TRANSATLANTIC TELEPHONE CABLE — AFFEL 277 



band of frequencies which can be transmitted. Loading was, how- 

 ever, a feature of the proposed single-channel telephone cable of the 

 thirties. 



2. — Incident to the other work, better materials were devised for use 

 as insulators. These included a material termed paragutta, a mixture 

 of guttapercha and rubber, which served effectively as a substitute for 

 the guttapercha of the earlier cables. More recently, anotlier material, 

 which was originated by the Imperial Chemical Industries, called poly- 

 ethylene (polythene) , has been found to have greatly superior qualities 

 as an insulating material for cables. 



A very useful improvement in cable structure for long cables con- 

 sisted in providing a definite path for the return currents by adding a 

 thin coaxially located sheath of copper outside the insulation, instead 

 of permitting the return currents to follow the ocean path. It also 

 lowers the attenuation of the cable for high-frequency currents. An 

 incidental advantage is that if there are spurious currents in the 

 ocean which may get into the cable circuit, the coaxial return con- 

 ductor effectively shuts them out and permits the application of higher 

 amplification on the cable. 



3. — But the most important advance in thinking leading to the 

 final concept of a multichannel cable consisted in the application to 

 undersea cables of the techniques that had grown up over the years 

 for overland cables, namely, the introduction of amplifiers or repeaters 

 in order to boost the feeble cable currents at intervals before they have 

 a chance to become sufficiently attenuated to be lost in the background 

 disturbing currents of a cable circuit. 



The proposals of Dr. O. E. Buckley and his associates, in particular 

 O. B. Jacobs, included certain fundamental features that led to the 

 present successful telephone cable system : 



1. — Kelatively closely spaced, low-power repeaters, which favored 

 the development and application of vacuum tubes having a long life. 



2. — The use of separate cables for opposite directional transmission, 

 which minimized the problem of two-way transmission. 



3. — ^A flexible repeater structure incorporated under the cable armor 

 to minimize laying difficulties and having necessary physical strength 

 to function satisfactorily at depths up to several miles and with ten- 

 sions of thousands of pounds often involved in normal cable-laying 

 operations. 



The details of this amplifier structure, how it retains its flexibility 

 to pass over a sheave several feet in diameter, at the same time main- 

 taining its integi'ity from the standpoint of water seepage, will be 

 told in more detail later. These amplifiers are installed at intervals 

 of some 40 nautical miles across the Atlantic, and the power necessary 

 to actuate their vacuum tubes is fed over the cable conductor which 

 also carries the communication currents. 



