With the four-wire circuit, since the two directions of transmission 

 are kept isolated from each other throughout, there is no need for 

 providing balance except at the terminals of the circuit and this makes 

 possible the use of higher repeater gains and, therefore, a higher net 

 efficiency of transmission with such circuits for long distances than 

 would be possible with the other form of circuit (generally called two- 

 wire circuit). Circuits of this four-wire type are now generally used in 

 toll cables for distances more than about 100 to 150 miles. 



TERMINAL 

 OFFICE 



INTER- 

 MEDIATE 

 OFFICE 



TERMINAL 

 OFFICE 



smu-KsmJ NE^oR*^ 



TO 

 SWITCH- 

 BOARD 



im^^ijm 



NETWORK i OOP ^ OOP . 



SWITCH- 

 BOARD 



I — ^Si^^^MU 



rm^^mu^ 



4-WIRE REPEATERS 



Fig. 14 — Schematic of a four-wire circuit using two one-way transmission paths. 



Mention was made in the first chapter of this statement of the re- 

 finements in manufacture and in installation procedures which were 

 necessary in order to produce suitable interurban toll cables, partic- 

 ularly cables designed for the use of phantom circuits. With the 

 extension of toll cables to great distances equipped at frequent intervals 

 with telephone repeaters, additional refinement in design and in con- 

 struction was necessary in order to prevent crosstalk between the 

 different circuits in the cable. This includes the physical separation 

 in different parts of the cable of conductors used by four-wire circuits 

 for the opposite directions of transmission. Even with all the refine- 

 ments which have been worked out, crosstalk remains today one of the 

 major factors to be considered in the engineering of long telephone 

 circuits. 



The velocity of propagation of telephone currents over circuits is 

 high so that in all of the early telephone development the length of 

 time required for propagation over the longest circuits used was not 

 sufficient to introduce any new difficulties in the problem of providing 

 good telephone transmission. The velocity of transmission, however, 

 varies with the type of circuit, is lower on loaded circuits than on non- 

 loaded circuits, and in loaded circuits in cable in common use is as low 



24 



