12 BELL SYSTEM TECHNICAL JOURNAL 



steel rings of the same width but so placed as to overlap the joints of the 

 inner rings. So assembled, the rings form an articulated cylinder about 

 seven feet long. To exclude water, there is placed over this cylinder an 

 annealed copper tube with water-tight seals at its ends. The details of 

 the seal are of the greatest importance. It combines a strictly hermetic 

 seal, in which the conductors are brought out through glass, with a plastic 

 seal through which diffusion of water vapor would be extremely slow, 

 should the glass seal fail. Joined to the copper cylinder, and extending 

 over the cable core for several feet, is a tapered copper sheath which serves 

 to distribute bending strain and protect the conductor joint at the seal. 

 Containers of the type described have been tested at pressures considerably 

 higher than would be encountered in a transatlantic cable. They have also 

 been subjected to repeated bending around a six-foot drum without failure. 



Within the repeater housing the elements of the repeater are separately 

 contained in plastic cylinders about six inches long, loosely fitting inside the 

 inner steel rings. Connections between these units are made with flexible 

 conductors. 



A repeater must, of course, be supplied with power and, as it is imprac- 

 ticable to provide a primary source of power in such a small housing, power 

 must be fed to the repeater over the cable from a direct-current supply. 

 The supply voltage is one of the limiting considerations in the design of 

 such a cable system. It must not be so high as to endanger the insulation 

 of the cable or repeater elements. An operating potential-to-ground of 

 2000 volts oppositely poled at the ends of the cable was assumed. Power 

 would be supplied on a constant current basis so that fluctuations of earth- 

 potential would not cause variations of current-supply. The repeater 

 elements were designed to withstand the anticipated voltage-to-earth. 

 Tests of cable-core and joints over a long period of time have shown no 

 observable change under this impressed voltage. 



The difliculties of lifting a deep-sea cable for repairs are such as prac- 

 tically to prohibit frequent access to the repeaters for maintenance. Hence, 

 the repeater must be provided with elements which will rarely, if ever, 

 require attention. A period of 20 years without replacement of parts was 

 assumed as a reasonable requirement. 



The problem of life and maintenance is principally the problem of a 

 rugged long-lived vacuum tube. Ordinary vacuum tubes have limited 

 service-life on account of evaporation of material from thermionic cathodes. 

 By making the level of transmitted signals relatively low, the space current 

 may be kept very small. By making the cathode surface relatively large, 

 this srnall current can be obtained at a temperature so low that the cathodes 

 of the tubes may be expected to last for a very long time. This is a dif- 

 ferent approach to the tube problem than has ordinarily been made. New 



