176 THE ELECTRIC ARC 



that both of these causes may aid in making it easier for 

 electrons to ionize by impact. 



We may then sum this up by saying that the direct cause 

 of ionization in the arc is the impact of electrons on the 

 molecules of the gas and that this ionization is aided either 

 by the high temperature of the arc, or by the presence of 

 some radiation, such as ultra-violet light, or by both of 

 these. 



An explanation has been given by Sir J. J. Thomson 1 

 for the large discharge passing from hot calcium oxide 

 when in a vacuum, in which he assumes that repeated bom- 

 bardment of molecules by electrons will break the molecules 

 apart, even when one impact would be quite unable to do 

 so. Evidently the same explanation could be given for the 

 ionization in the arc. There are, however, serious ob- 

 jections to this explanation in the case of discharge from 

 hot calcium oxide 2 and until further evidence is found in 

 favor of that view it would seem to be a less probable 

 explanation for the ionization of the arc than that which 

 has been suggested. 



Velocity of the Ions. The current through the arc 

 is carried very largely by electrons. These move much 

 more rapidly than positive ions, since their masses are 

 much smaller, while their charges are the same. We do 

 not, however, have any data as to the exact values of the 

 velocities of either the positive or negative ions in the arc. 

 As far as the carbon arc is concerned we do not have any 

 experiment showing definitely whether the positive or 

 negative ions move the more rapidly. There have been 

 several different methods used for the purpose of deter- 

 mining this question, but none of them is entirely satis- 



1 Nature, 73, 495; 1906. 2 Phys. Rev., 32, 492; 1911. 



