MEASUREMENT OF POTENTIAL DIFFERENCE 267 



In using any form of spark gap it is essential that all chance 

 of accidental circuit variations be eliminated. If this is not done, 

 the observer may be misled by the breaking down of the gap due 

 to high-voltage oscillations. The sphere gap is especially 

 susceptible to circuit variations; consequently all accidental 

 spark discharges from the testing circuit must be avoided. 



The following table from the Standardization Rules of the 

 American Institute of Electrical Engineers is based on the ex- 

 perimental work of Peek, Chubb and Fortescue. 



SPHERE-GAP SPARK-OVER VOLTAGES 

 (At 25C. and 760 mm. barometric pressure) 



Sparking distance in millimeters 



The values in the above tables are corrected for temperature 

 and barometric pressure as follows. To find the spacing at 

 which it is necessary to set a gap to spark over at some re- 



