980 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



cycles of alternate humid and dry environments over the years. Humid 

 conditions exist during the summer season, followed by a dry exposure 

 during the winter months when the central offices are heated. Experience 

 has shown that a relay exposed for six days to 90 per cent relative hu- 

 midity at 85°F will be comparable to those observed in service in humid 

 localities. Although the six day exposure is admittedly an accelerated 

 test, the dimensional changes produced are approximately the same as 

 those caused by the accumulating effect of fluctuating humidity during 

 the entire season. Similarly, a period of six days exposure to 120°F pro- 

 duces the same effect of drying as that which occurs during the heating 

 season. 



By making careful measurements on an adjusted relay of such impor- 

 tant parameters as operate current, releasing current, contact separation, 

 contact spring tension, armature back tension, stud gaps, etc., and then 

 subjecting the relay to repeated cycles of humid and dry conditions, 

 repeating the measurements after each exposure and noting the changes, 

 a good appraisal of the relay can be made. A repetition of the test will 

 reproduce the same pattern of results as the first cycle unless permanent 

 deformation of the materials in the relay has taken place. 



0.001 0.002 0.003 0.004 0.005 0.006 0.007 



STRAIN IN INCHES PER INCH 



Fig. 2 — Strength of relay pile-up screws. 



