Purified Textile Insulation for Telephone Central 

 Office Wiring 



By H. H. GLENN and E. B. WOOD 



This paper outlines methods by which silk and cotton insulation can be 

 purified and improved. It gives the results of tests on the insulation 

 properties of these materials before and after purification and explains the 

 testing procedures. One of the findings is that the purified cotton may be 

 substituted for ordinary' commercial silk. 



IN a contemporary paper, The Predommating Influence of Moisture 

 and Electrolytes upon Textiles as Insulators, Messrs. Williams and 

 Murphy have shown that the electrical properties of textiles are 

 closely associated with the moisture content and impurities in the 

 textiles. In particular, water-soluble salts become ionically con- 

 ducting in the presence of moisture and the ions migrate along the 

 paths of initially low resistance to the electrodes with which they 

 react chemically, causing serious corrosion. The resulting corrosion 

 products, themselves electrolytes, accelerate the process of current 

 transfer and may easily lead to a complete failure of the insulating 

 textile at the point of greatest concentration. Conversely, if the 

 impurities are removed, the insulating properties of the textile are 

 improved initially and, furthermore, are not subject to cumulative 

 deterioriation due to concentration of conducting salts and electrolytic 

 corrosion products at the weaker points. It is the purpose of this 

 paper to show how these principles are borne out by field observations 

 and laboratory tests, and to show in a general way the extent to which 

 the insulating properties of silk and cotton can be improved com- 

 mercially with particular application to telephone central office wiring. 

 Since the early days of telephone development work, silk and 

 cotton have been the standard insulating materials for wire insulation 

 in telephone central office apparatus, supplemented in later years 

 by enamel insulation. Relatively low voltages have always been 

 used in the telephone plant, 24 to 48 volts being the usual voltages 

 which are carried continuously in cables, while intermittent a.c. and 

 d.c. potentials generally do not exceed 100 to 150 volts. Therefore 

 it has been generally accepted that telephone cables once installed 

 and properly protected from accidental high voltages, could be 

 depended upon to have a substantially indefinite life. In general the 



1 Presented at the Winter Convention of the A. I. E. E., New York, N. Y., Jan. 

 28-Feb. 1, 1919; Abridgment published in A. I. E. E. Journal, February. 1929* 

 p. 146. 



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