1 64 THE ELECTRIC ARC 



need be only great enough, so that the positive ions by 

 their bombardment of the cathode should raise it to the 

 necessary temperature. 



There are, however, objections to this view. The most 

 important of these is that there are many substances which 

 can be used as the cathode of an arc and yet melt and 

 vaporize before a temperature is reached at which negative 

 ions are given off. The most noteworthy example of this 

 occurs with the arc between mercury terminals. Mercury 

 vaporizes, especially in a vacuum, at temperatures far 

 below that at which negative ions pass from a metal. 



In reply to this last objection it was shown by Stark 1 

 that even in the case of mercury there is a point at the 

 cathode which is very hot and at this point there is a con- 

 tinuous spectrum. This fact was also pointed out by 

 Arons. 2 Apparently the rise in temperature occurs so 

 suddenly that the mercury does not become a gas until 

 after the atoms are set in such violent vibration that they 

 give out light. 



If we should assume that this difficulty may be thus 

 explained, there is still the fact that in a vacuum the metals 

 which melt the most easily can be made the cathode of the 

 arc, while those, like iron and copper, which do not melt 

 easily can not be so used. We should expect that iron 

 would give off ions at least as readily as mercury, if the 

 phenomenon depended only on raising the metal to a high 

 temperature. 



There are also certain minor difficulties met with in this 

 explanation. For example, the measurements made by 

 Richardson on the discharge from hot carbon 3 would indi- 



1 Physik. ZS., 5, 750; 1904. 2 Wied. Ann., 58, 89; 1896. 



J Phil. Trans., 201 A, 497; 1903. 



