610 BELL SYSTEM TECHNICAL JOURNAL 



with the textiles. The presence of water-soluble impurities in textile 

 insulations is therefore very important and their thorough removal 

 by controlled processes of washing has resulted in vast improvements 

 in both cotton and silk. 



For some reason not yet fully understood, silk is much superior to cot- 

 ton as an insulating material over the usual range of atmospheric humid- 

 ities. This is true in spite of the fact that cotton absorbs less water 

 than silk at a given humidity, and that, over the usual range of use, silk 

 is more sensitive electrically to a given increment of water than cotton. 

 This is responsible for the extensive use of silk in the electrical industry 

 at several dollars per pound in place of cotton, which may be had for 

 about a tenth as much. The purification of cotton, however, has made 

 it good enough to replace silk for a large number of purposes, especially 

 in telephone cords, and the saving thus effected amounts to several 

 hundred thousand dollars per year for the Bell System. In addition 

 to this practical result, such studies as these have suggested interesting 

 scientific possibilities in the use of electrical measurements for deter- 

 mining the structures of colloids. 



Paper 



An allied product is paper, which is used in enormous quantities in 

 the construction of the common type of telephone cable. This cable 

 consists of a bundle of wires individually insulated from one another by 

 strips of paper helically served about each. Before being enclosed in a 

 lead sheath the bundles of insulated wires are thoroughly dried and 

 thereafter throughout their use have to be protected from entrance of 

 atmospheric moisture by hermetically sealing the lead covering. The 

 functioning of the cables depends absolutely upon the maintenance of 

 an extremely dry atmosphere within the cable. Our chemists have been 

 called upon for elaborate studies of the effects of minute increments o- 

 moisture upon the insulating qualities of paper and the effect of temf 

 perature upon the electrical characteristics of paper containing various 

 small proportions of moisture. Incident to this task it has been neces- 

 sary to develop a humidity recorder sensitive to as little as 10 parts per 

 million of water vapor in the air. Such a commercial recorder, pro- 

 duced by Leeds and Northrup at our instance, is in successful use as a 

 guide in controlling the atmosphere of cable-drying ovens. Improved 

 devices for determining the brittleness of cable paper and for judging 

 its predisposition to lose flexibility upon baking have also received at- 

 tention. 



Still another dielectric problem is that of condensers, which are unique 



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