192 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



wave (1), while for wave (2) spark over results at 104 kv. The 

 spark over at CO cycles is 75 kv. The time that wave (1) is abov^e 

 the continuously applied breakdown voltage is 0.95 microseconds, 

 wave (2), 2.70 microseconds. The impulse and continuously applied 

 needle gap si)ark-over curves are given in Figure 2G. An examina- 

 tion of Figure 2G shoAvs that the impidse spark-over voltage of 

 needles is always higher than the continuously applied spark-over 

 voltage, for a 10-cm. gap is 180 kv., or 2.4 times the GO-cycle or con- 

 tinuously applied voltage. 



The time to spark over a gap varies with the spacing and shape of 

 the electrodes. Foi- a given GO-cj^cle voltage setting the time re- 



Fio. 33. — Target made by lightning. No ni-dle 



quired to form a spark is greatest for gaps between points and least 

 for gaps between well-rounded surfaces. For .spheres, the time lag 

 is so small that discharge takes place before the impulse voltage can 

 rise appreciably above the continuously applied or 60-cycle spark- 

 over voltage. This is shown in Figure 25, where the drawn curve 

 is the CO-cycle curve, while the impulse spark-over voltages for waves 

 (1) and (2) are represented by crosses and triangles. 



The needle gap requires the maximum time of any gap, as con- 

 siderable air must be ionized before spark over can result. Another 

 way of considering it is that the corona increases the capacity and 

 places resistance in series with it as it forms. It requires time to 

 charire a condenser through resistance. 



