244 Dr. F. Horton on the 



In this formula X x is expressed in kilovolts per centimetre. 

 If /is the measure of the force in electrostatic units, 



/V«=60. 



Now Zeleny, working with liquid points, obtained the 

 empirical result * 



/ v /«=56*9. 



The similarity of these two results both in form and 

 magnitude of the constant is very striking, especially when 

 the extent of the approximations is considered. 



In conclusion, I should like to express my thanks to 

 Professor Townsend both for suggesting the research and 

 for kind help in carrying it out. 



XXVIII. On the Action of a Wehnelt Cathode. 

 By Frank Horton, Sc.D.'t 



SINCE the discovery by Wehnelt of the large electron 

 emission which takes place when a lime-coated cathode 

 is heated in a vacuum, several theories have been put forward 

 to explain its action. One of the earliest of these, and one 

 which received the support of the discoverer of the effectf, 

 was that the electrons proceed, not from the glowing oxide, 

 but from the platinum strip or wire upon which it is heated, 

 and that the action of the lime is confined to a reduction of 

 the amount of energy required to liberate the electrons from 

 the metal. More recently the view has been put forward by 

 Fredenhagen§ that the electron emission from glowing lime 

 occurs as a result of the recombination of calcium and oxygen 

 which have been separated by electrolysis. A modification 

 of this theory has been suggested by Gehrts ||, who assumes 

 that the calcium and oxygen are separated, not by electro- 

 lysis, but by thermal dissociation of the lime at the high 

 temperature. This view simplifies the explanation of the 

 manner in which the activity begins. This is supposed by 

 Fredenhagen to be due to the small thermionic current from 

 the platinum support electrolysing the lime in passing through 

 it, and thus giving a supply of calcium and oxygen to start 

 the greater emission from the lime itself. 



* J. Zeleny, Phys. Rev. n. s. iii. p. 88, Feb. 1914. 



f Communicated by the Author. 



% See Wehnelt, Phil. Mag. [6] x. p. 80 (1905). 



§ K. Fredenhagen, JBer. d. Kgl. Sachs. Ges. d. Wiss. lxv. p. 42 (1913). 



|| A, Gehrts, JBer. d. Deutsch. Phys. Ges. p. 1047 (1913). 



