300 



BELL SYSTEM TECHNICAL JOURNAL 



ohms per mile at 50 kc, the resistance at the lower temperature being 

 about 96 ohms per mile and at the higher temperature about 116 ohms 

 per mile. This amounts to about ± 10 per cent variation from the mean. 

 In addition to the wide annual variations, there are daily variations of 

 as much as 50° Fahrenheit at times, that is, almost half as much as the 

 normal annual variation. The practical importance of these large re- 

 sistance changes lies in their large contribution to changes in attenuation 

 as will be brought out more fully in connection with variations of attenua- 

 tion with temperature. 



150 



10 



20 



70 



80 



90 



30 40 50 60 



rREQUENCY- KILOCYCLES 



Fig. 4 — Resistance per mile vs. frequency — 19 gauge pairs 



Underground and buried cable are, of course, not subjected to such wide 

 annual variations and daily variations are almost entirely eliminated by 

 the attenuation of heat changes by the soil. Cable in ducts usually lies 

 well below the freezing line and this depth at the same time protects it from 

 the summer's heat. The normal range for cable in ducts is from about 

 freezing to about 70 degrees, F. Cable buried only a foot or so underground 

 would have a considerably larger annual temperature range but a great deal 

 of such cable is buried two to three feet deep. 



Curves in Fig. 4 show the actual a-c. resistance variation with frequency 

 and in Fig. 5 are shown temperature variations of resistance at typical 

 frequencies for 19-gauge toll pairs in a reel-length of standard toll cable. 



