PULP INSULATION FOR TELEPHONE CABLES 13 



quality performance. To displace the old standard, pulp must meet 

 this competition and give a greater return for the money invested. 



Telephone cable circuits are normally subjected to only a low di- 

 electric stress which permits their being placed in close proximity to 

 one another and the primary requirement of the insulation is that it 

 be distributed in a thin layer of uniform application, with the wire 

 well centered so that each conductor when packed into a cable is com- 

 pletely insulated from its neighbors throughout its length. The mean 

 radial thickness of the pulp insulation for the 26 A.W.G. wire which is 

 in common use is less than one hundredth of an inch and for 24 A.W.G. 

 which is the next larger size of wire usually used for telephone cables 

 this value is about 0.011 inch. The pulp is prepared and applied to 

 the conductor in such a manner that the fibres pack together to form 

 a cover with sufficient strength and elasticity to withstand the handling 

 the insulated wire must receive and yet be as light as possible in weight 

 per unit volume in order to obtain the best electrical characteristics. 



At the time this development was started 24 A.W.G. wire was the 

 finest regularly used and the earlier pulp cables were confined to this 

 gauge. Pulp insulated wire is structurally more like textile insulated 

 wire than air-spaced paper ribbon insulated wire. The insulation is 

 firm with no appreciable air gap between it and the wire, and bundles 

 of wires nestle together differently when grouped into a given space. 

 Furthermore, it was found that when pairs of conductors were stranded 

 together in the usual manner of concentric layers each reversed in direc- 

 tion, the unit thus formed was considerably less flexible than the pres- 

 ent standard construction. This is apparently caused by the greater 

 frictional resistance between layers sliding over each other as the cable 

 is bent, thus causing sharp kinks for even moderate bends. While 

 this feature is less pronounced for small cables, it is, of course, ob- 

 jectionable and an improvement in the handling qualities is effected by 

 stranding several layers in the same direction rather than employing 

 the single reverse layer construction. For the large size cable, a 

 design whereby the pairs are first grouped into units of fifty-one or 

 one hundred and one, all the pairs in these units being stranded in 

 the same direction and the units then stranded together into a cable, 

 gives a construction which seems to offer the most satisfactory arrange- 

 ment. Thus, for example, a 1212 pair cable is made up of 12 units of 

 101 pairs each, arranged with four units in the center and eight in a 

 surrounding layer, and an 1818 pair cable is laid up with two units in 

 the center surrounded by six units in the first layer and ten units in 

 the second layer. Fig. 7 shows a short section of 1818 pair 26 A.W.G. 

 cable with the units separated. One might expect these rather large 



