716 BELL SYSTEM TECHNICAL JOURNAL 



To the extent that metal coating gives greater capacitance than does 

 wetting the insulator the value of D is magnified by these measure- 

 ments. On the other hand, to the extent that the capacitance is 

 reduced by the high quality of glass, the measurements give too low a 

 value of D for alkali glasses. 



As these effects approximately balance each other, the measured 

 values are fairly representative of D for the alkali group. 



The difference in the two values measured at different times is mainly 

 due to the moisture content of the pins, the higher value corresponding 

 to the higher moisture content. 



Consider the insulator of Fig. 1 made of alkali glass and mounted on 

 wood pins. Item C will be small, very small when the pin is dry; and 

 while appreciably larger when the pin is wet, it is still considerably 

 smaller than its value for a metal pin of the same dimensions. D, on 

 the other hand, is large, even when the pin is dry; so in this example, 

 D may be said to be considerably more important a factor than C. 



In the case of this same design molded from a borosilicate glass, D 

 becomes relatively even more important a factor. 



2. Influence of Design. The general remarks which applied to 

 C as to insulator diameter and length and as to pin diameter apply to D. 



3. Influence of Insulator Material. For a given design, the lower 

 the dielectric constant of the insulator material, the lower D will be. 



4. Influence of Pi?i Material. Some study of this subject has been 

 made, but the results are not sufficiently conclusive to report. 



Item E — Displacement Current Losses in Crossarms 



1. General Characteristics. This item will first be considered as it 

 applies to insulators on metal pins. The path of the displacement 

 current is as follows : the current passes from one line wire through the 

 capacitance of the one insulator to its pin, thence through the crossarm 

 to the other pin and finally through the capacitance of the other insu- 

 lator to the other wire. The capacitances of the two insulators are thus 

 in series. The adjective "displacement" has been applied to the 

 current because its magnitude is mainly determined by the insulator 

 capacitance. 



The losses produced in the crossarm by this current obviously depend 

 on the electrical equivalent of the crossarm and on the insulator ca- 

 pacitance. If the former were a pure resistance with a magnitude small 

 compared with the reactance of the series capacitances, then E would 

 increase approximately as the square of the frequency. Experi- 

 mentally, E is generally found to increase at a rate lying between the 

 first and second power of the frequency over the range studied. 



