A STUDY OF TELEPHONE LINE INSULATORS 



721 



tions, errors caused by the presence of other wires have amounted to 

 several hundred per cent. 



The variation of F with frequency is much the same as E. 



Also, like E, F is less in wet than in dry weather. 



The general remarks made in the discussion of Item E regarding the 

 influence of insulator design and material apply also to F. 



For the well transposed lines used for carrier circuits and for the 

 reasonably well balanced insulator capacitances that the standard 

 construction gives, /^contributes little to the total wet-weather leakage. 



Item G — Displacement Currents Flowing Over Insulator Sur- 

 faces THROUGH High Resistance 



1. General Characteristics. Over the carrier range of frequencies 

 this item is the most important source of leakage in wet weather. It 

 probably contributes more than all other sources combined. On this 

 account, it may not be amiss to repeat here an already known theory 

 which fits the results of the present study fairly well. 



WIRE o " ^AAAAA- 



PIN o 



VWV\A 



n/WW^ 



Fig. 21 — Electrical equivalent of an insulator for the production of (G). 



Consider an insulator, the outside of which is wet. Divide this 

 surface into elements of area and take, for example, one of them near 

 the bottom of the insulator. Assume a small displacement current 

 to flow from the pin to this element through the small capacitance 

 which exists between them. Now for this current to reach the wire, 

 it must flow through the thin film of moisture lying between the element 

 chosen and the wire. This film offers a high resistance to the current, 

 not high enough, however, to seriously limit the current, but, neverthe- 

 less, sufficient to produce heat losses. These losses when integrated 

 over the entire insulator surface become important and qualitatively, 

 at least, account for the characteristics of item G. 



Fig. 21 shows in much simplified form an electrical equivalent of this 

 action. An inspection of this figure will throw some light on the sub- 

 ject. 



For one thing, it is clear that the apparent wire-to-pin capacitance 

 will decrease with increasing frequency. Wet-weather tests invariably 



