1 62 THE ELECTRIC ARC 



drop in potential in a region where there is only one kind 

 of ions. If this drop is sufficiently large, the positive ions 

 will attain high velocities, and by impact on the boundary 

 surface at C they also will ionize. We shall then have a 

 two-sided action. The negative ions coming from C will 

 ionize in the space ac and the positive ions from ac will 

 ionize at C. 



As the negative ions thus formed begin to fill the space 

 Cc, the drop in potential will become smaller. If, however, 

 it becomes small enough, ionization at C will cease and 

 the potential difference will again become greater. For a 

 condition of equilibrium the potential difference must be 

 large enough so that positive ions will ionize at C. 



Ionization at Surface of Anode. We find that the phe- 

 nomena at the anode are similar to those at the cathode. 

 The essential difference is that the negative ions ionize by 

 impact much more easily than the positive. The drop in 

 potential at the anode is consequently less than that at the 

 cathode. If no ions came from the anode, there would be 

 a large drop in potential near it, as in the corresponding 

 case with the cathode. This would cause the negative ions 

 to move with great velocity and their bombardment of the 

 surface layer at A would produce new ions. As before, a 

 condition of equilibrium exists when the drop in potential 

 is sufficiently large to cause many of the negative ions to 

 produce new ions at the boundary surface. 



Ionization at Cathode of Arc. The arc is in many ways 

 similar to the discharge in a vacuum tube. There is a drop 

 in potential at the anode, a gradual fall through the gas and 

 another sudden drop at the cathode. The essential differ- 

 ences between the two phenomena are that in the arc the 

 drop in potential at the cathode is much smaller than it is 



