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BELL SYSTEM TECHNICAL JOURNAL 



Introduction 

 (PEN wires were almost universally used for the transmission of 

 speech in the days when telephony was young but gradually as 

 the need arose the art of cable making was evolved. Today, except in 

 the rural districts, the open-wire lines have been almost entirely re- 

 placed by aerial or underground cable. The conductors in the early 

 metal covered cables were insulated with one or two servings of cotton 

 but in the late eighties Bell System engineers developed a spirally 

 wrapped paper insulation so much better electrically and lower in 

 cost that it was shortly adopted as the standard insulation for telephone 

 cables by the growing industry.* Now after some forty years of 

 service this type of insulation is being rapidly displaced for inter- 

 office and subscriber loop cables by a pulp insulation applied directly 

 to the conductor by a process which brings the paper mill into the 

 cable plant and combines the paper making and insulating operations 

 into one process with the elimination of a number of costly intermediate 

 steps. In addition, this process makes possible the use of a less 

 expensive material as an insulating medium. 



In order to establish a background for the logical consideration of 

 the pulp insulation development it is desirable to cover briefly the 

 materials, equipment and methods that have gradually been developed 

 for the rapid application and economic use of paper ribbon insulation 

 and indicate the limitations involved. 



For many years the standard paper in this country for insulating 

 conductors for lead sheathed telephone cables was made from a stock 

 composed of all old rope or old rope and a small admixture of cotton, 

 the fibres of the rope being chiefly manila from the plant Musa Textilis 

 or hemp from the plant Cannabis Sativa. Papers of such com- 

 position, slit into long narrow strips, were applied helically around the 

 wire to form the insulated conductor. Experience had proved them 

 to be highly suitable as an insulating medium, both as to structural 

 permanency and electrical characteristics and to be sufficiently flexible 

 and strong mechanically to admit of ready application to the conductor 

 in manufacture and to withstand subsequent handling in service. 

 With the mounting demand for insulating papers, however, came the 

 urge for the finding of a suitable less expensive fibre and the year 1920 

 saw the adoption, for the larger sizes of paper only, of a formula com- 

 posed of about 40 per cent chemical wood pulp and the remainder 

 rope stock. This wood fibre is of the spruce or other coniferous tree 

 species prepared by the sulphate or "Kraft" process. It is required 

 to have a high cellulose content and to be as free from water soluble 

 salts as the best manufacturing practice will permit. Extensive tests 



