244 BELL SYSTEM TECHNICAL JOURNAL 



insulation of these cables has been satisfactory, but breakdowns have 

 occurred which could not be attributed to faulty operating conditions 

 or to manufacturing defects. A study of this subject showed that 

 it was possible under certain conditions to get discolored or faded 

 spots in the insulation and corresponding corroded or pitted spots in 

 the tinned copper conductors. It was also observed that the textile 

 insulation at such spots showed a strong concentration of water 

 soluble salts. Also, cables in which such conditions occurred measured 

 relatively low in insulation resistance with the current leakage con- 

 centrated at these points. These observations led to the conclusion 

 that silk and cotton would be decidedly improved as insulating 

 materials if they were made less susceptible to deterioration under 

 telephone service conditions. 



Aside from the consideration of improving silk and cotton to assure 

 greater insulation stability, considerable thought has been given to 

 the possibility of improving the insulating characteristics of cotton 

 to such a degree that it could be substituted for the more expensive 

 silk. The importance of this work with respect to its bearing on the 

 cost of telephone service can be better appreciated from the fact that 

 about 2,000 pounds of silk are required daily to provide for the growth 

 of the country's telephone requirements, which if replaced with cotton 

 would reduce raw material costs by a very substantial sum. 



The desirability of reducing the quantity of silk required in the 

 telephone plant does not arise entirely from this phase of the economic 

 question. The problem of supply and demand has at times entered 

 into the matter. For example, shortly after the close of the World 

 War the supply of insulating silk was limited and the price prohibitively 

 high. Substantially the same condition arose a few years later, 

 which leads to the conclusion that silk is inherently much more 

 subject to violent fluctuations in available supply and cost than 

 cotton. Therefore, with demands for telephone equipment rapidly 

 increasing, we have decidedly greater assurance of an adequate supply 

 of insulating material at reasonable cost if cotton instead of silk is used. 



Purification Process 



With the foregoing as an introduction to indicate the economic 

 advantages to be gained by improving the electrical characteristics of 

 cotton and silk, the following is intended to show what has been 

 accomplished by the commercial application to silk and cotton thread 

 of the processes referred to by Messrs. Williams and Murphy for 

 removal of objectionable impurities. 



Since such impurities are soluble in water, it will be inferred that 



