280 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1965 



that are applied to the input, with the exception, of course, that they 

 are much larger in amplitude. 



These developments contributed greatly to long-distance multi- 

 channel telephony in the early thirties. Until they were available, 

 the only kind of repeatered transatlantic-cable system that could be 

 imagined was one in which the repeaters could be mounted on moored 

 platforms at sea or in submerged buoys fed with power from local 

 batteries and visited at intervals for maintenance and repair — an 

 arrangement considered impractical. 



The advances made in the long-distance systems for land use by 

 the early thirties involved not only vacuum-tube repeaters at frequent 

 intervals on open wire lines and cables, but the use of carrier systems 

 in order to obtain many talking channels through one set of conductors 

 and repeaters. Carrier systems obtain their name by virtue of the fact 

 that each talking channel is associated with its own "carrier" current, 

 the different carrier currents having different frequencies. By the 

 use of a number of carrier currents, the combined intelligence of 

 several speech channels can be joined at the sending end of a circuit 

 to pass over one set of conductors and through a single repeater. At 

 the receiving end, the different carriers, with their intelligence content, 

 can be separated by filters or tuned circuits responsive to tlie different 

 frequencies, as they are in the process of selecting particular radio 

 stations in the tuning-in process. 



The design of vacuum tubes reached a point permitting considera- 

 tion of tubes whose effective life would be extremely long. There grew, 

 therefore, the broad concept of a transatlantic cable system using two 

 nonloaded coaxial cables, one for each direction of transmission, into 

 which the repeaters would be spliced at regular intervals. The term 

 "coaxial" is here used to indicate a type of cable in which the return 

 conductor consists, as previously noted, of a thin sheath of copper sur- 

 rounding the insulating material. 



The use of loading to improve the transmission characteristics of the 

 cable was omitted since it was hoped that, by the use of a sufficient 

 number of repeaters, i. e., a relatively short interval between them, a 

 wide band of frequencies could be transmitted, providing many tele- 

 phone channels, using a range of frequencies beyond which loading 

 would no longer be effective. 



Detailed discussion of the relative merit of two cables versus one is 

 outside the scope of this paper. As a matter of present-day technical 

 achievement, the twin cables, i. e., one-way operation in each, may 

 be more economical on deep-water routes where traffic capacity is grow- 

 ing rapidly. In the practical case, the present choice was also in- 

 fluenced strongly by the need for a repeater of small size which would 

 cause a minimum of physical irregularity in the cable structure and 

 therefore permit more practical deep-sea handling. 



