Sec. 3-1] 



MAGNETIC TEA SSIH ( 'Hits 



211 



Faster response (and presumably less disturbance of the field) can 

 be obtained with a thermoelectric method. According to Laporte 

 and Vasilesco, 1 a thermoelement junction in an evacuated vessel is 

 placed in the magnetic rf field. The cold junction is outside the field 

 and kept at constant temperature. The emf is measured with a galva- 

 nometer or microammeter. Figure (3-1)39 shows the thermocouple 



75 



?50 



25 



E 



CD 







2 4 6 8 



Flux density (peak value), gauss 



Fig. (3-1)38. Transfer character- 

 istics for a thermal magnetometer 

 in an rf magnetic field. 



25 



20 



-15 



.10 



5 10 15 20 25 



Flux density (peak value), gauss 



Fig. (3-1)39. Transfer characteristics, 

 for two different frequencies, of a 

 thermoelectric magnetometer in an rf 

 field. 



output as a function of the applied field strength for an iron-con- 

 stantan thermoelement. In the linear part of the characteristic, the 

 sensitivity is of the order of 1 mV /gauss. 



c. Optical Methods, faraday effect at optical frequencies. 

 The principle of this transducer system is illustrated in Fig. (3-1)40. 



Fig. (3-1)40. Faraday-effect magnetic transducer, schematic diagram 



A monochromatic parallel light beam passes an optical polarizer P, 

 a vessel C of length L filled with a material which exhibits magneto- 

 rotation, and an analyzer A adjusted at an angle of 90° with respect 

 to the polarizer, so that no light will fall upon the detector D. If a 



1 M. Laporte and V. Vasilesco, J. physique radium, 6, 289 and 85 S (1935). 



