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 /i 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, 



(20) 



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



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 

 I 100 



j 90 



'-■ 80 



) 

 : 70 



1.00 

 1.30 



1.04 

 1.50 



1.08 

 1.70 



1.12 

 1.90 



1.16 1.20 



2.10 2.30 



'Rr 



1.24 

 2.50 



1.28 

 2.70 



1.32 

 2.90 



1.36 SCALE I 

 3.10 SCALE 2 



'Vr. 



Fig. 4 — Shielding efficiency of one, two, and three concentric c} linders 

 (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) /j. = 47rl0~^ henries/cm. 

 In general // = 47rjuolO^'' where mo is the intrinsic permeability referred to empty 

 space as unity. 



t Arnold and Elmen, " Permallo\-," Journal Franklin Institute, May, 1923, 

 pp. 621-632. 



