LONG DISTANCE TELEPHONE CIRCUITS 537 



some an illustration, we shall consider only circuits from New York 

 to other regional centers and primary outlets. Chicago has a larger 

 number of connections to these points but New York is chosen because 

 of its interest in connection with international service. These circuits 

 are indicated on the map in Fig. 6. 



Of the 64 circuit groups to regional centers and primary outlets, 

 43 are less than 870 miles (1400 kilometers) in length, 95 per cent of 

 the circuit mileage being cable. The makeup of a few of these groups 

 chosen for the purpose of illustration is given in Table IV. 



The remaining circuit groups indicated in Mg. 6, 870 miles (1400 

 kilometers) and more in length, and 21 in number, have the makeups 

 and electrical characteristics indicated in Table V. 



It will be noted that the average circuit in this classification is 

 made up of about one half four-wire toll cable circuit and the other 

 half of carrier telephone superimposed upon open wire. Since carrier 

 circuits have, in general, electrical characteristics comparable to the 

 four-wire cable circuits and have a relatively high velocity of propaga- 

 tion, the combination of four- wire with carrier results in very satis- 

 factory electrical characteristics, even for the longest circuits. 



A point of interest in connection with the table is the time of 

 propagation. This is, in all cases, well within the provisional limit 

 of 100 milliseconds adopted by the C. C. I. for the time of propagation 

 of the continental terminating circuits of an intercontinental con- 

 nection. 



Future Requirements of Very Long Cable Circuits 



At the present time cable up to lengths of 1800 miles (3000 kilo- 

 meters) is used in the regular routine in the United States and gives 

 a satisfactory performance. Tests have been made on longer four-wire 

 cable circuits up to lengths exceeding 3600 miles (^6000 kilometers). 

 These tests show that for such lengths, and particularly for the much 

 greater lengths which may result from the development of inter- 

 continental telephone service, the present design of toll cable circuits 

 would not be entirely satisfactory. 



More Effective Echo Suppressors 



Circuits 3600 miles (6000 kilometers) long when equipped with 

 ordinary echo suppressors fail to give the net loss of 9 db which has 

 been set up as a design objective. A more effective type of echo 

 suppressor is necessary to work such a circuit at as low a net loss as 

 9 db. Experiments have been made with an echo suppressor of a type 

 which changes its sensitivity automatically, depending upon the 



