Magnet ic Tra nsd ucers 



The most widely used magnetic transducer (a transducer that re- 

 sponds to magnetic field strength) is the induction coil or search coil 

 (3-11). The stationary search coil (3-1 la) requires a variation with 

 time of the magnetic field; it is useful whenever the field can be 

 turned on or off or when it varies periodically with time. Where this 

 cannot be done (e.g., the magnetic field of the earth or that in mag- 

 netic circuits containing ferromagnetic materials which show a resid- 

 ual flux density), the moving coil (3-116) furnishes adequate results. 

 The movement can be a translation, a rotation, or an oscillation. A 

 particularly useful induction system is the crystal-driven oscillating 

 loop; it is capable of measuring an area of not more than 0.15 mm 2 . 

 The output from the search coils is a voltage which may be constant, 

 alternating, or a pulse. Special search coils have been devised for 

 measurements in inhomogeneous fields (3-1 lc). 



Transducer systems based upon the effect of a magnetic field on 

 moving electric charges are described in 3-12. The magnetic field 

 may act on the electrons in a moving metal disk (3- 12a) or upon the 

 charged carriers of electricity in a moving conducting liquid (3-126), 

 or it may act on moving free electrons (3- 12c) or on electrons and 

 positive holes moving in a crystal lattice (3-1 2d). 



The variation with magnetization of the permeability of certain 

 ferromagnetic materials forms the basis of a further group of trans- 

 ducers (3-13). The permeability variation may lead to a change of 

 inductance (3-13a), or it may cause a variation in magnitude and 

 direction of the magnetic flux density (3-136); it may cause a dis- 

 tortion of a wave shape and, in further sequence, the generation of 



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