254 BELL SYSTEM TECHNICAL JOURNAL 



However, for the following reasons, 75 per cent and 87 per cent at 

 100 degrees Fahrenheit were selected as offering definite promise of 

 giving consistent results. 



The main considerations in the choice of humidity conditions were, 

 first, that the humidity should be high enough so that insulation 

 resistance measurements of sufficient accuracy could be made using a 

 band of threads as described above and a commercial galvanometer 

 of reasonably high sensitivity; second, that the humidity be lower 

 than that at which polarization effects would introduce serious error. 

 The humidities chosen are within the range found suitable for cotton 

 and silk under these limitations. Furthermore, these conditions are 

 readily obtainable by the use either of saturated salt solutions or 

 sulphuric acid solutions, thereby increasing the flexibility of the test. 

 The temperature of 100 degrees Fahrenheit was chosen arbitrarily as 

 one which could be maintained in the shop at any time of the year 

 without artificial cooling. 



Application to Apparatus 



From an economic standpoint the most important conclusion to be 

 drawn from the graphs is that cotton can be improved by washing 

 to such an extent that it becomes a better insulator than the ordinary 

 commercial insulating silk in general use. Since the cost of washing 

 silk and cotton is nominal, usually less than 5 per cent of the cost 

 of the material, the engineer given purified textiles may either take 

 advantage of marked improvement in quality of electrical character- 

 istics by using washed silk, or may substitute washed cotton for silk 

 and realize substantial economies without degrading the product. 

 As an example of how this applies to Bell System apparatus, central 

 office distributing frame wire with annual requirements of more than 

 400 million conductor feet is now insulated with two coverings of 

 silk where three were formerly required. The resultant wire is superior 

 electrically to the old wire and the annual saving in silk amounts to 

 about 70,000 pounds. 



As another example, telephone cords of various types have been 

 reduced substantially in cost with no impairment in quality by 

 substituting two washed cotton braids for the cotton and silk braids 

 formerly used. Altogether, various types of textile insulated wire 

 aggregating annual requirements in excess of two billion conductor 

 feet have either been changed to employ washed textile insulation or 

 are scheduled for change as soon as possible because of corresponding 

 economies in manufacturing cost or improvement in electrical prop- 

 erties. 



