MAGNETIC SHIELDING OF TRANSFORMERS 



425 



The curves of Fig. 4 show the shielding efificiency of one cylinder 

 and of two and three concentric cylinders with air space between. 

 The shielding efficiency is given as a function of the ratio between the 

 outside and inside radii of the shield, that is, i?„/ri, where Rn is the out- 

 side radius of the outside cylinder and n is the inside radius of the 

 inside cylinder. These curves show the relative shielding efficiencies 

 of 1, 2 and 3 cylinders and give numerical values for ^l equal to 5000. 

 The relative dimensions of the cylinders are such that the ratios 

 between the inside and the outside radii of the cylinders and of the 

 air spaces between the cylinders are in geometric progression. These 

 curves show that when a very high shielding efficiency is desired it is 

 not only advantageous but necessary to use two or more cylinders. 

 Thus with one cylinder the maximum shielding efficiency that can be 

 considered practical when /x = 5000, is approximately 50 db. The 

 maximum theoretical limit for one, two and three cylinders is 62 db, 

 124 db and 186 db respectively when n = 5000. 



200 400 600 800 1000 1200 1400 1600 1800 2000 



SEPARATION IN MILS (S) 



Fig. 5. — Shielding efificiency of two concentric cylinders versus the air-gap between 

 them. Thickness of the wall of each cylinder kept constant. Zero frequency. 



In Fig. 5 is shown the shielding efficiency of two concentric cylinders 

 vs. the thickness of the air space between the cylinders. The thick- 

 nesses of the walls of the two cylinders are equal. Two thicknesses 

 of the walls of the cylinders have been considered, namely, .014" and 

 .042". An interesting fact is brought out by comparing the curves of 

 Fig. 5 with curve "5" of Fig. 4. For example, when the air space 

 is .042", the upper curve of Fig. 5 gives a shielding efficiency of two 

 cylinders with an air space between them, the thickness of which is 

 the same as that of the cylinders. The ratio between the outside 



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