at the Electrodes in Vacuum-Tube Discharge* 629 1 



The intersection of each line with the axis of ordinates 

 gives the value of the drop, for the pressure given, below 

 which the current can no longer be maintained. This value 

 increases more and more rapidly with decreasing gas-pressure. 

 For very low pressures, at which, according to the observa- 

 tions of Graham (I. c.) the potential-gradient in the gas is very 

 small, and where the drop at the anode is small compared 

 with that at the cathode, the above equation should give the 

 minimum difference of potential required to maintain a 

 current. For i = the calculated value of V is given for 

 different pressures* : — 



p- 



V. 



General 

 Quality of Discharge. 



millim. 

 10 



01 



001 



o-ooi 



volts. 

 280 



610 



3900 

 37000 



Unstriated. 

 Striated or dark. 

 Cathode rays. 

 Eontgen rays. 



The writer has not been able to find any direct observations 

 with which to compare these results. They are in keeping 

 with observation in that, as the pressure decreases without 

 limit, the difference of potential required to produce a current 

 increases without limit. It is probable therefore that the 

 very large potentials required to produce a discharge in high 

 vacua are of the form of a drop at the electrodes. As the 

 pressure of the gas decreases, the drop of potential at the 

 cathode, for the same current, increases. This might be due 

 to a decreased density of ionization causing increased velocity 

 of impact, or, it might possibly be due to an increased difficulty 

 in discharging, in that, as noted before, the presence of a 

 large number of molecules in a given space would shorten 

 (by increasing the number of collisions per second) the time 

 required to bring the ion to a minimum velocity. 



Effect of the Area of the Cathode on its Drop. — Experiments 

 were tried to determine if the rate of increase of potential 

 with the current be simply dependent on the current-density. 

 For this purpose a first experiment with two steel electrodes, 



* The constants from which these values are deduced are liable to 

 considerable variation, so that these results represent at best a rough 

 approximation to the actual values. 



Phil Mag. S. 6. Vol. 2. No. 12. Dec. 1901. 2 T 



