A STUDY OF TELEPHONE LINE INSULATORS 713 



ured in dry weather the leakage conductance so found is Hkely to ex- 

 ceed by a substantial amount the wet-weather value of item C. 



Another procedure consists in calculating the leakage of the metal- 

 coated insulator. This method requires a knowledge of the wire-to-pin 

 capacitance and the phase angle of the insulator material. 



Fig. 14 shows the measured value for the insulator of Fig. 1 when 

 molded from a clear flint glass. The phase angle of this particular glass 

 is not known. However, the wire-to-pin capacitance is known. Using 

 this known value, the leakage has been calculated for two measured 

 values of phase angle for flint glasses, between which the value of this 

 particular glass probably lies. 



Similarly, the measured and calculated leakage are shown for this 

 same design of insulator when molded from a borosilicate glass of 

 known phase angle. 



Neither method is capable of high accuracy, but they need not be 

 for our present purpose of determining the order of magnitude of item 

 C. 



Consider item C at 50 kc. where its relative importance is greatest. 

 At this frequency the metal-coated flint-glass insulator gave a measured 

 leakage which is only about 10 per cent of the total wet- weather leakage 

 of such an insulator as commonly used uncoated. So it appears that 

 even at this high frequency, item C is less than 10 per cent of the total 

 leakage of this particular design and material. This particular sample 

 of flint-glass is not the best of the common alkali glasses nor is it the 

 worst; so this round number of 10 per cent is a fair average value to 

 use for alkali glasses. 



The corresponding value for the borosilicate glass of which the sam- 

 ple was representative is about 4 per cent. 



2. Influence of Design. The absolute magnitude of item C is in- 

 fluenced somewhat by design because of the effect of shape on capaci- 

 tance. An idea of the magnitude of this effect can be obtained by 

 comparing Fig. 14 with Fig. 15. Both designs were cast from the same 

 batch of glass so that material plays a small part in comparing the two 

 designs. 



In general, for a given size of pin, the capacitance is decreased by 

 enlarging the insulator diameter, particularly at the wire groove. 

 Similarly, for a given outside diameter, the capacitance is decreased by 

 making the pin diameter less. In general, too, the shorter the insula- 

 tor, the less the capacitance. 



Through the ordinary range of shapes the absolute magnitude of 

 item C will not vary more than about three to one, and expressed as a 

 percentage of the total wet-weather leakage, it is doubtful if item C, in 



