MAGNETIC SHIELDING OF TRANSFORMERS 



435 



currents, iei (Fig. 1). Curve B shows that, after the effect due to the 

 open ends of the cylinder is eliminated by means of covers, the shielding 

 efficiency is approximately proportional to the logarithm of the 

 frequency. 



Although copper has a very low shielding efficiency at low frequencies 

 when used alone, tests show that under certain conditions it is very 

 effective when used in conjunction with permalloy. This is illustrated 

 by a comparison between the curves C and E of Fig. 1 1 . The copper 

 cylinder is similar to the one referred to above except that the thickness 

 of the wall is only 1/32". The permalloy cylinders are those for which 

 the shielding efficiency is given by curve C of the same figure. 



Another striking example of the use of copper in conjunction with 

 permalloy is furnished by Fig. 14. The curve A gives the observed 



500 1000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 14 — Observed effect of copper between core and permalloy shield, 



shielding efficiency of a permalloy box when there is an airspace 

 between the core and the box of 1/32" {d, = 1/32"). If 1/32" thick 

 copper plates are inserted between the core and the box (see Fig. 14), 

 there is a great improvement in the shielding efficiency as a comparison 

 between the curves A and B shows. This improvement is 10 db at 

 50 cycles although the effect of the copper plates alone would be of 

 the order of one db, as is evident from the curves on Fig. 13. At 

 higher frequencies the improvement is still better. It is approximately 

 15 db between 100 and 4000 cycles. Approximately the same results 



