the Faraday Dark Space of Vacuum- Tubes. 573 



about *4 mm. the current at which the transformation took 

 place was smaller than in this case, while at lower pressures 

 Only the striated condition was observable. These observa- 

 tions are of possible interest in connexion with the theory of 

 J. H. Jeans*, who deduces the curve for a striated discbarge 

 from the conditional equations of Prof. J. J. Thomson 

 {he. cif.) under the assumption that the density of the ions 

 shall remain finite. 



The explanation, given above, of the difference in conduc- 

 tivity along, and at right angles to, the direction of the 

 discharge may be briefly applied to the striated discharge. 

 W'kcm found the curve of conductivity at right angles to 

 possess maximum values in the luminous parts and minimum 

 in the dark spaces, which is exactly opposite to the curve of 

 conductivity along the discharge (as given by the potential 

 gradient), where the dark spaces possess maximum conduc- 

 tivity. Thus we find maximum and minimum for one 

 direction, coinciding respectively with minimum and maxi- 

 mum taken at right angles thereto. 



We may assume that the ions possess a zero velocity at 

 right angles to the direction of the discharge. Then, sup- 

 posing the degree of ionization constant, the regions of 

 maximum velocity along the discharge will be, due to this 

 velocity, regions of maximum conductivity along, and of 

 minimum conductivity at right angles. 



If we suppose the velocity of the ions constant throughout, 

 then regions of maximum conductivity in both directions will 

 coincide with regions of maximum ionization, and with each 

 other. The latter condition is impossible because contrary to 

 experimental results. If we suppose maximum ionization in 

 the luminous and minimum in the dark spaces, the velocity 

 in the latter must be sufficient to compensate for the minimum 

 ionization there, so that the conductivity along the discharge 

 may be greater in the dark spaces, as the gradient requires. 



If, lastly, we suppose, according to Prof. Thomson, maxi- 

 mum ionization in the dark spaces, the velocity there must be 

 sufficient to produce minimum conductivity at right angles. 



All three possible cases require a greater velocity of. the 

 ions in the dark spaces than in the luminous. This is of 

 interest in connexion with the theory of Spottiswoode and 

 Moulton t, that the current is transmitted by successive dis- 

 charges across the dark spaces, from which a greater velocity 

 of the ions in these spaces would logically follow. 



* J. H. Jeans, Phil. Mag. xlix. p. 245 (March 1900). 



f Spottiswoode & Moulton, Phil. Trans. 1879, part 1, p. 201. 



