Sec. 1-2] 



MECHANICAL IS PUT TRANSDUCERS 



55 



UP 9) 



Fig. (1-2)36. Derivative of mutual inductance between two 

 single-turn coaxial coils versus displacement between them. 



where fi is the magnetic permeability in Mks units, a the conductivity 

 of the plate material, and /the current frequency. 1 



The eddy-current transducer is in general used for small displace- 

 ments, i.e., for a total range from = j = 10 _4 to = [ = 10 _2 in. Displacements 

 of 10~ 5 in. can still be detected. A small repulsive force arises between 

 the primary coil and the metal plate. For a perfectly conducting 

 plate of an area large compared to that of the coil, the force is 



F -- -n 2 ,i l -^~ x 10- 11 dyne 



dx 



1 See R. W. P. King, H. R. Mimno, and A. H. Wing, "Transmission Lines, 

 Antennas, and Wave Guides," McGraw-Hill Book Company, Inc., New York, 

 1945. 



