38 J. E. Moore — Electrical Discharge from the point 



very low, and (a) is the cathode, the discharge stream from (a) 

 covers the whole surface of the electrode. The "dark space" 

 still appears around the electrode at about 3 cm from it on the 

 concave side. On making (h) the cathode, a sharp, very well 

 defined stream is seen to spring from it, and there is no ap- 

 pearance of the " dark space" about the electrode. At this 

 pressure, the area of the electrode (J), covered by the discharge 

 stream, is only a small fraction of the total area of the electrode. 

 The walls of the tube being cylindrical, the stream is of cir- 

 cular cross-section, and springs from the portion of the elec- 

 trode lying immediately around the point of intersection of the 

 axis of the tube with the electrode. As the pressure in the 

 tube is gradually decreased, the area of the electrode covered 

 by the discharge stream from it becomes less and less, until 

 the stream totally disappears. At this pressure, upon making 

 (a) negative, the discharge stream from it still covers a con- 

 siderable portion of the electrode. At very low pressure, dis- 

 charge can be sent through the tube by making (a) the 

 cathode, but cannot be made to pass, with the same potential- 

 difference, when (b) is made the cathode. This experiment 

 was repeated by means of a tube identical with that shown in 

 fig. 12, excepting, in place of electrode (a), is substituted one 

 whose diameter is just equal to the internal diameter of the 

 large tube, and whose radius of curvature is 3*75 cm . The dis- 



13. 



14. 



Sec.AB 



charge stream from the large electrode when it is negative is much 

 more clearly marked than the discharge stream from cathode (a) 

 in the tube shown in fig. 12. This is due to the fact that 

 in the last tube all disturbance arising from discharge from 

 the convex side of the electrode is avoided. The same de- 

 scription of the inequality of cross-section, and final vanishing, 

 at extremely low pressure, of the discharge streams from the 

 two electrodes, when first one and then the other is made 

 cathode, as given for the tube shown in fig. 12,- applies to the 

 discharge streams from the two electrodes in this tube. 



To illustrate the effect of unsymmetrical walls upon the 

 form of cross-section of the discharge stream from the 



