710 BELL SYSTEM TECHNICAL JOURNAL 



Probably more important than either hardness or chemical stability, 

 which directly affect surface leakage, is the transparency of the material 

 which affects leakage only indirectly, through the medium of insects, 

 apparently small spiders. These spiders build nests on the inner 

 surfaces and seem to prefer dark or dimly lighted spaces for their homes. 

 Therefore, the more transparent the insulator, the less attractive home 

 it makes; and of opaque materials, probably white ones are accordingly 

 less attractive than dark colored ones. 



Other factors which enter into the spiders' choice appear to be those 

 of space and of protection from the elements. These latter factors are 

 functions of insulator design rather than material. 



There has been little opportunity to study these factors on the in- 

 sulator test lines, due to the lack of spiders. Only one specific case has 

 been found where their effect was marked. This was a case where 

 small borosilicate insulators were given an opaque metal coating. 

 After this coating had been on several months the direct surface leakage 

 increased to several times the value for the similar uncoated ones. An 

 investigation showed spider nests under many of the coated insulators. 



On the other hand, several types of larger insulators have shown no 

 such effect when coated, although some of these have been exposed for 

 several years. These results are not conclusive. They merely indi- 

 cate that design, as well as transparency of material, is a factor. 



An experience of the telephone plant in the use of opaque insulator 

 material (porcelain) showed a serious reduction of efficiency after a 

 few years of exposure, apparently explained by the action of insects, 



4. Specific Conductivity of Film. The specific conductivity of the 

 rain itself before it reaches the insulators is determined by the nature 

 and amount of impurities collected in its fall. Both the kind and 

 amount of impurities must vary greatly in different localities, for ex- 

 ample, industrial centers as compared with open country. Then again, 

 the amount in any given locality must vary throughout any storm on 

 account of the cleansing action of the rain on the atmosphere. 



On reaching the insulators the rain will suffer a further increase in 

 conductivity depending on the impurities it finds there. Smooth 

 vertical surfaces should be advantageous in reducing the collection of 

 dust. 



After a prolonged dry period in which the surfaces have become dust 

 coated, the conductivity may be quite high at the start of rain. As 

 the rain continues a certain amount of cleansing action occurs on the 

 unexposed surfaces, depending on the splashing. A decrease in leak- 

 age corresponding to this cleansing action has occasionally been ob- 

 served. 



