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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



Referring to Figure 21) the spark-over voltage of a given needle 

 gap is ^1, and always practically constant if the time of application 

 is not limited. Spark over may take place after the continuously 

 applied voltage e^ has been on for some time t.^. If a voltage increas- 

 ing at a rapid rate, as represented by A in Figure 29, is applied, 

 spark over will not take place when the continuously applied spark- 

 over voltage e^ is reached, as the time ^^ is required at this voltage. 

 The spark will begin to form when the voltage reaches the value 

 ^1, however. The voltage will, therefore, rise above e^, and spark 

 over will take place after the time to has elapsed and the voltage 

 has risen to e^- When the voltage is applied at a more rapid rate 

 along wave i?, the spark, as before, will begin to form when voltage 

 e^ is reached. The voltage wall continue to rise and reach some 



Time /n /^/cro-secof?c/s 



Fig. 29. — Diagrammatic illustratiou of why the lightning or Impulse spark-over 

 voltage is higher than the continuously api)lied 



value ^3 during the time t^ before the spark over occurs. Thus, on 

 account of the time lag, when voltage is applied at a very rapid rate, 

 as by an impulse, spark over does not occur Avhen the continuously 

 applied breakdown voltage is reached. The voltage "overshoots" 

 this value during the time that rupture is taking place. The excess 

 in voltage is greater, and the time lag less, the greater the rate of 

 application. The time lag for any given gap or insulation has thus 

 not a fixed value but depends on the wave shape of the impulse or 

 rate of application of the voltage. In making a study of such phe- 

 nomena it is necessary to use certain dehnite wave shapes. Figure 

 30 shows the impulse voltage-time characteristic for needle gaps. 

 The impulses used in this test were single half cycles of sine waves. 

 Note that the impulse spark-over voltage is not greatly above the 

 continuously applied or GO-cycle voltage when the time is over 5 

 microseconds; that is, when the time of application is comparatively 



