718 



BELL SYSTEM TECHNICAL JOURNAL 



A knowledge of the electrical equivalent of the crossarm, together 

 with the insulator capcitaace, permits E to be calculated. 



The constants of the crossarm between steel pins six inches apart, as 

 measured on a dry day, are given in Fig. 17. From this measurement, 

 the magnitude of E has been calculated for individual insulator capaci- 

 tances of 4 and 20 m. m. f., and the results are shown in Fig. 18. The 

 capacitances of most of the insulators studied lie within these two 

 limits. 



Fig. 18 also shows the value of E as measured for the insulators of 

 Fig. 19. This measured value of E was obtained as follows: The leak- 



Fig. 19 — British post office design. 



age of the insulators was measured first with the crossarms short- 

 circuited by tying a wire from each pin to the other. Then the wires 

 were removed and the measurement repeated. The difference between 

 these measurements is E, if other sources are assumed to have remained 

 unchanged. This assumption appears justified from the following test 

 of its validity. The measurement of E, together with the measured 

 constants of the crossarm, enables the insulator capacitance to be cal- 

 culated. Such a calculation has been carried out and the value of 

 capacitance so obtained checks very closely the value obtained by 

 direct measurement. 



