172 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



SPARK-OVER 



The sparkin<j^ distance between electrodes has been used for meas- 

 uring' high volta«>es. 



A study ot" spaikin*^ distance is also of considerable theoretical 

 interest and of practical interest in electrical design. It is neces- 

 sary to know the sparking distances in order to supply sufficient 

 clearances. The actual length of the spark for a given voltage de- 

 pends upon the electrodes used and tlie temperature and barometric 

 pressures. 



A given voltage will spark the greatest distance between needle 

 points and the minimum distance between planes and large spheres. 



Fio. % 



This follows because the electrical stress is very unevenly distributed 

 on the dielectric for needles and quite evenly distributed for spheres. 

 For needles, the stress is greatest at the points where a corona 

 brush always occurs at voltages much lower than the sparking 

 voltages. 



Tests at the very high voltages show that the laws and curves 

 established at the lower voltages still hold. This is best illustrated 

 in the curves that follow. 



NEEDLE GAP 



The needle gap spark-ovei- curve is given in Figure 2 up to 

 1,500,000 volts effective. It will be noted that the curve is continu- 

 ous from the low to high voltages. There is considerable variation in 



