PURIFIED TEXTILE INSULATION 247 



shown in Figs. 6, 7 and 8 and described later. The same comparison 

 is shown in Fig. 2 except that these graphs show the insulation re- 

 sistance of wire insulated with the washed and unwashed textiles. 

 In addition to the insulation resistance requirement, it is required 

 that the energy losses at talking and carrier current frequencies must 

 be maintained at the minimum point consistent with the space 

 limitations permitted for the conductors. The effect of purification 

 of the textiles on this characteristic expressed in capacitance and 

 conductance, measured at 1,000 cycles per second between the wires 

 of twisted pairs is shown in Fig. 3 and Fig. 4. The data represented 

 by these graphs converted into transmission loss units are illustrated 

 in Fig. 5. As the same thickness of insulation was used in all cases, 

 the graphs are on a comparative basis. It should be noted that the 

 graphs are illustrative of the effects of purification on the electrical 

 properties of cotton and silk as insulation and should not be considered 

 as applying quantitatively to telephone circuits. 



From a telephone transmission point of view, perhaps the most 

 significant fact to be observed is the large reduction in capacitance 

 and conductance at relative humidities of 75 per cent and higher. 

 These characteristics which largely determine transmission efificiency 

 are relatively low for both silk and cotton at 65 per cent and below, 

 but in commercial textiles in general use for insulating purposes they 

 increase very rapidly as the relative humidity increases. The char- 

 acteristics of purified textiles are not as markedly different from those 

 of unpurified textiles at 65 per cent relative humidity as at higher 

 humidities, but their rate of increase as the humidity increases is 

 greatly reduced. This fact is of particular importance in the main- 

 tenance of a standard level of voice transmission through toll offices 

 where suitable repeater gains and balance must be maintained. 

 Losses, if fixed in value and not excessively large, can be compensated 

 for, but if they change with every change in atmospheric moisture 

 content the compensation problem becomes serious. 



Method of Testing 



Two fundamental characteristics of silk and cotton made it necessary 

 to do a large amount of experimental work before a practicable shop 

 test method could be established to determine whether or not the 

 textiles were washed to the point of meeting the requirements estab- 

 lished. One of these characteristics is the high electrical resistance 

 of both washed and unwashed textiles at the lower relative humidities 

 and the other the extreme sensitivity to change, with minor change in 

 relative humidity especially at the higher humidities. The first 



