198 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



be used only for the measurement of constant (d-c) magnetic fields, 

 and the resistance should be measured with direct current. 



The response characteristics of bismuth transducers depend 

 strongly upon the chemical composition (i.e., impurities) and the 

 method of preparing and handling the wire. Using very pure bis- 

 muth, compressing the wire while at a temperature close to the 

 melting point and annealing the spiral are recommended. 1 The 

 response also depends somewhat upon the geometric form of the 

 spiral and the direction of the magnetic field. The bismuth wire 

 shows, in general , an anisotropic behavior. Conn and Donovan 2 

 describe a method of preparing a bismuth wire by drawing bismuth 

 in glass capillaries, dissolving the glass, and forming the wire into a 

 loop. This method reduces the anisotropy. The magnetic sensi- 

 tivity — slope, (ARjR)/AB — of thin bismuth films is very small but 

 can be slightly increased by repeated heating and cooling. 3 



The characteristic of the prepared probe changes considerably 

 with temperature. In the linear part of the characteristic and at 

 room temperature, a change in temperature of 1°C causes a change 

 in resistance of about 0.4 per cent (corresponding to a change of the 

 magnetic field of 80 oersteds). The sensitivity increases considerably 

 at lower temperature. For this reason and since the thermoelectric 

 effect of bismuth against other metals is very high and likely to intro- 

 duce errors, operation of the transducer at constant temperature and 

 measurement of the resistance at low current (to avoid heating) are 

 required. With adequate provisions for constant temperature or 

 with temperature-effect compensation, the transducer can be used 

 for magnetic fields from 500 oersteds up to the highest field strengths 

 with an accuracy of about 1 per cent. Smith 4 describes a bridge in 

 which two bismuth spirals of approximately 30 ohms are arranged 

 in opposite bridge arms. For temperature compensation the two 

 other bridge arms are made from nickel which has approximately the 

 same resistance-temperature coefficient as bismuth. The sensitivity 

 S = AiJAB, i.e., the change of current in the bridge diagonal indi- 

 cated by the meter divided by the change of the magnetic flux density 

 in the bismuth plate, is of the order of 1 mA per 1,000 gauss. 



For a theoretical summarizing paper and references concerning other 

 materials showing magnetoresistive effect, see J. P. Jan, "Galvanomagnetic 



1 G. Bublitz, loc. cit. 



2 G. K. T. Conn and B. Donovan, loc. cit. 



3 J. A. Becker and L. F. Curtis, Phys. Rev., 15, 457 (1920), and F. K. 

 Richtmyer and L. F. Curtis, Phys. Rev., 15, 465 (1920). 



4 G. S. Smith, Trans. AIEE, 56, 441 (1937). 



