584 



BELL SYSTEM TECHNICAL JOURNAL 



An interesting picture is presented in Figure 11 which shows the 

 attenuation variations with time on a particular line (about 110 miles 

 in length) during the period in which a storm arose to cause the 

 attenuation to increase. Later, when the insulators dried, the cor- 

 responding drop in attenuation was that shown. From these vari- 

 ations it is quite obvious that means such as afforded by the pilot 

 channel are needed to insure that the talking circuits provided by 

 the carrier channels remain at substantially constant volume. 



Figure 11 — \'ariations in attenuation of a particular open-wire circuit 



In addition to the improvement in stability effected by the use of 

 pilot channel apparatus, substantial advances have been made in the 

 design and application of special types of line insulators in which 

 the high-frequency losses, particularly in wet weather, have been 

 appreciably reduced, resulting in still further improvement in stability. 

 The attenuation data given above are for the lines equipped with 

 the older standard types of telephone insulators, which are still 

 employed on the majority of circuits in the telephone plant. How- 

 ever, the newer types of improved insulators are now being applied 

 and their use makes it possible to reduce the wet to dry weather 

 attenuation variation by a factor of about 3 to 1 and to reduce the 

 absolute value of attenuation at the higher frequencies by as much 

 as 25 per cent, l^'urthor information describing the de\elopmcnt 



