Action of a Wehnelt Cathode. 245* 



Fredenhagen's theory has been tested by experiments with 

 Nernst filaments, which give a convenient form of oxide 

 cathode without the complication of a metallic support ; and 

 a paper has recently been communicated to the Royal Society 

 in which it is shown that the emission from such an oxide 

 cathode is quite independent of the possibility of electrolysis,, 

 and tbat the emission from the material of the filament at a 

 given temperature is the same whether it is heated in the 

 usual manner by conducting an electric current, or is powdered 

 up and heated upon a platinum strip. Further experiments 

 have been made to test these theories of the origin of the 

 activity of a Wehnelt cathode, and two of these experiments 

 are described in the present paper. The first was made to 

 test whether the emission from lime depends upon the nature 

 of the material upon which it is heated ; the second experiment 

 is a test of the separation of calcium and oxygen by the passage 

 of a thermionic discharge from lime, and of the connexion 

 between the recombination of these elements and the electron 

 emission/ 



The discharge-tube used in these experiments and the 

 method of heating the Nernst filament and of covering it 

 with lime are described in an earlier paper*. The tempera- 

 tures were measured by means of a Fery optical pyrometer, 

 for the use of which I am indebted to Professor T. Mather, 

 of the City and Guilds College, London. 



I. A comparison of the Electron emission from Lime heated 

 upon a Nernst filament with that from Lime heated upon 

 Platinum. 



In a recent paper * the author has described an experiment 

 which shows that a large electronic emission is obtained from 

 lime when heated upon a Nernst filament, but attempts to 

 compare the emission with that given by an ordinary Wehnelt 

 cathode were unsuccessful on account of the difficulty of 

 maintaining the filament glowing in a vacuum at a tempe- 

 rature low enough to enable measurements to be made in the 

 absence of luminous pencils of cathode rays. The appearance 

 of these luminous rays is always accompanied by great 

 overheating at the points of lime from which they start ; so 

 that it is impossible to ascertain to what temperature of the 

 cathode the measured current corresponds. 



At temperatures rather lower than that at which these 

 luminous rays are seen, a much fainter luminosity occurs in 

 the discharge-tube and, at the low pressures used in these 



* Proc. Camb. Phil. Soc. xvii. p. 414 (1914). 



