1909] on Electrical StrkiUons. 585 



If, however, the force increases so that the corpuscles produce ions 

 by collision quite a different state of affairs will occur ; suppose that 

 when the corpuscles get to a place P, their velocity is sufficient to 

 produce ionization. On the anode side of P positive and negative 

 ions will be produced, the positive ones will crowd towards P, the 

 negative ones will move away from it ; the consequence will be that 

 there will be an excess of positive electrification on the anode side 

 of P : now positive electrification implies a diminution in the electric 

 force as we move towards the anode, thus the electric force will fall. 

 When it has fallen below the value required for ionization the nega- 

 tive electricity will as before begin to accumulate in the front of the 

 stream, and the electric force will again increase to the value required 

 for ionization when the process will be repeated. We shall in this 

 way get a periodicity in the electric force such as is observed in the 

 striated discharge. Thus on this view the concave side of the bright 

 head of the striation acts as a cathode, the corresponding anode being 

 the convex side of the bright head of the adjacent striation on the 

 anode side. Between these two places we have a complete discharge, 

 forming a unit by the combination of which the whole discharge is 

 built up. The ions which carry the current through any unit are for 

 the most part manufactured in the units themselves, so that these 

 units will behave, as Goldstein and Spottiswoode and Moulton have 

 observed the striations to behave, as if they were to a considerable 

 extent independent of each other. The effect of pressure on the 

 distance between the striations can easily be understood from this 

 point of view, for the lower the pressure the greater will be the dis- 

 tance which particles projected with high velocity will travel before 

 their velocity is destroyed. Again, the variations in the electric field 

 are due to the accumulation of electrical charges in the tube. These 

 accumulations may be regarded as electrified disks whose cross-section 

 is that of the tube ; the distance from the disk at which these forces 

 fall to a certain fraction of their maximum value will depend upon 

 the diameter of the disk ; the larger the diameter the greater this 

 distance, so that when the diameter of the tube is small the fluctua- 

 tions in the intensity of the electric force will be much more rapid 

 than when it is large, and thus we should expect the striations to be 

 much nearer together in a narrow tube than in a wide one. 



To explain the variations in the luminosity which accompany 

 these fluctuations in the electric field we must consider the variation 

 in the kinetic energy possessed by the positive ions when they recom- 

 bine. The recombination of ions does not in general seem to be 

 accompanied by luminosity, unless the ions possess a definite amount 

 of kinetic energy. We certainly can have a gas with great electrical 

 conductivity, and in which a large number of ions are recombining 

 without any visible luminosity ; it seems as if the ions must have a 

 definite amount of kinetic energy for visible light to be developed on 

 their recombination. Now in the space between two striations the 



2 Q 2 



