90 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



coil. It is crimped to hold the coil mechanically in place and then welded 

 at the end for electrical connection. At the end of the coil as well as in 

 all other places where it is possible, a mechanical wrap is made in addi- 

 tion to spot welding. An additional precaution is taken by inserting an 

 insulated molybdenum support rod through the center of the cathode 

 coil. The filling gas is argon at a pressure of 10 mm Hg. To provide ini- 

 tial ionization, 1 microgram of radium in the form of radium bromide 

 was placed on the inside of the tube envelope. All materials were pro- 

 cured in batches of sufficient size to make the entire lot of tubes and care- 

 fully tested before being approved for use. The tubes were fabricated in 

 small groups and a complete history was kept of the processing of each 

 lot. 



For detailed study of tube performance, a number of electrical tests 

 were made. These involved measurements of breakdown voltage, operat- 

 ing voltage as a glow discharge at low current, current required to cause 

 the transition to a thermionic arc, the time required at the cable current 

 to cause transition to the low voltage arc, and the sustaining voltage at 

 the full cable current. 



All tubes were aged by operating at 250 milliamperes on a schedule 

 which included a sequence of short on-off periods (2 min. on, 2 min. off) 

 followed by periods of continuous operation. A total of 150 starts and 

 300 hours of continuous operation were used. Following this aging 

 schedule the tubes were allowed to stabilize for a few days and then sub- 

 jected to a 2-hour thermal treatment or pulse at 125°C. It was required 

 that no more than a few volts change in breakdown voltage occur during 

 this thermal pulse before a tube was considered as a candidate for use 

 in repeaters. 



After aging and selection as candidates for repeaters, tubes were 

 stored in a light-tight can at 0°C. Measurements were made to assure 

 stability of breakdown voltage and breakdown time. 



The quality of each group of 12 tubes was further checked by continu- 

 ous and on-off cycling life tests. The fact that none of these tubes has 

 failed on the cycling tests at less than 3,500 hours and 1,500 starts and 

 no tube on continuous operation has failed at less than 4,200 hours gives 

 assurance that system tubes will start once and operate for the few hours 

 necessary to locate a defective repeater. Long-term shelf tests of repre- 

 sentative samples at 70°C and at 0°C give assurance of satisfactory 

 behavior in the system. 



CONTAINER AND SEALS 



The design of the flexible enclosure for the flexible repeater unit is 

 basically the same as it emerged from its development stages in the 



