FLEXIBLE REPEATER DESIGN 



83 



and the fact that the physical and electrical requirements dictated the 

 use of relatively high dielectric stress in these capacitors, a program of 

 study covering a wide range of dielectric materials was undertaken about 

 1940. This study showed that none of the usual solid or semisolid materi- 

 als used to impregnate paper capacitors were suitable for continuous use 

 at sea bottom temperatures. Typical results of this program are shown 

 in Figs. 12 and 13. These curves show the performance of capacitors 

 operating at approximately 1.8 times normal dielectric stress at both 

 sea bottom and room temperatures. It is evident that even semisolid im- 

 pregnants are inferior to liquids at the lower temperature. The need for 

 the maximum capacitance in a given space restricted the field still fur- 

 ther, so that the final choice was a design using castor-oil-impregnated 

 kraft paper as the dielectric. 



It is well established that the life of impregnated paper capacitors is 

 inversely proportional to the fourth to sixth power of the voltage stress ; 

 or 



h = 



where p ranges from 4 to 6. This fact permits the accumulation of a large 

 amount of life information in a relatively short time. In order to insure 



in 

 lij 



I- 



z 

 o 



z 

 z 

 < 



LU 



a. 



t- 

 z 



UJ 



o 

 a. 



LU 



a 



100 

 90 

 SO 

 70 

 60 

 50 



40 



30 



20 



10 



1/2 



1 n/2 



TIME IN YEARS 



21/2 



Fig. 13 — Accelerated life tests on paper capacitors with various impregnants 

 at 40°F. 



