700 BELL SYSTEM TECHNICAL JOURNAL 



These items ^ will now be discussed individually and will be treated 

 generally as they exist under wet rather than dry weather conditions. 

 In this connection most of the experimental evidence that will appear 

 was obtained from tests on insulator test lines located near Phoenixville, 

 Pa, The measuring equipment employed there has already been de- 

 scribed in another paper ^ and for lack of space will not be further dis- 

 cussed here although certain improvements have been made since that 

 paper was prepared. 



Item A — Direct Leakage To Pin 



This item refers to that leakage through the insulator material which 

 is directly due to the conductivity of the material, for example via path 

 a b, Fig. 1. Item A has been listed chiefly for completeness because it 

 is fairly well known that for most materials commonly employed, this 

 source of leakage is very small. In the present study it has been found 

 to be negligible in magnitude. 



Item B, therefore, becomes the controlling source of leakage for 

 direct current and accordingly deserves consideration. However, it 

 will be given even more consideration because its magnitude, being 

 so closely dependent on the shape, size, location, and condition of the 

 insulator surfaces throws light on those factors which will later be 

 shown to play a very large part in leakage for alternating currents. 



Item B — Direct Surface Leakage 



1. General Characteristics. Item B refers to that leakage over the 

 insulator surfaces which is directly due to the conductivity of those 

 surfaces, for example, via path a c d e, Fig. 1. 



The outstanding characteristic of this type of leakage is the enormous 

 changes in magnitude it exhibits under changing weather conditions. 



For example. Fig. 4 shows the results of measurements made on a 

 telephone line in Texas equipped with insulators of the type shown in 

 Fig. 3. This test, made in clear weather, shows the large change in 

 leakage produced simply by the condensation of moisture on the insula- 

 tor surfaces. The peak value may be seen to be about 2500 times the 

 smallest value measured.^ Had it rained along the entire length of line 

 the peak might readily have been 10 times greater and the correspond- 



^ The existence of several of these factors appears to have been well appreciated 

 by Mr. R. D. Mershon as early as 1908, although Mr. Mershon's measurements were 

 made at frequencies below 100 cycles where many of the items are extremely small in 

 magnitude. See "High Voltage Measurements at Niagara," by R. D. Mershon 

 and Discussion, A. I. E. E. Trans., Vol. 27, 1908, pp. 845-929. 



^ "Methods of Measuring the Insulation of Telephone Lines at High Frequencies," 

 E. I. Green, A. I.E. E. Trans.,_ Vol. 46, 1927, pp. 514-519. 



^ Such tests indicate the negligible effect of Item A. 



