498 Prof. Skinner on Conditions controlling the Drop of 



Table II. — Effect of Diameter of Discharge Tube on the 



Drop at a Co-axial Cylinder Cathode. 



Drop at Cathode in Volts. 



Diameters : — 1st Tube ll'O, 2nd 7, 3rd 5*5 mm. 



Gas 



1 





















Pressure 



1. 



3-6 







1-2. 







0-5. 





02. 



(nun.). 



1 

 1st 





















Current. 



2nd 



3rd 



1st 



2nd 



3rd 



1st 



2nd 



3rd 



1st 2nd 



Tube. 



Tube. 



Tube. 



Tube. 



Tube. 



Tube. 



Tube. 



Tube. 



Tube. 

 645 



Tube. Tube. 



050 







... 







605 



573 



0-60 









307 



408 



445 



650 



582 



685 



776 



0-70 



316 



317 



344 









672 



600 



723 





0-80 







345 



... 







695 



630 



732 







0-82 



















... 



950 



9eo 



090 



310 



31G 



348 









755 





755 







1-00 





316 



350 









820 



G75 



795 







1-20 



324 



318 



355 



450 



485 



502 



860 











1-50 



345 























the negative glow in the two smaller tubes, expanding with 

 decreased pressure, touched the walls, while that in the 

 largest remained still unconfined. At 0*5 mm. the curves 

 are all irregular, but interest centres on the fact that the drop 

 for the largest tube is for all currents above that for the one 

 of intermediate size, while at the higher currents it rises 

 above that for both. Now at this pressure (0*5 mm.) the 

 negative glow in the largest tube just begins to touch the 

 walls, and to the expansion of the glow with increasing- 

 current the form of the curves is probably to be attributed. 

 At lower gas-pressures the battery could not maintain a 

 current sufficient for making a complete set of observations, 

 but it was found on decreasing the pressure that at a certain 

 point the cathode drop for the different tubes gave again 

 normal sequence, the smaller tube in each case possessing the 

 greater drop, as shown by the observations taken at 0"2 mm. 

 These results are explicable when we consider as the 

 ionizing agent the negative ion shot forth with a high velocity 

 from the immediate vicinity of the cathode. The ionization 

 from which the cathode draws its current is largely produced 

 in the negative glow. Beyond the negative glow the ion 

 does not possess sufficient velocity to further ionize the gas. 

 If now in its course the ionizing particle impinges on the 

 wall of the tube it must rebound with a loss of energy. 

 With an incident velocity sufficiently large it should, after 

 impact on the wall, possess a velocity such as to produce 

 ionization, as is probably the case in the cathode dark space ; 



