Thermionic Currents with Potentials. 555 ; 



A wire coated with CaO and tested only for negative 

 emissions showed an instability of currents with applied 

 potentials. There were apparently three possible ranges for 

 the current-potential curves. The current broke suddenly 

 from one range to another. The behaviour may be understood 

 best by reference to fig. 3. These double and triple range- 

 phenomena were obtained with two other CaO cathodes and. 

 with a BaO cathode. 



Fisr. 3. 



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It seems to the writer that the facts established in the 

 above experiments fit very well into already proposed theories, 

 if some further assumptions are made as to conditions at the 

 surface. The lack of saturation, the initial growth, and the 

 over-shooting effects are the most general facts to be 

 adapted. 



Richardson's theory of negative thermionic emission regards 

 a discontinuity in the potential energy of an electron at the 

 surface of the metal. Since this theory is well established, the 

 probability of the existence of the discontinuity is great. The 

 discovery and direct measurements of the cooling effect by 

 Wehnelt * and by Richardson and Cooke f, and of the heating 



* Ann. tier Pkysik, [3] vol. xxviii. p. 537 (1909). 

 t Phil. Mag. April and Sept. 1913. 



