A NEW TYPE OF UNDERGROUND TELEPHONE WIRE 447 



long period under the severe moisture conditions to wliich it would 

 be subjected in the ground. 



Experience with the cable burial ])roblem had led to the development 

 of a cable laying plow, the neat operation of which in plowing cable 

 into the ground at depths ranging up to thirty inches without trenching 

 or backfilling in the ordinary sense has been described elsewhere.'* 

 The adaptation of this method to the burial of wire at an appropriate 

 depth required that it be simplified so that it would be less expensive, 

 and involved such considerations as the very much smaller size and 

 tensile strength of the wire, its greater vulnerability to mechanical 

 injury, the need for reducing and simplifying traction requirements, 

 and the like. 



On the point of serviceability, it remained for our research chemists 

 first to develop a rubber compound that could be relied on to maintain 

 suitable insulating properties over a period of years under the severe 

 moisture conditions under ground. 



With these fundamentals in hand, the development engineers under- 

 took to study the mechanical and electrical problems involved and 

 design a wire that would have appropriate transmission and handling 

 characteristics. In addition, they had to devise methods of splicing 

 the wire; to adapt plow equipment to its installation; to develop 

 loading arrangements for use on the longer lengths; to study methods 

 of tracing the path of the wire and locating faults, etc. In short, the 

 job was to develop buried wire as a practicable plant instrumentality. 



The Insulated Wire 



The wire as actually developed employs 17-gauge annealed and tin- 

 coated copper conductors, insulated in parallel twin construction with 

 a special rubber compound designed to withstand long water immersion 

 without serious deterioration of the electrical properties. The wire 

 is adapted to a continuous process of extrusion and vulcanization.* 



While the insulation of this wire is very resistant to water absorption, 

 it is, in common with most high grade rubber insulating compounds, 

 quite sensitive to sunlight so that it must be carefully guarded from 

 any unnecessary exposure to direct rays of the sun and from any 

 extended exposure to indirect rays. 



Splicing 



One of the principal problems in using a wire of this kind is that of 

 splicing, as it is quite obvious that the splice must be essentially as 



2 C. W. Nystrom, Telephony, Volume 98 (June 21, 1930). 



^ S. E. Brillhart, Mechanical Engineering, Volume 54, pages 405-9 (1932). 



