at the Electrodes in Vacuum- Tube Discharge. 621 



anode as it moves through the Faraday dark space is covered 

 with a thick luminous disk which is simply, so far as lumi- 

 nosity is considered, the positive column shortened. This 

 luminosity should have the same ionizing effect in the Faraday 

 dark space a3 in the positive column itself. 



As previously suggested by the writer*, another factor 

 probably enters to cause a difference in conductivity of the 

 gas along and across the main stream in the Faraday dark 

 space, namely, the higher velocity of the negative ions in 

 this space (imparted to them by the drop of potential at the 

 cathode) over that maintained by them in the luminous 

 column. In the luminous column the current is maintained 

 solely by the electric intensity at the point considered. If, 

 in the Faraday dark space, it is maintained partially, or 

 wholly, by the inertia of the ions driven into it by forces 

 without, the electric intensity necessary in that space will be 

 correspondingly smaller. At the same time, owing to the 

 motion of the ions along the stream, the conductivity across 

 the stream will be less (for the same ionization) than at points 

 where the velocity of the ions is less. 



We may explain in the same way the difference in the 

 drop at the anode in the two spaces referred to : Other con- 

 ditions being the same, the greater the velocity of t/ie dis- 

 charging ions the greater the drop at the anode. If the velocity 

 of the discharging ions be zero, as in the highly ionized gas 

 of the negative glow, the drop at the anode vanishes. It 

 will be seen that a number of phenomena may be explained 

 from this standpoint. 



The effect of increased ionization in decreasing the drop at 

 the anode may in this case be explained as due to a decreased 

 velocity of the discharging ions, in that, for the same current, 

 an increase in the number of ions in unit volume necessitates 

 a proportionate decrease in their velocity. 



Further observations on the drop at each ele3trode as the 

 anode is moved into the cathode dark space indicate, that, at 

 the cathode as at the anode, the drop of potential between 

 metal ami gas may bo attributed to the velocity of the dis- 

 charging ions. In Tables II. and III., values of the drop 

 at both electrodes under the same conditions are given. The 

 exploring wire was placed as near the stationary electrode as 

 possible. By passing the current first in one direction, then 

 reversing it, the drop at anode and at cathode could be 

 successively and rapidly measured. In Table II. the anode, 

 placed in the negative glow and possessing a drop of 1*2 volts, 

 bad no effect on the drop at the cathode ; but when moved 

 * Phil. Mag. December 1900, p. 563. 



