424 



BELL SYSTEM TECHNICAL JOURNAL 



In (18) and (19) / is the frequency in cycles, p is the radius in cms., 

 g is the intrinsic conductance in mhos/cm., and n is the intrinsic 

 inductance in henries/cm.* 



A in (16) is the sum of the attenuation losses in the successive 

 shells. For any one shell 



A = 8.686a/ db, 

 where a = ^Tgfxf and / is the radial thickness in cms. 



(20) 



Calculated Curves 

 By means of the equations (2) to (11) the shielding efficiency of 

 various types of cylindrical shields has been calculated. The permea- 



150 

 140 

 130 

 120 



no 

 ) 

 1 



j 100 



) 



J 90 



'' 80 



) 



r 70 



: 60 

 j 



, 50 



] 40 



\ 30 



20 



10 







Fig._4 — Shielding efficiency of one, two, and three concentric cyhnders 

 (for zero frequency). 



bility considered is 5000 which is readily obtainable at low frequencies 

 and field strengths by means of permalloy .f The calculations are 

 given in the form of curves in Figs. 4, 5 and 6. 



* Thus in empty space (or dielectrics, approximately) n — 4x10"' henries/cm. 

 In general yu = 47r;uolO~' where mo is the intrinsic permeability referred to empty 

 space as unity. 



t Arnold and Elmen, "Permallo}'," Journal Frankliii Institute, May, 1923, 

 pp. 621-632. 



