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BELL SYSTEM TECHNICAL JOURNAL 



which will oppose the flux following the low reluctance path in the 

 walls of the case, or what is the same thing, decrease the effective 

 permeability of this path and will, in that way, decrease the shielding 

 efficiency. In a case made of high permeability material the latter 

 eddy currents, i\?, obviously should be reduced as much as possible. 



Fig. 1 — Eddy currents produced in a shield by an alternating magnetic field. 



If the resistivity of the material is increased, both sets of eddy currents 

 will be decreased. If, however, the material is laminated with the 

 sheets parallel to the wall of the case as indicated by section A'-A" 

 in Fig. 1, the undesired eddy currents will be reduced without affecting 

 those which are beneficial. 



The relative effectiveness of the low reluctance path and eddy 

 currents in securing good shielding efficiency against magnetic fields 

 depends mainly upon the frequency of the magnetic field. As a 

 general rule, we can say that at low frequencies, the effect of the low 

 reluctance path predominates, while the shielding effect of the eddy 

 currents, ie\, increases as the frequency increases. 



This way of looking at the effect of permeability and conductivity 

 in a magnetic shield is intended to be purely descriptive and probably 

 would not be practicable for a mathematical treatment. It is, 

 however, very suggestive to the design engineer. 



As an illustration of the way in which the mechanical construction 

 of the case may affect the shielding efficiency it is at once clear that 



