12 BELL SYSTEM TECHNICAL JOURNAL 



General Physical Characteristics 

 Pulp insulation is a new product and has certain inherent charac- 

 teristics. These characteristics may be modified somewhat by choice 

 of materials and methods of manufacture but they cannot be entirely 

 controlled. A brief survey of these characteristics may be of interest 

 to give a better picture of the possibilities and limitations of the 

 product. This survey co\ers only 24 and 26 A.W.G. wire as these are 

 the sizes which have been run almost exclusively to date. It should 

 be noted, however, that wires ranging in a size from 19 to 28 A.W.G. 

 have been covered successfully. 



Some of the physical characteristics of the insulation are shown 

 below in tabular form giving the possible range of values obtainable. 

 They are controlled by the beating of the pulp, the amount of pulp 

 fed to the machine, the dryness of the sheet in the polishers and the 

 speed of drying. 



Diameter of Insulated Wire, Inches 0.030 to 0.050 for 24 A. W. G. 



0.026 to 0.040 for 26 A. W. G. 

 Weight of Dry Pulp, Grams per Foot 0.045 to 0.12 for 24 A. W. G. 



0.040 to 0.095 for 26 A. W. G. 

 Density — Ratio of Fibre to Total Volume. . 35% to 55% — Independent of 



Gauge. 



The tensile strength and flexibility of the insulation can be varied 

 through rather wide limits by different treatments during manu- 

 facture. The elongation is quite comparable to that of ordinary 

 paper and is not susceptible of much variation. The insulation is 

 made sufficiently strong and flexible to withstand the various opera- 

 tions incident to cable fabrication and subsequent handling yet not so 

 tough that it cannot be readily removed from the wire at the point of 

 splicing. 



The surface of the insulation has a rather rough blotting paper 

 appearance, though some variation is possible by changes in the 

 beating. The cross-section is circular with the conductor in the 

 center in the ideal case, but because of limitations imposed by practical 

 operating considerations there is a tendency toward some eccentricity 

 and flattening of the insulation. 



Pulp Insulated Cables 



Design 



The smallest wires now used in commercial telephone cables are 



24 and 26 A.W.G. and it has been found that pulp is particularly 



suitable for insulating such fine wires. Here it is in direct competition 



with non-wood content strip paper that has been giving satisfactory 



