THERMIONIC ELECTRON EMISSION 



471 



This has been justified by Schottky ^^ and Laue " who concluded that 

 the effect of these charges is negligible. For convenience, the zero of 

 potential is taken not inside the metal but at a point where the electrons 



1.5 



-2.0 



-4.0 



-4.5 



2.5 3.0 



LOGiqV 



0.5 1.0 1.5 2.0 



Fig. 24 — Currents between parallel plates limited by space charge. 



5.0 



have just overcome the surface forces. Because of the assumption 

 made above, this point is taken at the cathode surface. 



In order to obtain a solution, some assumption regarding the velocity 



