1 68 THE ELECTRIC ARC 



is through not more than 0.03 volt. If ionization by the 

 electrons coming from the cathode can be produced by 

 anything like this potential difference, then the drop which 

 we actually find at the cathode must be determined by the 

 potential difference needed for ionization with the positive 

 ions and not by that needed for the negative ones. 



Ionization at the Anode. There are two distinct sets of 

 conditions to be found at the anode, depending on whether 

 the cathode rays hit directly on the anode or not. The 

 first conditions exist only when the arc is in a very high 

 vacuum. The negative ions then go directly from the 

 cathode to the anode. There is then no luminosity between 

 the electrodes which is at all comparable with that ordi- 

 narily existing in the arc and there is apparently but very 

 slight ionization of the gas between the electrodes. 



In all probability there is then no ionization at the 

 surface of the anode. Apparently the conditions are 

 identical with those in a vacuum tube with currents such 

 as were studied by Skinner 1 and by myself. 2 In these 

 investigations it was found that the anode drop varied 

 from one or two volts to over one hundred volts. It appar- 

 ently depended on the average velocity of the negative ions 

 when they reached the anode, the anode drop decreasing 

 as the velocity of the ions increased. This is what would 

 be expected if the current near the anode is carried by ions 

 of one sign only as is shown on page 161. 



Whether this is the whole explanation of the anode drop 

 with the arc in a vacuum under very low pressure is, no 

 doubt, open to question. The fact that Stark, Retschinsky, 

 and Schaposchnikoff 3 found that it was nearly independent 



1 Phil. Mag., (6), 2, 637; 1901. a Phys. Rev., 29, 351; 1909. 

 * Ann. d. Phys., 18, 244; 1905. 



