158 THE ELECTRIC ARC 



Causes Producing Ions. Ions thus formed do not con- 

 tinue indefinitely. They quickly recombine and again 

 form uncharged atoms. Thus, to have a continuous supply 

 of ions, there must be a continuous production. Among 

 the causes producing them may be mentioned radio-active 

 substances, Rontgen rays, ultra-violet light, the impact of 

 ions on atoms, chemical action, especially at high tem- 

 peratures, and incandescent solids. Of these, two only 

 will be of interest to us at this time, namely, the impact 

 of ions on atoms, and incandescent solids. 



lonization by Impact. Let us first consider ionization 

 by impact. If ions exist where there is an electric field 

 they tend to move with an accelerated velocity. Under 

 ordinary conditions, they can move but a short distance 

 before colliding with an atom or molecule of gas. If the 

 atom or molecule is hit with sufficient velocity, it is broken 

 into parts which are charged and which are indeed new ions. 

 Thus, cathode rays and the electrons produced by ultra- 

 violet light or incandescent solids ionize the gas through 

 which they pass and render it conducting. It has been 

 shown by Townsend that positive ions will produce the 

 same effect. 1 



The velocity of an ion at any instant depends on the 

 force which has been acting on it and upon the distance 

 covered since its last collision, or in other words upon the 

 potential difference through which it has passed since it 

 was at rest. Consequently, to have sufficient velocity to 

 ionize by impact, an ion must pass through a certain 

 potential difference. If the mean free path is too short, 

 or the electric force too small, no effect will be produced. 

 The measurements made by Townsend indicate that the 



1 Phil. Mag., (6), 6, 598; 1903. 



