PRACTICAL APPLICATION OP CARRIER SYSTEMS 47 



there arc usually very simple, we shall consider the Pittsburgh- 

 Chicago section. There are at present three carrier telegraph systems 

 actually in operation between Pittsburgh and Chicago. They pro- 

 vide a total of twenty-eight lull duplex channels. These give service 

 which could not be given otherwise as all the open-wire facilities 

 between Pittsburgh and Chicago are completely equipped with 

 direct current composited telegraph sets (to give all possible tele- 

 graph channels). The layout of the carrier telegraph systems between 

 these points is shown in Fig. 5. 



The above example is representative of the conditions under which 

 carrier telegraph will be installed. In cases where open wire or 

 cable facilities are available which can be composited with the ordi- 

 nary direct current methods, the telegraph facilities can be obtained 

 as a by-product most cheaply in this way. As soon as these facilities 

 are all in use or an insufficient number of spare circuits remains, 

 carrier telegraph can properly be used provided the returns from the 

 special contract telegraph service are sufficient to meet the annual 

 charges on the apparatus itself. 



One of the carrier systems listed in the above tables is the Key 

 West-Havana carrier system. The details of the telephone and 

 telegraph channels obtained for the submarine cables were described 

 in detail in the paper on the "Key West-Havana Submarine Tele- 

 phone and Cable System" published in the journal of the A. I. E. E., 

 dated March, 1922. On account of the considerable length of this 

 cable and its high attenuation, the carrier equipment is special al- 

 though resembling in principle the carrier telegraph apparatus used 

 in our ordinary land installation. Without the carrier equipment 

 it would have been possible to obtain only one telegraph channel 

 on each of the three submarine cables by means of direct current 

 composite sets. With the carrier apparatus it is possible to obtain 

 4 telegraph channels in addition to the single telephone channel. 



Typical Cases — Telephone: 



The most important application at the present time of carrier 

 telephone apparatus is probably between Pittsburgh and Chicago 

 where the existing open wire leads are so congested that the additions 

 of further circuits would require extensive construction work and 

 possibly an entirely new pole line. An engineering study of this 

 situation resulted in the drawing up of plans for an aerial toll cable 

 which will largely replace the open wire. Work is already well 

 advanced on the installation of this cable and it will be completed 



