A STUDY OF TELEPHONE LINE INSULATORS 



715 



Item D — Dielectric Absorption in Pins 



I. General Characteristics. Item D applies only to pins of dielectric 

 material (usually wood) or to metal pins with cobs of dielectric material. 



Like C, D is roughly proportional to the frequency and its impor- 

 tance is greatest at the upper end of the carrier range of frequencies. 



Again, like C, D increases in wet weather due to the accompanying 

 increase in wire-to-pin capacitance. It also increases because of ab- 

 sorption of moisture by the pin. 



Fortunately, due to the small value of C, particularly when dielectric 

 pins are used, a rough measure of D can be obtained by again making 

 use of metal-coated insulators. 



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FREQUENCY-KILOCYCLES 

 Fig. 16 — \'ariation of (D) with frequency for standard D. P. design. 



Fig. 16 shows two measurements of the leakage in dry weather of 

 metal-coated insulators on wood pins. These insulators are similar 

 in shape to that of Fig. 1, and were molded of a borosilicate glass. 



Due to the low capacitance when wood pins are used and to the high 

 quality of glass, item C can be neglected here. 



Item E, which is to be discussed in the next section, enters here to 

 some extent, but the cross-section of the crossarm is so much greater 

 than that of the pins that this item, while it cannot be neglected, is 

 probably small. 



The measured leakage then closely represents that due to dielectric 

 absorption in the pins under the conditions of the test, namely, metal- 

 coated borosilicate insulators. 



