PULP INSULATION FOR TELEPHONE CABLES 15 



wire most suitable for the space it was to occupy in cable form. There 

 is some latitude here in the distribution of a given amount of fibre but 

 taking into account both the mechanical and electrical requirements, 

 the diameter for the insulated conductor finally selected as the most 

 satisfactory for the series of standard cables of 24 A.W.G. was 0.041 

 inch and for 26 A.W.G. — 0.033 inch, and the aim in manufacture is 

 to produce an insulation as uniformly close to these dimensions as 

 possible. These diameters are measured by a volume displacement 

 method. Short samples, as representative as possible of the wire 

 under consideration, are inserted for a given distance into a small 

 bore tube of mercury and the displacement noted. The gauge is 

 calibrated so that mean diameters are read directly on the scale. 



The above specific sizes of pulp insulated conductors apply only to 

 cables designed for a particular set of characteristics. As in the case 

 of ribbon paper cables, the amount of insulation for a given gauge of 

 conductor may be varied within reasonable limits, so as to produce 

 cables of other characteristics. 



Electrical Characteristics 



It was reasoned that pulp insulated cables would probably be 

 inherently higher in mutual capacitance than similar sizes of paper 

 ribbon cables because, considering the insulated wire itself, in the 

 case of helically applied strip insulation the volume of air beneath the 

 paper is about equal to the volume of the paper itself, while for pulp 

 insulation there is very little air space between the insulation and the 

 wire. This fundamental difference could be somewhat compensated 

 for, however, by the introduction of more air into the spaces between 

 the fibres of the pulp insulating medium than is found in the paper 

 ribbon itself, but it was not expected that it would entirely neutralize 

 the effect of lack of air space next to the wire. It was appreciated, 

 however, that the aim should be to get as low density insulation as 

 possible still consistent with obtaining a continuous, flexible and 

 strong covering on the wire and emphasis was placed on this phase 

 from the start of the development. 



The very first experimental cables manufactured compared favorably 

 in mutual capacitance with corresponding sizes of strip paper cables. 

 The wire was insulated in a manner resulting in an apparently well 

 centered, round insulation and the covering was low in weight of 

 fibre per unit volume. The insulation after being formed around the 

 wire was quickly dried in a hot tube resulting in less shrinkage and 

 tightening down around the wire while the moisture was being driven 

 out than if slowly dried. The insulation was unsatisfactory, however, 



