A STUDY OF TELEPHONE LINE INSULATORS 



727 



Fig. 25 shows a similar estimate for the C, W. design. Here the 

 bonded pin thimbles shield the wood pins from any electric field and 

 thus eliminate dielectric absorption D from the pins. Similarly, by 

 short-circuiting the crossarms, the losses occurring there are eliminated. 

 Accordingly, both items D and E are made negligible and do not appear. 



Of the remaining factors, the direct surface leakage B contributes 

 about 12 per cent of the total (at 30 kc, for example). The losses in 

 the glass C are liberally estimated at about 5 per cent. Finally, the 

 surface losses G contribute over 80 per cent of the total. 



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 FREQUENCY- 



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 KILOCYCLES 



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Fig. 26 — Estimated allocation of leakage for C. S. insulator. 



Comparing the C. W. performance in this test with that of the D. P., 

 we find that most of the improvement shown by the C. W. has resulted 

 from the elimination of items D and E. Due to the single skirt design 

 of the C. W. and the pin thimbles, item B has been increased in magni- 

 tude. The loss in the glass C has been decreased by the use of borosili- 

 cate glass. However, the most important item G has been only slightly 

 reduced, and if C. W. insulators in this test were aged as far as the D. P. 

 the C. W. might show no improvement with respect to G. The pin 

 thimble construction tends to increase the insulator capacitance and 

 thus tends to make G larger for the C. W. than for the D. P. design. 

 The use of a borosilicate glass with its lower dielectric constant counter- 

 acts this action somewhat. 



The estimated division of losses for the C. S. design is given in Fig, 26. 

 The use of metal has eliminated any dielectric absorption D from oc- 

 curring in the pins. The bonding of the pins by wire has eliminated 

 crossarm losses E. 



