LIGHTNING PEEI^ 



191 



long there may be a considerable variation of this time without an 

 appreciable change in the spark-over voltage- The continuously 

 applied (GO-cycle, or r/— c, where heating does not occur) spark-over 

 voltage is the lowest voltage at which spark over can take place. 



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7//T7e Af/cro- seconds fro/r? Zero to max a/ong sine ^o^e. 



Pio. 30. — Time and voltage to spark over needle ga[)s with a sine wave impulse 



Waves (1) and (2) (fig. 31) show actual wave shapes used in ob- 

 taining data given in Figures 25 and 26. Wave (1) is equivalent 

 to a single half cycle of a sine wave; wave (2) rises to a maximum 



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K|7. 31. — Continuously applied and impulse voltages to just cause spark over 

 of a 10-cm. gap between needles 



at the same rate as wave (1) but has a flat top, or is of much longer 

 duration. Of the two waves shown in Figure 31 the voltage required 

 to spark over a given gap is higher for the one of shorter duration 

 or for wave (1). In this figure, where actual test data are given 

 for a 10-cm. gap between needles, spark over results at 180 kv. for 



