714 



BELL SYSTEM TECHNICAL JOURNAL 



the worst combination of poor design and poor material studied, could 

 reach as high as 20 per cent. 



The foregoing remarks apply to insulators on metal pins. When 

 wood pins are used, the capacitance is less and C is correspondingly less. 

 If the pin is dry, C is extremely small ; if the pin is wet, C is still consid- 

 erably less than it is for the same insulator on a metal pin. 



a. 



_i 



D 

 (/) 



Z 



u. 



O 



a 



o 



I 



O 



o 



I 



liJ 

 o 

 < 



< 



in 



10 20 30 



FREQUENCY-KILOCYCLES 



40 



50 



Fig. 15 — Variation of (C) with frequency for standard toll design. 



3. Influence of Material. The range of the absolute value of C for 

 various kinds of glass is much greater than it is for various designs. 

 Phase angle of the material is frequently used as a criterion but Hoch^ 

 has shown that for insulator purposes, the product of phase angle and 

 dielectric constant is a better criterion. Using the latter, a range of 

 more than 20 to 1 is found for the glasses studied. 



From the standpoint of item C alone, on account of its small relative 

 magnitude there is little justification in going to high grade glasses, 

 especially as these are more costly, unless other sources are first reduced 

 sufficiently to make C really important. 



'See "Power Losses in Insulating Materials," E. T. Hoch, Bell System Technical 

 Journal, November 1922, pp. 110-116. 



