64 BELL SYSTEM TECHNICAL JOURNAL 



and complicated circuits is a very considerable problem both from the 

 standpoint of technique and of organization. In this paper, these 

 problems will not generally be discussed, but certain features will 

 briefly be indicated. 



The service maintenance of the circuits includes periodic tests of 

 transmission efficiency with transmission measuring sets designed for 

 rapid and efficient use by the plant maintenance forces. The fre- 

 quency of tests varies according to the requirements of each circuit 

 group. 



To expedite the testing and adjustment of the circuits the longer 

 cable circuits are subdivided into circuit units, these units usually 

 being in cable about 160 to 240 kilometers in length, including the 

 conductors and equipment involved in one section arranged for the 

 automatic compensation of temperature variations. When trouble 

 occurs on a long circuit, the circuit unit in which the trouble is located 

 is immediately replaced and the location of trouble within the circuit 

 unit then can be carried out without further interruption of service. 

 The responsibility for establishing and maintaining each circuit group 

 is given to a control office which is provided with private communica- 

 tion channels to all parts of the circuit. 



An important feature in the maintenance of long toll circuits is 

 the physical relations between the telephone circuits and circuits for 

 the transmission or distribution of electric power. The Bell Tele- 

 phone System and the power companies of the United States as repre- 

 sented by the National Electric Light Association are very actively 

 cooperating in a study of the best means of so coordinating the plant 

 of telephone and power companies as to avoid interference under the 

 various types of conditions important in practice. By means of this 

 work it has been possible to find in every case a satisfactory solution 

 permitting each utility to extend and increase its service along natural 

 lines and providing proper protection of the telephone service. 



International Connections 



General 



The connections between the telephone systems of the United 

 States and the telephone systems of other countries are indicated in 

 Fig. 53. 



The territory of the United States has direct contact with only 

 two other nations, Canada on the north and Mexico on the south. 

 The common language and the close commercial relations between 

 Canada and the United States have naturally resulted in a well de- 

 veloped arrangement of lines connecting the telephone systems of the 



