MAGNETIC SHIELDING OF TRANSFORMERS 433 



of a permalloy cylinder which has a permeability of approximately 

 5000. This cylinder is 4.5" high, inside diameter 2.5", and thickness 

 of wall equal to .07". By increasing the thickness of the wall the 

 efficiency would be only slightly increased. This is evident from a 

 study of Fig. 4. However, by placing a second cylinder over "JB" a 

 substantial improvement is obtained (Curve C). Still greater shielding 

 efficiency is obtained by placing a third cylinder over the former two 

 as shown by curve D. The dimensions of the second and third cylin- 

 ders are such that the ratios between the outside and inside radii of 

 the three cylinders and of the air-gaps between them are approximately 

 in geometric progression. The height of the second and third cylinders 

 is also 4.5" and the effective permeability at low frequencies and field 

 strengths approximately 5000. 



Since the effective permeability of the permalloy used in the above 

 shields is close to 5000 we can compare these data with the theoretical 

 curves of Fig. 4, which were calculated with the permeability assumed 

 equal to 5000. This comparison shows that the theoretical analysis 

 of the shielding of infinite cylinders against steady magnetic fields 

 may be applied to the shielding of transformers. Due to such factors 

 as a magnetic core inside the shield, eddy current shielding, end 

 effects etc. only an approximate check can be expected. At 50 cycles 

 per second the measured values for one cylinder and for two and three 

 concentric cylinders are 40, 64, and 80 db respectively. Corresponding 

 calculated values as given by Fig. 4 are 41.5, 66, and 89 db respectively. 



It is evident from the effect of the copper cylinder between two 

 permalloy cylinders as shown by curve E (Fig. 11) that three per- 

 malloy cylinders with copper cylinders between the inner and middle 

 and between the middle and outer will give a shielding efficiency of 

 the order of 100 db (voltage ratio EejEi = 10^) from 50 to 4000 cycles. 



Effect of Covers 

 The information given in Fig. 12 shows the importance of covers. 

 This figure gives the shielding efficiency of a cylinder which consists 

 of two layers of .014" permalloy sheet. Curve A gives the shielding 

 efficiency without any covers and curve B shows the advantage of 

 adding covers which overlap 1/2" and consist of two layers of .014" 

 permalloy sheet. The two curves C give the shielding efficiency of 

 the same cylinder provided with flat plate covers, Ci representing 

 covers .014" thick and C2 covers .028" thick. The relative size of 

 the coil and cylinder is such that there is a clearance of approximately 

 1/16" between the core and the magnetic shield. It is evident that 

 in this particular instance the covers are very important. Regarding 



