712 



BELL SYSTEM TECHNICAL JOURNAL 



Such a field exists between the line and tie wires on one side and the 

 pin on the other. The insulator material lies more or less in this field 

 and, therefore, dielectric absorption is naturally to be expected. While 

 it is convenient to refer to a single insulator it should be remembered 

 that two insulators are in series between wires at any one point. 



The chief characteristic of the leakage resulting from this phenom- 

 enon is its variation with frequency. Approximately, it increases 

 directly with the frequency. In the voice range its magnitude is gen- 

 erally negligible, but may become appreciable in the carrier range, 

 particularly at the upper end. 



Item C, like item B, increases in wet weather, but to a far less degree. 

 This increase, which is brought about by the enlargement of the field 



m 



^ 



.08 



.06 



10 



g .04 



O 



a. 

 o 



' 02 



o 

 < 



< 



UJ 



10 20 30 



F REQUENCY- KILOCYCLES 



40 



50 



Fig. 14 — Variation of (C) with frequency for standard D. P. design. 



caused by the wetting of the insulator surfaces, cannot be directly 

 separated from the increases in the several other sources. Indirectly, 

 however, a general idea of the magnitude of item C in wet weather can 

 be obtained in the following two ways. 



Both methods are based on the observation that the increase in 

 capacitance between the wire and pin produced by metal-coating the 

 outside of the insulator is invariably greater than the corresponding 

 increase caused by wetting the uncoated insulator. For example, the 

 increase due to coating the insulator of Fig. 3 exceeded the maximum 

 increase observed for any rain by at least 40 per cent. More commonly 

 this figure would be 100 per cent both for this and other shapes. 



Therefore, if such metal-coated insulators (mounted on bonded metal 

 pins for reasons appearing in the discussion of items D and E) be meas- 



