284 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



LONG-LIFE TUBES 



The vacuum tube finally adopted was a conservatively designed so- 

 called suppressor-grid pentode whose details were finalized in 1941 

 and which has not since been changed m any significant way. Essen- 

 tial features of its construction are unusually low cathode tempera- 

 ture and liberal internal spacing between elements, with generally 

 rugged and shock-proof construction. Some of these tubes have been 

 under continuous test for over 13 years and 18 have been in opera- 

 tion for about 5 years in repeaters on the Key West-Havana cables 

 referred to previously. All indications are favorable for something 

 better than 20-year life expectancy for North Atlantic service. 



OTHER COMPONENTS 



In addition to the tubes, about 60 other circuit elements have to be 

 accommodated inside the submerged repeater housing. These include 

 resistors, capacitors, inductors, transformers, and crystals. Like the 

 tubes, these elements have been designed and fabricated so as to avoid 

 all possible risk of failure in service. They have also had to meet 

 requirements of ruggedness and reasonably small size. As in the case 

 of the tubes, cost has fortunately been a less important factor than it 

 is in many other applications. In the choice of details of design of 

 the components, conservative design was followed. For manufacture, 

 particular effort was made to select and train personnel for skill and 

 pride in product. Extraordinary inspection techniques were em- 

 ployed. 



CABLE 



The cable consists of a solid dielectric coaxial structure covered by 

 the usual jute and armor for protection. In its broader aspects this is 

 not unconventional, and the term "solid dielectric" is used to differen- 

 tiate it from other types of coaxial structure in which the insulating- 

 material is often permitted to include large portions of air or other 

 gas. 



The composite central copper conductor consists of a central solid 

 wire surrounded by a single layer of spiraling abutting tapes, care- 

 fully formed to fit close about the wire with a minimum of voids. Such 

 a composite structure has better flexibility and protection against 

 bi-eakage than a solid conductor and has better alternating-current and 

 direct-current resistance than a stranded conductor of the same outside 

 diameter. The return conductor is also a flexible composite copper 

 structure consisting of a single layer of abutting spiraling tapes care- 

 fully formed into a tubular configuration. It is covered with an over- 

 lapping spiraling copper tape for teredo protection. A teredo is a 

 small marine animal which has a penchant for burrowing into the 

 insulation of submarine cables to create serious current leakages. 



