FLEXIBLE REPEATER MANUFACTURE 123 



that the glass seal contain a minimum of residual tensile stress. These 

 two problems were resolved collectively by performing the sealing opera- 

 tion on a single spindle glass sealing machine. Accurate positioning of 

 the glassware and sealing fires, together with precise timing and tem- 

 perature controls, achieved the desired results. 



Evaluation of residual stresses were made by inspections using a 

 polarimeter and by a thermal shock test. The maximum safe stress was 

 established at 1 .74 KG/mm^. The thermal shock test required successive 

 immersion of the unit in boiling water and ice water. The electrical char- 

 acteristics of these units exceeded all others made previously by Western 

 Electric. The ratio of reactance to effective resistance ("Q") was greater 

 than 175,000 — twice that ever previously produced and 17 times that 

 required in the average filter crystal. 



Stabilit}^ for frequency and resistance was assured by a 28-day aging 

 test. During this period, precise daily resonant frequency and resistance 

 measurements were recorded against temperature within 0.1° C. The 

 maximum permissible change was 0.0005 per cent in frequency and -f5 

 per cent to — 10 per cent in resistance. 



GLASS SEALS MANUFACTURED AT ALLENTOWN 



The glass seal used to close each end of the container for the repeaters 

 and equalizers is manufactured at the Allentown Works of the Western 

 Electric Gompany. 



The unit is essentially a glass bead-type seal. It insulates the central 

 conductor of the repeater from the container and serves as a final vapor 

 barrier between the cable and the interior of the repeater. As such, it 

 backs up several other rubber and plastic barriers as shown in Fig. 3. 



Fig. 4 shows the various components, subassemblies, and a cross-sec- 

 tion of the unit. The unit consists of the basic seal brazed in the Kovar 

 outer shell, to which is brazed a copper extension provided with two 

 brazing-ring grooves. One of these grooves is used in brazing the seal, 

 along with support members, into a length of container tubing in the 

 same manner as the seal is ultimately brazed into the repeater. Packaging 

 of the seal in this manner was necessary to pressure test the seal. Under 

 test, in a specially constructed chamber 10,000 psi of helium gas pressure 

 was applied to the external areas of the packaged glass seal and a mass 

 spectrometer type leak detector was connected through the tubulation 

 to the internal cavity of the packaged unit. In this manner, the interface 

 of the glass to metal seal, the brazed joints, and the porosity of the metal 

 were checked for leakage. The unit is left in this package for delivery 

 to provide protection during shipment. Before the seal could be used. 



