144 



BELL SYSTEM TECLINICAL JOURNAL 



Figure 7 shows the relative plasticities of cable gutta percha and 

 paragutta at several temperatures as determined by the Williams * 

 method, which can be taken to indicate the relative plasticities of 

 these materials at working temperatures. 



Brittle Temperature: It is extremely important that the temperature 

 at which submarine cable insulation becomes brittle should be far 

 below the range of sea bottom temperatures to be encountered in use. 

 This is one of the properties in which rubber and gutta percha greatly 

 excel any other available insulating material. Kohman and Peek ^ 



40 



50 



90 



100 



60 70 80 



TEMPERATURE -DEG. C 



Fig. 7 — Effect of temperature on the plasticity of cable gutta percha and paragutta. 



have described an apparatus for accurately determining this tem- 

 perature. The brittle temperature of paragutta is somewhat lower 

 than cable gutta percha, as can be seen from the results in Table II, 

 which give the range of brittle temperature values found for different 

 samples of several materials. 



Water Absorption — Electrical Stability 



The amount of water absorbed by rubber and gutta percha when 

 immersed in water is the result of a complicated mechanism. The 

 quantity and nature of water soluble or water absorbing impurities 



* Kohman and Peek, Jour. Ind. &" Engg. Chem., 20, 8 (1928). 



