^2 Mr. J. D. Morgan on Impulsive 



result when allowance is made for the variation, of H from: 

 day to day, is an indication of the reliability of the method. 

 Check experiments with a not very reliable Kew Magneto- 

 meter instrument gave the same value of H. 



In conclusion I again wish to express my thanks to 

 Dr. Hicks, who first of all suggested the use of the principle 

 involved in the Solenoid method. 



Dacca College. 

 7 tli August, 1920. 



XL. Impulsive Sparking Voltages in small gaps. 

 By J. D. Morgan, B.Sc * 



IN a paper entitled " Time Lag in the Spark Discharge"' 

 (Phil. Mag. vol. xxxvii., August 1919) Dr. Norman 

 Campbell discusses the fact that the impulsive sparking 

 voltage is in certain gaps greater than the static sparking 

 voltage. As explained in that paper, when the voltage 

 applied to a gap is caused to rise rapidly from zero, the 

 value it will attain before sparking occurs is greater than 

 that reached by a voltage which is applied gradually. The 

 ratio of these two values has been termed by Peek the 

 " impulse ratio/' and this expression is now generally adopted. 

 It is commonly believed that two conditions other than 

 voltage are involved in the process of spark production, 

 namely, time and initial ionization. Campbell recognizes 

 these two conditions, and shows how they can be used to 

 explain why the impulsive sparking voltage of a gap is 

 greater than the static voltage, or in other words, why the 

 gap has an impulse ratio greater than unity. Arguing 

 from his own and Peek's investigations, the conclusion 

 reached by Campbell is that there are two kinds of lag, 

 a regular and an irregular one, in the process of spark 

 production, though it is apparent that he sees a probable 

 connexion between them. 



Taking Campbell's mode of presenting the facts, there 

 would appear to be justification for distinguishing between 

 regular and irregular lags, but it is questionable whether 

 this manner of regarding the subject is likely to lead to the 

 most useful practical results. In the writer's opinion it is 

 more convenient to assume only one lag, and to regard the 

 sparking voltage of a given gap as dependent jointly on the 

 three variable conditions which have hitherto been recog- 

 nized. These conditions are (1) rate of rise of voltage, 

 * Communicated by the Author. 



