4 BELL SYSTEM TECHNICAL JOURNAL 



was obvious that such a step could not be accomplished by mere structural 

 changes. New materials were required. For years, a systematic search 

 had been made to improve the properties of electric and dielectric mate- 

 rials for use in submarine cables. By 1928 sufficient progress had been 

 made in the development of materials and in the structure of the cable 

 itself to permit seriously undertaking a transatlantic telephone cable. A 

 decision was than made to embark on a test of a section of such a cable 

 under practical conditions. 



In determining the requirements for this cable, it was decided to engineer 

 it for the Newfoundland-Ireland route rather than for the route via the 

 Azores. The longer link made the cable more difficult and its cost per 

 mUe higher, but the total cost and considerations of operation and main- 

 tenance favored the more direct route. 



The structure proposed for the Newfoundland-Ireland telephone cable 

 was of the single-core type with a continuously loaded central conductor 

 and a concentric return conductor similar to that of the Key West-Havana 

 cables, but different in materials and dimensions. Instead of a serving 

 of iron wire or permalloy to provide inductance, there were used four 

 layers of very thin perminvar tape. Perminvar is an alloy which, in the 

 form of loading tape, has a permeability and resistivity suitable for tele- 

 phone use and at the same time has very low hysteresis, which helps in 

 preventing distortion of speech due to magnetic modulation. The loaded 

 conductor was insulated with paragutta rather than with gutta percha. 

 Paragutta is a mixture of specially purified and deproteinized rubber, de- 

 resinated balata or gutta percha, and some wax. It has a dielectric con- 

 stant 15 per cent lower than the gutta percha in the Key West-Havana 

 cables, and leakance at telephone frequencies about one-fifteenth as great. 



The cable was designed on the basis of as high attenuation as would be 

 permitted by considerations of noise at the receiving end and usable power 

 at the sending end. Since the attenuation of such a cable increases rapidly 

 with frequency, only the noise at the high-frequency end of the speech 

 band is significant; here the noise is entirely of thermal origin, for static 

 and other external interference are eliminated by shielding. The sending 

 power is limited by magnetic hysteresis and there is little advantage in 

 applying more than about 50 volts. Most of the power can be concentrated 

 in the high frequencies by placing, at the sending end, part of the network 

 which corrects the distortion of the cable. By these means, it is possible 

 to set a permissible overall attenuation as high as 165 db for a top frequency 

 of 3,000 cycles. This far exceeds attenuation permissible in other wire 

 telephone practice. The cable was designed to give this attenuation with 

 the most economical disposition of materials within practical limits. Its 

 core comprised a loaded central conductor of 800 pounds of copper and 

 95 pounds of perminvar per nautical mile, 720 pounds of paragutta insula- 



