OPEN-WIRE TELEPHONE LINES 



743 



Considerable study has been given to methods of reducing the leak- 

 age conductance, particularly at carrier frequencies, and two new types 

 of insulators have been developed for this purpose. In these there is 

 used an improved dielectric (borosilicate glass) which has a low power 

 factor and a reasonably low dielectric constant, as well as good chemical 

 stability. Two expedients for eliminating losses in the pins and cross- 

 arm are employed. In the CW insulators, illustrated in Fig. 7, two 



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Fig. 8 — Leakage conductance of open-wire pairs equipped with different types of 



insulators. 



metal shells or thimbles are bonded together and placed over the 

 wooden pins. In the CS insulator, also shown in Fig. 7, steel pins are 

 employed and the two steel pins of the pair are bonded together. In 

 the past few years CS insulators have been applied to the open-wire 

 lines of the Bell System in increasingly large numbers. 



With these two types of insulators a substantial reduction in the 

 total leakage conductance is brought about. The best available figures 

 for the limiting values of leakage conductance for these types are 

 shown in Fig. 8. A further advantage obtained through the applica- 

 tion of the new insulators is that of stabilizing the attenuation at carrier 



