

Curves of the Oscillating Spark. 721 



stationary vapour o£ some form of luminosity caused by the 

 discharge current. Similar work of my own * resulted in a 

 conclusion more in accordance with the original view of 

 Schuster and Hemsalech, that each streamer is the drawn-out 

 image of a puff of luminous vapour given off from the 

 electrode at each oscillation and proceeding at a high rate 

 towards the centre of the gap. It was found that the metal 

 vapour is in many cases (if not in all) electrically charged, 

 and it is thus in addition to diffusion impelled towards the 

 centre of the gap by the electric field of the s~:>ark. 



If we accept this view of the constitution of the streamers, 

 the results of the present paper go to show that the electrical 

 propulsion of the vapour is not likely to account for very 

 much of the velocity which the streamers exhibit, for the arc 

 p.d. which lasts throughout much the greater part of the 

 oscillations is a comparatively small one, and the glow p.d., 

 which might be expected to produce an effect if it lasted, is 

 of excessively short duration. We must consequently infer 

 that the main cause of the production of each streamer is the 

 high temperature to which the surface of the electrode is 

 raised by the impact on it of the ions which carry the current 

 across the gap. The theory of the mechanism of conduction 

 in the spark given above now enables us to explain the often 

 noted but unexplained fact that the streamers do not show with 

 equal intensity at each electrode, but that in successive oscil- 

 lations they are alternately more prominent at one electrode 

 than at the other. The more prominent streamer is thus 

 always associated with an electrode of the same sign, which 

 according to the observations of Schenckf, Walter J, Royds, 

 and Milner, is found to be negative. Now we have seen 

 that in consequence of the different penetrating powers which 

 we must ascribe to the positive and negative ions, combined 

 with the rapidity of the oscillations, the characteristic feature 

 of the spark which distinguishes it from the arc is the cora- 

 parativeby low temperature of the surface layer of the 

 instantaneous anode. The strength of the streamer is 

 conditioned by the temperature of the surface layer only of 

 the electrode, and it should therefore be greater at the 

 instantaneous cathode, as is found to be the case. 



August 8th, 1912. 



* Phil. Trans. A. ccix. p. 71 (1908). 

 f Astrophys. Journ. xiv. p. 116 (1901). 

 X Ann. d, Thys. xxi. p, 223 (1906). 



