1242 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



incidental cables) to the use of lower-cost transmission-improvement treat- 

 ments, even though they are not so good as loading in certain respects. 



Cable Program Circuit Loading 



During the 1930's and early 1940's, there were extensive applications of 

 loading on the cable sections of nation-wide chain networks used for trans- 

 mitting AM broadcast program material. Now that high-grade program 

 transmission circuits may be obtained by carrier methods on broadband 

 cable carrier systems, the future demand for 8-kc loaded cable program 

 circuits will be largely limited to special situations where the carrier program 

 circuits are not economical. 



It is expected also that there will be a moderate, continuing demand for 

 the recently developed loading that provides a 15-kc band for the trans- 

 mission of FM program broadcast material, principally on studio-transmitter 

 circuits, and on end links in toll cable networks, where carrier program 

 circuits may be uneconomical. 



Continuotts Loading 



Over the years, a substantial amount of exploratory development work 

 on continuous loading for ordinary types of paper-insulated cable has been 

 done, but with negative results so far as commercial applications in the Bell 

 System are concerned; it has not yet been found feasible to compete with 

 coil loading in service performance and cost. 



However, continuous loading has had a few appHcations in single core 

 submarine cables, in deep water installations where coil loading is not 

 feasible. The three 1921 cables between Key West and Havana are the only 

 continuously loaded cables to become a part of the Bell System. They use 

 iron wire as the loading material. Several years later, permalloy tape 

 continuous loading developed by the Bell Telephone Laboratories made 

 possible a great increase in the message-carrying capacity of transoceanic 

 telegraph cables. During the middle 1920's, an aggregate of about 15,000 

 nautical miles of the new type, high speed, cable was installed for use by 

 non-affiliated telegraph and cable companies. 



Late in the 1920's, a perminvar type loaded cable suitable for voice- 

 frequency telephony between Newfoundland and Ireland was developed by 

 the Bell Telephone Laboratories. The business depression of the early 

 1930's intervened to cause a temporary postponement of the project; later 

 on, an indefinite postponement resulted from improvements in transatlantic 

 radio-telephony. 



From the foregoing, it is clear that the importance of continuous loading 

 has been low relative to that of coil loading in the growth of the Bell Tele- 

 phone System. 



