MOTIONS OF ELECTRONS 181 



electron may therefore not acquire sufficient energy 

 between successive impacts to admit of its ionizing the 

 gas through which it bumps its way. Its ability to 

 ionize depends upon its acquiring the necessary kinetic 

 energy during a motion comparable with the mean free 

 path of the gas. But this kinetic energy is equal to 

 the change in potential corresponding to the distance 

 which it moves along the path between the two plates. 

 If this change is sufficient then ionization will occur. 



The phenomenon when ionization occurs is that with 

 which we are familiar on a large scale in the case of 

 lightning. On a small scale it is illustrated by the 

 spark discharge between the electrodes of a machine 

 such as Galvani was using, or in the discharge between 

 the electrodes of a so-called induction coil such as is 

 used in the ignition system of an automobile. In all 

 such cases the discharge starts because of the presence 

 in the medium of a few electrons. These, by their col- 

 lisions with neutral molecules, produce other electrons 

 and also positive ions. The number of electrons which 

 are traveling toward the positive electrode therefore 

 increases rapidly. The positive ions which are formed 

 by the collisions naturally move toward the negative 

 electrode, where they combine with its excess electrons 

 and neutralize not only their own deficiency of electrons 

 but also the excess of the electrode. 



Such recombination may also take place en route. 

 In fact, during a continuous transfer of electricity 

 through a gas it is possible for the same atomic nucleus 

 to change partners several times. Thus, after a re- 

 combination with an electron has again made it neutral 

 and while pursuing its own haphazard motions, it may 



