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BELL SYSTEM TECHNICAL JOURNAL 



the shielding efficiency very little. That is, practically the same 

 results will be obtained with a silicon steel core, having a permeability 

 of 400 as with a permalloy core having a permeability of 5000. On 

 the other hand if the silicon steel core is replaced by an air core the 

 change in the shielding efficiency will be of such an order as is indicated 

 by Fig. 7. The reason for the small effect of changing from a per- 

 malloy core to a silicon steel core as compared to the changing from 

 a silicon steel core to an air core is, of course, due to the fact that in 

 the first case there is a decrease in permeability of 12.5 : 1 while in 

 the second case the corresponding reduction in permeability is 400 : 1. 



High Efficiency Shields 



A shielding efficiency of from 20 to 50 db is, for many purposes, 

 sufficient in connection with the shielding of transformers. Occasions 

 arise, however, when a shielding efficiency much greater is desired. 



120 



^^^^^^2^^^t^:2lo^^ 



500 1000 

 FREQUENCY IN CYCLES PER SECOND 



Fig. 11 — Observed shielding efficiency of various high-efficiency shields. 



To accomplish this by means of a simple case, magnetic material 

 having a permeability much greater than is now readily available 

 would be needed. The curve B of Fig. 11 gives the shielding efficiency 



