Till- i.o.inr.n scnM.iKixr. rr.i.v.cK.irii ciiur m6 



wh«Tr R. = copper resistance per unit leni;th 



K, =e<ldy current resistance per unit liiii;ih 

 R, =sea return resistance per unit liiiutli 

 /?*= hysteresis resistance per unit Icngtli 



The copper resistance Re is that determined !)>• a direct current 

 measurement of the loaded conductor since the resistance of the 

 loading tape is so hi^h and its length is so great that the current 

 Howing longitudinally through it may be safely neglected. 



The eddy current resistance Rt is given approximately by the 

 fornuiia, 



R,= 



p(d-iy 



where / is the lliickness or diameter of the loatling ta[)e or wire, d, 

 the outside diameter of the loaded conductor, /, the fref|uenc\-, p, 

 the resistivity of the loading material, fi, its magnetic permeability 

 and HI. a constant which depends on the form of the loading material 

 and is in general greater for tape than for wire loading. Although 

 it is possible to compute a %alue of in, the \'alue found in practice is 

 <ilwa>s larger than the theoretical \alue which is necessarily based 

 on simple assumptions and does not take into account such a factor 

 as variation of permeability through the cross-section or length of the 

 loading material. Accordingly it is necessary to determine m ex- 

 fierimentally for any particular type of loaded contluctor. 



The sea-return resistance may be safely neglected in the com- 

 putation of slow speed non-loaded cables, but it is a factor of great 

 consequence in the behavior of a loaded cable. By sea-return re- 

 sistance is meant the resistance of the return circuit including the 

 eflFect of the armor wire and sea water surrounding the core of the 

 cable. Although the exact calculation ' of this resistance factor is 

 too complex to be discussed here, the need for taking it into account 

 may be quite simply explained. Since the cable has a ground return, 

 current must flow outside the core in the same amount as in the con- 

 ductor. The distribution of the return current is, howe\er, dependent 

 on the structure of the cable as well as on the frequencies in\T)lved 

 in signalling. If a direct current is sent through a long cable with the 

 earth as return conductor the return current spreads out through sucl^ 

 a great volume of earth and sea water that the resistance of the 

 return path is negligible. On the other hand if an alternating current 

 is sent through the cable the return current tends to concentrate 



'See Carson and Gilbert, Jour. Franklin Inst., Vol. 192, p. 705, 1921: FJeclrician, 

 Vol. 88, p. 499, 1922; B. S. T. J., Vol. I, No. 1, p. 88. 



