16 



BELL SYSTEM TECHNICAL JOURNAL 



from a continuity and tensile strength standpoint and could not be 

 considered as suitable for commercial cable. 



Special effort was then directed towards producing an insulation 

 better mechanically, with the result that the early commercial cables 

 were satisfactory in this regard but were from 20 to 25 per cent higher 

 in mutual capacitance than the standard ribbon paper cable. This 

 impairment in transmission efficiency was considered prohibitive for 

 cables to be used for interoffice trunks and was definitely objectionable 

 for any class of service. However, the indicated savings in cable 

 first cost warranted continuing the development and over a period of 

 years marked progress has been made in reducing this excess of 

 capacitance and yet retaining an insulation sufficiently strong and 



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Fig. 8 — Curve showing improvement in mutual capacitance since early 1928. 

 Ordinates are percentages by which capacitance of 24 A.W.G. pulp insulated cables 

 exceeds that of ribbon. 



flexible to handle reasonably satisfactorily in the fabricating of the 

 cable and installing it in the plant. The attached chart. Fig. 8, shows 

 graphically the progress that has been made in reducing the mutual 

 capacitance of 24 A.W.G. cable since early in the year 1928. Although 

 a substantial improvement has been made in lowering the mutual 

 capacitance to within less than 4 per cent of the corresponding ribbon 

 paper cable, a further reduction would have considerable value 

 warranting more effort in that direction. For 26 A.W.G. cable the 

 excess in capacitance is even less than for 24 A.W.G. and furthermore 

 it is not so objectionable from a transmission standpoint as in the case 

 of the larger gauge. 



The principal factors which have brought about this reduction in 

 capacitance are improvements in the treatment of the pulp itself, 

 refinements in machinery operation to permit the use of a lower 



