Curves of the Oscillating Spark. 713 



beginning of tbe spark, before the current Las risen appre- 

 ciably, the p.d. falls from the initial sparking value with great 

 rapidity to about 35 volts ; and this value is maintained 

 constant throughout the first oscillation while any measurable 

 current is passing. In the subsequent oscillations, for a 

 very short period while the current is changing sign, a rapid 

 rise of p.d. to about 300 volts occurs, followed by a fall to 

 35 volts again before the current has become of measurable 

 strength. These two characteristic potential differences will 

 be referred to subsequently as the " arc " (35 v.) and the 

 "glow" (300 v.) p.d's. 



Figures intermediate between the spiral and the cross can 

 be obtained by connecting Y to I, i. e. between the two 

 inductance-coils L x and L 2 . (See fig. 5, PL XX.) By 

 diminishing L 2 these become transformed more and more 

 into the cross of fj£. 4, so that there can be no doubt about 

 the interpretation of the latter as being the form charac- 

 teristic of the gap when any inductance p.d. due to the 

 varying current is eliminated, 



A large number of observations with capacities varying 

 from '02 to *002 inf., and inductances varying from 500 to 

 3 mcrh., all showed the same general features. The variations 

 in the magnitude of the p.d*s v both the arc and the glow, are 

 surprisingly small with the alterations in the conditions. The 

 arc p.d. shows a slight increase with the increase of the 

 spark-length j and probably a slight decrease at the highest 

 frequencies attainable, while the glow p.d. is practically 

 independent of both these conditions. 



Rosehansky concluded from his investigations that as the 

 frequency is increased the spark tends to behave as a pure 

 resistance *. In my earlier observations I found that at very 



Fig. 4. 



"PotenZ/ial 



"7" 



high frequencies the figures always took the form shown in 



fig. 4. This is obviously not a resistance effect, since the 



* Phys. ZeiUch. ix. p. 627 (1908). 



