MAGNETIC SHIELDING OF TRANSFORMERS 



431 



These curves have been drawn discontinuously because the shielding 

 efficiency is a function not only of "J" but of other factors such as 

 permeability, size of the cylinders, etc. Curve "C" on the other 

 hand is primarily a function of "d'' and has, therefore, been drawn 

 continuously. This curve gives the difference between the shielding 

 efficiency with an air core and with a permalloy core and shows the 

 advantage of increasing the air-gap between the core and the cylinder. 

 Thus, for example, in this particular case approximately 8 db better 

 shielding efficiency is obtained with an air-gap of 1/16" than if there 

 is no air-gap. 



0.028 



d IN INCHES 

 556 0.084 



BOX 



Fig. 10 — Observed effect of air-gap between core and shield. Rectangular shield. 



Frequency 70 cycles. 



Similar curves are given in Fig. 10 for rectangular boxes of the same 

 height and same wall thickness as the cylinders. The measurements 

 were made at 70 cycles per second. Due to the larger contact area 

 between the core and the shield the effect of the air-gap is much 

 greater here than with cylinders. 



As the efifective permeability of the walls of the shield increases 

 the effect of an air-gap increases, other things being equal. In general 

 it may also be said that as the shielding efficiency increases (due to 

 increased thickness of the walls, for example) the effect of an air-gap 

 increases. 



If there is an appreciable air-gap between the core and the shield 

 a large variation in the effective permeability of the core will affect 



