Sec. 3-1] 



MAGNETIC TRANSDUCERS 



189 



Fig. (3-1)10. "Solid" flux ball, 

 partly seen in cross section to 

 show the arrangement of the 

 single layers. 



where D p is the layer spacing, and D z is the spacing between adjacent 

 turns (effective wire diameter). 



A disadvantage of the flux ball is its physical dimensions, which 

 are considerabty larger than those of a flat coil, and which make it 

 impossible to measure fields in narrow 

 gaps or in close proximity of a mag- 

 netic pole. 



Herzog and Tischler 1 have shown 

 that an ordinary cylindrical coil can be 

 employed instead of the flux ball if 

 the ratio of the length L to the outer 

 diameter D has the value LjD = 0.72. 

 The voltage-time integral at the out- 

 put of this coil depends only upon the 

 change of field intensity in the center 

 of the coil. The authors found that in 

 using coils of this shape the results 

 of measurements in an inhomogene- 

 ous field are practically independent 

 (within less than 1 per cent) of the size of the coil. 



d. Magnetomotive-force Transducer Coil. A magnetic potential 

 difference or magnetomotive force M between two points A and B 

 is defined as 



M = [ Hds 



it is independent of the direction and length of the path as long as s 

 does not surround a current-carrying conductor. The transformation 

 of a magnetomotive force into an electric signal can be obtained, in 

 analogy to the transformation of magnetic field strengths into elec- 

 tric signals, through the use of a special coil, the "magnetomotive- 

 force meter." 2 



The instrument consists of a long coil (about 1 m in length), as 

 shown in Fig. (3-1)11. Two layers of fine wire are wound on a band 

 or belt of a flexible plastic material, both in the same direction, as 

 shown schematically in Fig. (3-1)12. For practical reasons, con- 

 nections are made to the center of the coil. 



If the two ends of the coil are brought to two points in a magnetic 

 field between which a magnetomotive force M exists, and if the field 



1 R. F. K. Herzog and O. Tischler, Rev. Sci. Instr., 24, 1000 (1953). 



2 W. Rogowsky and W. Steinhaus, Arch. Elektrotech., 1, 141 (1912); for a 

 summarizing review and references, see H. Neumann, Arch. tech. Messen, J 

 64-1, May, 1934. 



