430 



BELL SYSTEM TECHNICAL JOURNAL 



A comparison between the curve for permalloy and that for silicon 

 steel on Fig. 8 gives a striking example of the advantage of using 

 permalloy instead of steel. 



Effect of Air -Gap Between Core and Shield 



It has been pointed out previously that the magnetic core decreases 

 the reluctance through the interior of the case and in that way de- 

 creases the shielding efficiency. When the size of the core is such as 

 to almost fill the case, that is, when the air-gap between the core and 

 the case is small, this effect becomes large. In Fig. 9 are given some 



IN INCHES 

 )63 0.094 0.125 0.156 



y25 

 O 



:^io 



NO. 3 NO. 4 



CYLINDER 



Fig. 9— Observed effect of air-gap between core and shield. Cylindrical shield. 



Frequency 70 cycles. 



data in this connection for cylindrical shields. Six permalloy cylinders 

 were used in this illustration. They are so constructed as to fit over 

 each other, the smallest fitting snugly over the transformer core 

 ("d" = 0). Each cylinder consists of two layers of .014" thick 

 permalloy sheet. The cylinders have been numbered 1 to 6 from 

 the smallest to the largest, respectively. Curve "A " of Fig. 9 shows 

 the shielding efficiency with a non-magnetic core and curve "B" 

 gives the corresponding information with the permalloy core having a 

 permeability of approximately 5000 at 70 cycles and low field strengths 

 which are the conditions under which the measurements were made. 



