THEORY OF THE ELECTRIC ARC 169 



of the current under these conditions would indicate that 

 it is not, but it is doubtful whether any more complete 

 explanation can be given until some method is found for 

 determining the velocity of the negative ions when in the 

 immediate neighborhood of the anode. 



Ordinarily the ions are stopped before reaching the anode 

 and we then have the second set of conditions; namely, 

 those which occur when there is a large amount of ioniza- 

 tion and of luminosity between the electrodes. Under 

 these conditions there is ionization at the anode. If there 

 were not, the drop would be very much larger than it is. 



The amount of the anode drop under such conditions 

 appears to be determined by the potential difference neces- 

 sary to produce ionization at the surface of the anode. 

 That is, the drop increases until it becomes sufficiently 

 large to cause the negative ions by their impact to disrupt 

 the atoms at the surface of the anode. Of these new atoms 

 the positive ones move toward the cathode and by their 

 electrostatic influence keep the potential difference from 

 becoming any larger. 



The phenomenon is much the same at the anode as at 

 the cathode, ionization at the anode being produced by the 

 impact of the negative ions, and that at the cathode by 

 the impact of the positive ones. It is not necessary, how- 

 ever, for the temperature of the anode to be high, while it 

 is necessary that the cathode should be very hot. More- 

 over at the cathode there is a production of negative ions 

 at the surface and of positive ones at a short distance from 

 it, while the positive ions produced at the anode play no 

 part in the production of negative ones. 



This view is in harmony with the fact that the anode 

 drop is nearly independent of the amount of the current. 



