54 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



25 



20 



fr 



sr 



*H 



10 



0.2 



0.4 0.6 



0.8 1.0 

 X/A 



1.2 



1 .4 



Fig. (1-2)35. Mutual inductance between two single-turn coaxial 

 coils versus displacement between them. 



coil. Practical values of sensitivity are ofthe order of 10 mV/0.001 in. 

 If the metal plate cannot be considered a perfect conductor, Eq. (1) 

 is to be modified by the (empirical) equation 



1 



0.0066(5 



12 



where d is the penetration depth ofthe eddy current, i.e., the depth 

 where the current is 1/e of that on the surface (the plate being thick 

 enough so that the current on the back surface is essentially zero). 

 The penetration depth is computed from 



6 = 



yfn^ /W 



