CHAPTER IX. 



THEORY OF THE ELECTRIC ARC. 



UNTIL the ionic theory of discharge through gases was 

 developed, there was no explanation of the phenomena of 

 the electric arc which was of value. But in recent years 

 several explanations based on this theory have been sug- 

 gested and an outline of these will be given in the following 

 pages. The first attempt in this direction was made by 

 the present writer in ipoo. 1 A few years later a more 

 complete explanation was given by Stark, 2 and a slightly 

 different one by Sir. J. J. Thomson. 3 



All of these .explanations assume that the current is 

 carried by ions and a discussion of them will require us to 

 understand, first, what is meant by ions; second, how they 

 are produced; and third, their effect on the potential 

 gradient between two electrodes, especially in a place where 

 ions of one kind only exist. 



Definition of Ions and Electrons. Certain causes ap- 

 pear to break atoms into parts, one part of each atom being 

 charged positively and one part negatively. These parts 

 have been called ions. They tend to attach themselves 

 to uncharged atoms, and these clusters of atoms are also 

 called ions, so that an ion may be defined as an atom, a 

 part of an atom or a cluster of atoms which has a positive 



1 Phys. Rev., 10, 151; 1900. 



2 Ann. d. Phys., 12, 673; 1903. 



3 Conduction of Electricity through Gases, ist. Ed., p. 416. 



156 



