A STUDY OF TELEPHONE LINE INSULATORS 



711 



Thus, in a locality where rain is infrequent but where dew, for ex- 

 ample, might wet the insulators, the leakage might conceivably be 

 quite high. 



This phenomenon can be produced artificially and emphasized by 

 placing roofs over the insulators, thus preventing any direct cleansing 

 action of the rain. 



Roofs of sheet metal six inches in height, five inches in width and 

 twelve inches in length were placed over insulators of the design in Fig. 

 1. The ends were left open to permit passage of line wire. 



While still new and clean these protected insulators showed less leak- 

 age than the unprotected ones of the same design. However, after a 



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Fig. 13 — Illustrates effect of sheds on d.-c. surface leakage. 



few months the protected insulators became covered with a thick layer 

 of dust blown under the roofs by wind. Then when it rained, enough 

 moisture reached the surfaces to wet the dust, but not enough to have 

 much cleansing action. The surface leakage then becomes very high, 

 as will be apparent from Fig. 13. 



Item C — Dielectric Absorption in Insulator Material 



1. General Characteristics. When a dielectric is subjected to a 

 varying potential field a certain amount of the electrical energy is 

 dissipated in the material in the form of heat, depending on the nature 

 of both the material and the field. This phenomenon is commonly 

 called dielectric absorption. 



