260 



ELECTRICAL MEASUREMENTS 



THE SPARK GAP METHOD OF MEASURING HIGH PEAK 

 VOLTAGES 



It is difficult to design indicating instruments for directly deter- 

 mining extra high voltages because of corona effects, disruptive 

 discharges, and extraneous electrostatic attractions. Also in 

 testing the dielectric strengths of insulations it is desirable to 

 know the maximum rather than the effective voltage to which 

 any sample is subjected. Consequently a method of meas- 

 urement depending on the dielectric strength of air has been 

 developed and has been employed for many years. The neces- 



FIG. 146. Needle-point spark gap. 



sary apparatus is termed a spark gap and its use as a means of 

 determining high voltages is sanctioned by the American Institute 

 of Electrical Engineers 3 . This method is frequently used in 

 acceptance tests of new apparatus where the dielectric strength 

 of the insulation is guaranteed. 



The construction of a needle-point spark gap is shown in Fig. 

 146. According to the standardization rules of the American 

 Institute of Electrical Engineers, "the spark points should con- 

 sist of new sewing needles supported at the ends of linear con- 

 ductors which are each at least twice the length of the gap. 

 There should be no extraneous body near the gap within a radius 

 of twice its length." 



The arrangement is such that the points of the needles may 

 be set at any desired distance apart. The function of the carbon 



