518 BELL SYSTEM TECHNICAL JOURNAL 



a disturbing voltage is reduced. Necessary impedance data are given 

 for numerous iron-surrounded cable constructions and working charts 

 are supplied for the convenient determination of the shielding obtain- 

 able with commercially available steel tape armored cables. 



On the basis of data presented in this paper, prediction of the 

 shielding to be obtained from steel tape armored cable sheaths or 

 those inclosed in iron pipes is concluded to be both feasible and 

 practical. With internal impedances measurable on short length 

 samples of a chosen construction, the accuracy of prediction is limited 

 principally by the precision to which the disturbing field and the 

 grounding resistances of the cable sheath may be determined. Either 

 of the constructions discussed is capable of effecting a high order of 

 shielding against low frequency induction and practically complete 

 protection from harmonic disturbances. Field observations on in- 

 stalled cables, both tape armored and in pipe conduit, have verified 

 the computational methods presented. 



Propagation of High- Frequency Currents in Ground Return Circuits.^ 

 W. H. Wise. The electric field parallel to a ground return circuit is 

 calculated without assuming that the frequency is so low that polariza- 

 tion currents in the ground may be neglected. It is found that the 

 polarization currents may be included by replacing the r in Carson's 

 well-known formulas by r-\jli{e — l)/2cXa-. The problem to be solved 

 is that of calculating the electric field parallel to an alternating current 

 flowing in a straight, infinitely long wire placed above and parallel to 

 a plane homogeneous earth. Carson's derivation of this field is based 

 on three restricting assumptions: (1) The ground permeability is 

 unity; (2) the wave is propagated with the velocity of light and without 

 attenuation; (3) the frequency is so low that polarization currents may 

 be neglected. The first of these restrictions is usually of no conse- 

 quence and the formula would be quite complicated if the permeability 

 were not made unity. As pointed out in a later paper by Carson, the 

 second restriction amounts merely to assuming reasonably efficient 

 transmission. The effect of the third restriction begins to be notice- 

 able at about 60 kilocycles. The object of the present paper is the 

 removal of the third restriction. 



Acoustical Requirements for Wide-Range Reproduction of Sound. ^ 

 S. K. Wolf. The extension of the frequency and volume ranges in 

 recording and reproducing sound has aroused a greater and more critical 



5 Proc. I. R. E., April, 1934. 

 « Jour. S. M. P. E., April, 1934. 



