708 BELL SYSTEM TECHNICAL JOURNAL 



borosilicate compositions, the rain water does not seem to wet the 

 surfaces but rather stands in individual separate drops. The phenom- 

 enon is so marked that the surfaces have the appearance of having 

 been oiled or waxed. 



Under such conditions the conducting path is broken up and is 

 discontinuous. Naturally its conductivity is very small. It would be 

 fine if this property of the material could be preserved. Unfortunately, 

 along with exposure to weathering there comes an increasing tendency 

 for the drops of water to spread out and unite to form a continuous 

 path. Apparently, this action results from the collection of impurities 

 on the surfaces, for such molecular phenomena are well known to be 

 very sensitive to any contamination. 



The direct surface leakage has been observed generally to increase 

 10 or more times after only a few weeks' exposure. In one particular 

 case where it rained the same day that the new insulators were installed, 

 their surface leakage was observed to be less than one per cent of that 

 measured on insulators of the same shape and material which had been 

 aged for about two years. 



This study of insulators has shown that while changes in design may 

 affect changes in direct surface leakage by a factor of, say, 10, changes 

 in kind of material affect this leakage by a factor of two or less after 

 long exposure. In general, then, the material may be said to be less 

 important than the design. Purely general considerations indicate 

 this conclusion in view of the surface nature of the phenomenon. Con- 

 sider an insulator of a given design exposed to the elements for several 

 years. More and more foreign matter will collect on the surfaces as 

 time goes on. It is obvious that as this process continues, the insulator 

 material is becoming less and less important and it is conceivable, at 

 least, that given time enough, this surface of foreign matter would 

 determine the insulator performance irrespective of the material be- 

 neath this surface coating. At this point, design is paramount. 



However, the material might be expected to influence the aging 

 process in some way. From this viewpoint smoothness and hardness 

 of surface, together with chemical stability, appear to be desirable 

 qualities. 



This study has been confined chiefly to various kinds of glass. In 

 general, these have all had smooth surfaces, but they have varied in 

 hardness and chemical stability. For example, borosilicate glasses 

 are said to be generally harder and chemically more stable than the 

 common alkali group. 



The relative performance of one sample from each of these respective 

 groups is given in Fig. 11. Both were molded to the design of Fig. 12. 



