetoresistive transducer. 



sheets of mica. In general the thickness of this probe does not exceed 

 1 mm. The ends of the wire are soldered to two copper bands, 

 insulated from each other, which serve both as connecting elements 

 and as a handle. The bifilar arrangement eliminates induction effects 

 when the coil is moved in or out of a magnetic field or when the field 

 changes. 



A typical transfer characteristic is shown in Fig. (3-l)22. 1 In the 

 linear part of the characteristic the resistance increases by about 5 

 per cent for an increase of the magnetic field of 1 ,000 oersteds. Below 

 400 oersteds the characteristic shows an irregular and anomalous 

 behavior. 2 



The resistance variation of bismuth in magnetic fields shows a 

 transient aftereffect which is not fully understood. The effect 

 depends upon the magnetic field strength and the current through 

 the bismuth probe. 3 Because of this effect the bismuth spiral should 



1 G. Bublitz, Arch. tech. Messen, V 391-2, May, 1938. 



2 G. K. T. Conn and B. Donovan, J. Set. Instr., 28, 7 (1951). 



3 See, for instance, P. P. Konig, Ann. Physik, 25, 921 (1908), and T. 

 Heurlinger, Physikal. Z., 17, 221 (1916). 



