at Low Pressures. 395 



When making a measurement of the Hall Effect the stopper 

 G was first rotated until the electrometer indicated that E and F 

 were at the same potential. The magnetic field was then applied 

 and reversed, and the change in the P. D. between E and F on 

 reversing the field was measured by the electrometer. 



The results obtained are shown in the accompanying diagrams 

 Nos. 1, 2, 3 and 4 (pages 396, 397). An examination of the 

 curves in these diagrams shows that the variation of the Hall 

 Effect along the discharge is very similar to that of the electric 

 intensity 1 . 



In the previous paper it is shown that in a uniform part of 

 the discharge 



Z=±HX{h 2 -h). 



According to this equation Z is proportional to X so that the 

 curves for Z and X would be exactly similar if this equation held 

 true in all parts of the discharge. 



In the striated positive column the Hall Effect falls to an 

 almost zero value between the striae, whereas the electric intensity 

 only falls to a comparatively small extent. 



It appears therefore that the difference between k 2 and k ± in 

 the dark spaces between the striae is very small. It is probable 

 that when a molecule is ionised a free electron or negative 

 corpuscle is split off so that the negative ion is of very small 

 mass compared with the positive ion ; k. 2 — & 2 is therefore at first 

 very large. After a short time however the negative corpuscle 

 probably becomes attached to a neutral molecule, so that the two 

 ions are then of almost equal mass and k 2 — k x is very small. It 

 seems therefore probable that ions are formed largely in the 

 striae, and that by the time they get into the dark spaces, the 

 negative ions which were corpuscles at first have become attached 

 to molecules. 



This view is in agreement with the idea that each stria and 

 its adjacent dark space constitute a to some extent independent 

 portion of the discharge analogous to that formed by the negative 

 glow and Faraday dark space. 



In conclusion I wish to say that my best thanks are due to 

 Prof. J. J. Thomson for his kindly interest and advice during the 

 carrying out of these experiments in the Cavendish Laboratory. 



1 For curves showing the variation of the electric intensity see H. A. Wilson, 

 Phil. Mag., June, 1900. 



