Action of a Wehnelt Cathode. 



24,7 



same potential differences between Hhe electrodes. The 

 difficulty is to allow for the effect of the field due to the 

 heating current. In the case of the Nernst filament there 

 was an alternating potential difference of about 50 volts 

 between the two ends of the cathode, while with the platinum 

 tube about 16 volts were used — the exact value in each case 

 being different at different temperatures. It was finally 

 decided to apply between the terminals of the discharge-tube 

 a potential difference of 207 volts in the case of the platinum 

 tube, and 168 volts in the case of the Nernst filament — these 

 being values easily obtained from the high-potential battery, 

 while the difference between them makes allowance for the 

 difference in the field due to the heating current in the two 

 cases. For both cathodes curves connecting the thermionic 

 current and the temperature were drawn, and from these 

 curves the following values of the emissions at different 

 temperatures are taken. The area of the cathode covered 

 by lime was approximately the same in the two cases. The 

 numbers given in the table are the largest values of the 



Temperature, 

 centigrade. 



Thermionic Current in 10 ° ampere. 



Lime on Filament. 1 Lime on Platinum. 



1100 

 1150' 

 1200 

 1230 



129 



202 

 340 

 46-2 



41 

 102 

 230 

 339 



thermionic current in both cases. On continued heating, 

 the emission from the lime-covered filament decreased more 

 rapidly than that from the lime-covered platinum, apparently 

 because the lime adhered more readily to the metal than to 

 the material of the filament. From both lime cathodes there 

 was a continued slight emission of gas which, if it were very 

 unequal in the two cases, would render a comparison of the 

 emissions impossible j for, with the strong electric fields used, 

 ionization by collisions would play an important part in 

 deciding the value of the measured thermionic current. 

 Throughout the experiments a carbon tube cooled in liquid 

 air was in connexion with the discharge-tube so as to keep 

 the pressure low, and as nearly constant as possible. 



From the table it will be seen that the numbers measuring 

 the electron emission from lime heated upon a Nernst 



