196 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



must be calibrated individually. The Hall coefficient changes with 

 the temperature and with the magnetic field strength. At room 

 temperature and at a field strength of 1,000 gauss, a temperature 

 increase of 10°C causes an increase of the Hall coefficient for germa- 

 nium by about 4 per cent, and for bismuth by about 10 per cent. 



Table 13. Hall Coefficients for Different Materials 



The influence of the magnetic field strength upon the Hall coefficient 

 is different for various substances. The Hall voltage in bismuth rises 

 first with increased field strength, reaches a maximum at 5 to 8 kilo- 

 gauss (depending upon temperature), and falls off to zero or negative 

 values between 10 and 20 kilogauss. 1 Anomalies in the behavior of 

 bismuth at low magnetic field strengths limit the applications at 

 fields below 0.3 gauss. 2 



The output voltage is in the milli- and microvolt region. The out- 

 put impedance depends upon the resistivity of the material and the 

 dimensions of the probe plate; the resistivity of those substances 

 which have a high Hall coefficient varies considerably with the tem- 

 perature and the magnetic field strength. A transducer system based 

 upon the Hall effect is described by Peukert. 3 The author uses a plate 

 ofbismuth,6.8 X 2.8 X 0.1 cm and a current 7 of 1.5 amp. Pearson 4 

 describes a transducer using n-type germanium with a range up to 

 20,000 gauss. The deviation from linearity (as determined by com- 

 parison with a rotating coil meter) is ±2 per cent up to 8,000 gauss 

 and -9 per cent at 20,000 gauss. The output impedance varies 

 between 140 and 170 ohms. 



/. Magnetoresistive Transducers. The resistivity of metals at low 



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



2 P. H. Craig, Phys. Rev., 27, 772 (1926). 



3 W. Peukert, Elektrotech. Z., 31, 636 (1910). 



4 G. L. Pearson, Rev. Set. Instr., 19, 263 (1948). 



