74 DR. S. R. MILNER ON THE NATURE OF 



In their work on the inductionless spark, Messrs. SCHUSTER and HEMSALECH have 

 observed an effect in the case of cadmium which is apparently somewhat similar to 

 the "above. Some of the metallic lines showed a wavy outline, which presumably 

 indicated that the vapour had a velocity which alternately decreased and increased as 

 it receded from the pole. This led them to suggest in 1900 the possibility that the 

 metallic vapour was charged and carried the electric current, but the effect was not 

 sufficiently marked in their photographs to enable them to make any definite assertion 

 in this respect. SCHENCK later, in 1901,* showed that the streamers could not be the 

 actual carriers of the current, as they do not proceed more than a short distance 

 across the spark gap before the electrical discharge becomes reversed. 



My photographs are therefore important, as the fact that the luminous vapour 

 actually come* hack towards the pole when the electric field of the spark reverses its 

 direction forms so much definite evidence that the particles of which it is composed 

 must be charged. This suggests that modifications may be called for in our view of 

 the nature of the streamers. While one view of their constitution is that they are due 

 to purely thermal diffusion and incandescence of the vapour of the electrode metal, 

 another one possible is that they are mainly produced by the luminescence of charged 

 atoms suddenly torn off from the surface of the electrode, and propelled towards the 

 centre of the spark gap by the intense electric field of the spark ; and the photographs 

 support the second of these conceptions. This view has the advantage of relieving us 

 of the necessity of making any hypothesis as to the temperature of the spark, for it is 

 not difficult to imagine that the sudden release from strain which would occur as the 

 atoms are torn off from the surface of the metal might give rise to the vibrations 

 which correspond to the characteristic metal lines of the spark, apart from any 

 thermal incandescence of the vapour. It is clear, however, that the electric force of 

 the spark is not by itself capable of tearing off the atoms from the surface of the 

 electrode, for the force exists before the spark begins to pass. But when the discharge 

 has actually begun the metal surface is being bombarded by the ions, which carry the 

 current, and this bombardment also is doubtless an essential factor in the disruption 

 of the surface and the production of the luminescence. This view perhaps suggests 

 a reason why the streamers are so much more vigorous at the negative than at the 

 positive electrodes, if we suppose that the positive ions which carry the current of the 

 spark are more efficient in causing disruption of the surface than are the (probably 

 smaller) negative ones. Such a result would at any rate be in accord with their other 

 known capabilities as regards ionisation. 



Effect of a Magnetic Field ott, the Streamers. 



In order to test these points still further, I made some observations on the effect of 

 a magnetic field on the course of the streamers. A strong magnetic field was arranged 

 at right angles to the length of the spark, and so that its direction coincided with 



* LOK. at. 



