go DK. S. R. MILNER ON THE NATURE OF 



for instance, the bismuth and the mercury streamers in figs. 14 and 29. The bismuth 

 streamers spread out fanwise from a point, while the mercury ones converge to a point 

 from a broad base.* The velocities also vary somewhat erratically in different sparks 

 with the same metal. Consequently, while numerical values of the velocities are 

 given in the notes on the individual photographs, it would be inadvisable to lay very 

 much stress on them as representing more than very roughly any absolute values 

 characteristic of the metals. They are, however, sufficient to admit some conclusions 

 to be drawn from them. It may be observed that these difficulties do not enter into 

 the comparison of the velocities of the streamers corresponding to the various lines 

 of the same spark, for which the photographs are well adapted. 



In considering the velocities a distinction has to be drawn, as was first pointed out 

 by ScHENCK,t between the velocity with which the front of the metallic vapour 

 approaches the centre of the spark gap and that of the vapour in the subsequent 

 individual streamers. Thus, taking as an example fig. 26, it will be seen that the first 

 streamer goes only a little way towards the centre of the gap, and then its velocity 

 falls off very rapidly, as is shown by the sudden way in which it curls round and stops 

 short. The next streamer goes a little further, and it is not until after two or three 

 oscillations that the successive streamers become fairly uniform. The velocity with 

 which the front of the metallic vapour approaches the centre of the spark, which is 

 that represented by the locus of the extremities of the streamers, is thus very much 

 less than that of the streamers in the earlier parts of their course. This effect is 

 shown more or less by all the photographs. The probable explanation of it is, I think, 

 that the puff of vapour in the first streamer is rapidly stopped by the resistance of the 

 air present between the poles. The velocities of the streamers quite close to the 

 electrodes are, however, very big, and it is quite possible that they are greater than 

 the molecular velocity of the surrounding air. If this is the case, each puff of vapour 

 will leave a region of smaller pressure behind it, and into this the vapour of the next 

 streamer can be propelled with comparative freedom from friction, and so be propa- 

 gated further towards the centre of the gap. 



This behaviour of the streamers makes it necessary also to distinguish between the 

 velocity of an individual streamer in the oscillating spark and the velocity of the 

 vapour in the spark without self-induction. In the inductionless spark the streamer 

 is a composite one formed of a number of superposed true streamers, and unless the 

 discharge is absolutely instantaneous and the superposition perfect the velocity 



* In photographs taken without a slit the shape of the streamers will be affected by any spreading out 

 sideways which the vapour composing them may undergo as it recedes from the pole. This, and the 

 effects of superposition and under exposure which are referred to below, doubtless account for most of the 

 differences observed in the shapes of the streamers in the sparks of different metals. In so far as these are 

 the causes of the different appearances the measurement of the central line of the streamers will give true 

 values of the velocities. 



t ' AstrophysicalJournal,' vol. H, p. 133 (1901). 



