136 BELL SYSTEM TECHNICAL JOURNAL 



since its mechanical properties are so unique, rather than to attempt 

 to synthesize a new chemical compound which would imitate it. In 

 order to overcome the excessive stiffness of the pure gutta hydrocarbon, 

 as well as its prohibitive cost, it was determined to attempt to blend 

 large quantities of rubber with it, since rubber is the nearest kindred 

 material and is commercially available at low cost. There resulted 

 thermoplastic products of fairly good mechanical characteristics which, 

 however, proved to be insufficiently stable electrically. 



Meanwhile, a thorough study was being made of the electrical and 

 physical characteristics of rubber and particularly of the causes of its 

 electrical instability upon prolonged immersion in water. Our hope 

 that such a study would not only reveal the nature of the defects of 

 rubber but also suggest means for remedying them has been realized 

 to a gratifying degree. 



Rubber, as is well known, is also derived from the latex of certain 

 trees, chiefly Hevea Brasiliensis. This tree has been cultivated in 

 large areas on the plantations in the Far East and the product is 

 obtainable commercially in excellently standardized grades. Its 

 principal constituent is a hydrocarbon scarcely distinguishable from 

 that of gutta percha by chemical means, but radically different from 

 it in physical properties, notably in that it has but a slight degree of 

 thermoplasticity and is far more distensible in the cold state. Aside 

 from the hydrocarbon, rubber also contains small amounts of resins, 

 proteins and other impurities, but the aggregate non-hydro-carbon 

 constituents in the better grades are usually less than 10 per cent in 

 contrast to 50 per cent or thereabouts for gutta percha and balata. 



Rubber is used almost exclusively in industry in a vulcanized form, 

 that is, in combination with a small percentage of sulphur. In this 

 form rubber has also been used to a limited extent for submarine cable 

 insulation, but has long been recognized as lacking sufficient electrical 

 stability for deep sea cables designed to carry a heavy traffic. It is 

 still used to a considerable extent with a fair degree of success for 

 insulation on short cables where the electrical requirements are not 

 severe. In tropical waters it has the advantage over gutta percha of 

 greater resistance to teredo attack and to damage by high temperature. 



Some years ago an extended study ^ was made of the causes of the 

 electrical instability of vulcanized rubber, which led to the conclusion 

 that the water soluble impurities are largely responsible. These 

 impurities can be removed comparatively readily and satisfactorily 

 in the process of manufacture, and a submarine insulation of a fair 

 degree of stability is thereby attained. 



^ Williams and Kemp, Jour. Franklin Inst., 230, 35 (1927). 



