Sparking Voltages in small gaps. 469 



smd in all eases the general relationship shown in figures 

 '2 and 4 was found, the differences always being one of 

 degree and not of kind. Where the gap is formed by- 

 electrodes of relatively large diameter or flat plates, the rise 

 of sparking voltage with increase of primary current is 

 usually small, and, when artificial means are employed to 

 produce copious initial ionization of the £^ap, may be very 

 small. But when pointed electrodes are used the variations 

 are usually large, and with the higher current values are 

 very irregular. The use with poiuted gaps of a third insu- 

 lated point reduces the sparking voltage and regularizes the 

 action of the gap. It also diminishes variation of sparking 

 voltage with primary current. Instances were noted with 

 a flat-plate, an annular, and a three-point gap, in which the 

 sparking voltage was apparently constant and independent 

 of the primary current, but these results were occasional 

 and could not be reproduced at will. No explanation of 

 this exceptional behaviour can be given. The gaps in which 

 the effect was observed behaved normally (i. e. showed an 

 increase of voltage with primary current) at other times. 



It follows from such considerations as those outlined in 

 this paper that the impulsive sparking voltage for a given 

 gap is a quantity depending on three variable conditions. 

 The rate of rise of voltage varies with the kind of apparatus 

 used for producing the spark. With the same apparatus, 

 keeping the circuit conditions constant, it can be varied by 

 varying the primary current. The initial ionization of the 

 gap varies with atmospheric conditions, but can be made 

 more or less constant by artificial means. The element of 

 time varies with the other two conditions and is different 

 for different gaps, as Campbell rightly points out. When 

 the potential gradient in the gap is uniform, the time for 

 which a certain minimum potential must be imposed on the 

 gap may be very small, but it may become relatively large 

 when the potential gradient varies greatly (as in a point-gap). 

 Due to the latter condition two gaps of different form but 

 "having the same static sparking potential and the same 

 initial ionization will have different impulsive sparking 

 voltages, as is well known. 



The subject is one of considerable importance, and has 

 direct practical application in connexion with e. g. research 

 on the ignition of gases by induction-coil sparks, ignition 

 apparatus for internal combustion engines, protective gaps 

 for certain kinds of electrical machinery, and overhead 

 transmission systems subject to rapid electrical impulses. 



Phil. Mag. S. 6. Vol. 41. No. 213. March 1921. 2 I 



