82 



BELL SYSTEM TECHNICAL JOURNAL 



resistance component found in impedance measurements on the same 

 circuit with one coil omitted at the thirteenth loading point from 

 the end at which the tests were made. It will be noted that in the 

 latter case the characteristics of the circuitsvary greatly with frequency. 

 It would therefore be very difficult as a practical proposition to build 

 up a network that would balance lines in this condition, and such 

 variations in the electrical characteristics of a circuit impair the 

 quality of telephone transmission, as the currents of different fre- 

 quencies are differently affected. The necessity for careful main- 

 tenance work in promptly replacing loading coils which may become 

 defective or preventing other irregularities from creeping into the plant 

 will therefore be clear. 



Transmission Regulation 



The resistance of small-gage cable conductors is one of the important 

 factors that determine the transmission losses of a circuit. The 

 resistance of a No. 19 A. W. G. pair is about 88 ohms per mile so that 



1500 



-40 i 



-ao I 



•120 

 • 160 



ShO>» OUITIIO COIL IN 13^ BKCIiON 



R(«i«rAii» 



Cquivalcnt inouctancc 



Fig. 27 — Cable Circuit with Loading Coil Missing at Thirteenth Loading Point from 



Terminal 



in a long circuit this factor of line resistance reaches considerable 

 proportions. Now as most of the cable is aerial, the resistance of 

 the conductors is of course affected by changes in temperature both 

 daily and seasonal and the transmission losses vary accordingly. 

 These changes in transmission values are of such magnitude that 

 automatic transmission regulators are being provided for certain 

 groups of longer circuits. All changes in the transmission equivalents 

 of the circuits from whatever causes must be carefully watched and 

 necessary adjustments made or the service will be seriously affected. 



