LONG DISTANCE TELEPHONE CIRCUTTS 523 



systems are different it is to be expected that the most economical 

 designs will differ. 



Cable Constants 



The toll cable whicli is standard in the Bell System plant has 

 capacitance of .062 microfarad per mile (.038 mf. per km.) for the 

 side circuits and about .100 microfarad per mile (.062 mf. per km.) 

 for the phantoms. There appears to be little to gain for voice- 

 frequency circuits by varying materially from these capacitance 

 figures. 



With respect to size of wire, No. 19 A. W. G. conductors (.9-milli- 

 meter diameter) are well suited for both two-wire and four-wire circuit 

 operation. No. 16 A. W. G. conductors (1.3-millimeter diameter) have 

 been employed to a considerable extent in the past. In new cables 

 conductors of this gauge are, in general, used only for relatively short 

 non-repeatered circuits, or for program circuits. At the present time 

 the possible economic advantage of using smaller sizes of wire than 

 No. 19 A. W. G. is so small that it is considered to be outweighed, 

 in general, by the greater complexity and variability of the circuits 

 which would result. 



Side Circuits and Phantoms 



With the exception of program transmission circuits, multiple twin 

 quads, utilizing both the side and phantom circuits, are used exclu- 

 sively. 



The ratio between the capacitances of phantoms and side circuits 

 is about 1.6, while the ratio of resistances is 0.5. Because of this the 

 phantoms which are loaded for the same cutoff frequency as the side 

 circuits have lower attenuation and lower impedance than the side 

 circuits. For repeatered circuits the phantoms and sides are so 

 operated that they give substantially equal transmission results, this 

 being desirable for flexibility reasons. 



Two-Wire and Four-Wire Circuits 



As is well known, two-wire repeatered circuits are more economical 

 for the shorter distances while four-wire circuits are necessary to 

 economically provide satisfactory transmission for longer distances. 

 It is possible for terminating business to design two-wire cable circuits 

 which will deliver good telephone service for distances of at least 

 900 miles (about 1500 kilometers). However, to meet the transmission 

 standards current in the United States four-wire circuits are generally 

 more practicable for distances more than two or three hundred miles 

 (a few hundred kilometers). 



