MEASUREMENT OF POTENTIAL DIFFERENCE 261 



resistances placed vertically above the supporting pillars is to 

 limit the current when the gap breaks down. The establishment 

 of surges in the circuit due to a sudden change in its constants is 

 thus avoided. Water-tube resistances are more reliable for this 

 purpose than carbon rods, which may have low resistances at 

 high voltages. 



The current after the gap has broken down should not be 

 greater than 1 amp. The needles are set in accordance with 

 the table given below. 



To use the gap, it is set to correspond with the appropriate 

 voltage and placed in parallel with the apparatus under test. 

 The applied voltage is then gradually raised until the gap breaks 

 down. The reading of the voltmeter on the low-tension side 

 of the testing transformer and the adjustment of the voltage 

 regulating apparatus at the instant of breakdown are noted. A 

 new set of needles is then inserted and the gap set for a voltage 

 about 20 per cent, too high. The former reading of the voltmeter 

 or the adjustment of the regulating apparatus is then repro- 

 duced and the voltage applied for the required time. 



The gap breaks down at the peak of the wave and therefore 

 gives a measure of the maximum voltage to which the insulation 

 has been subjected. It will do this irrespective of the wave 

 form, but the following table is for sinusoidal waves and effective 

 voltages. If the gap is to be set for peak voltages, the values in 

 the table must be multiplied by \/2. 



TABLE I. NEEDLE-POINT SPARK-OVER VOLTAGES WITH No. 00 SEWING 



NEEDLES 

 (At 25C. and 760 mm. barometer relative humidity 80 per cent.) 



The American Institute of Electrical Engineers sanctions the 

 use of the needle-point spark gap for voltages between 10 and 

 50 kv. 



