228 BELL SYSTEM TECHNICAL JOURNAL 



communication and the amount of communication between continents 

 and, indeed, such an effect has already been experienced, significant 

 reductions in rates having been made within the past year. In view 

 of what has already been accomplished it seems not unlikely that the 

 further introduction of loaded cables will completely revolutionize the 

 whole transoceanic communication situation. 



Historical 



Like nearly all other important technical advances, that of the 

 loaded telegraph cable is the outgrowth of contributions of many 

 investigators and engineers working in different fields of endeavor. 

 The history of the development of the idea of inductively loading 

 transmission lines, from its original conception to recent times, is 

 well known and need not be gone into here. An excellent theoretical 

 analysis of the problem of transmitting signalling impulses over an 

 ocean cable has been made by Malcolm, who in 1917, in his book on 

 "The Theory of Submarine Telegraph and Telephone Cables," went 

 so far as to predict that heavy continuous loading was the next great 

 advance in the telegraph-cable art to be expected. 



The practical accomplishment of the permalloy-loaded cable came 

 about as a result of researches conducted in the laboratories of the 

 American Telephone and Telegraph and Western Electric Companies, 

 now known as the Bell Telephone Laboratories. Our interest in the prob- 

 lems of submarine cables was a natural part of our mterest in all phases 

 of electrical communication and was at first concerned principally with 

 the application of vacuum tube amplifiers to ordinary telegraph cables. 

 Considerable progress in the development of amplifiers for this purpose 

 was made in the laboratory as early as 1914. The great demand for 

 cable communication which the World War brought about led to 

 further activity in this direction and to extensive tests of vacuum tube 

 amplifiers on the cables of the Western Union Telegraph Company. 



From these tests it was found that although the vacuum tubes 

 would provide any desired amplification of signal strength and although 

 by the combination of vacuum tubes and suitable electrical networks 

 the distortion of the received signals could be corrected to any desired 

 degree, relatively little actual gain in traffic capacity could be obtained 

 by these means, since the real limit to cable speed was not distortion 

 but interference. Although some improvement in cable speed could 

 have been achieved by refinement of means for duplex operation and 

 by improved means to eliminate extraneous interference, it was quite 

 apparent that to obtain any great advance over the existing art would 

 require a modification of the cable itself. 



