246 Dr. F. Horton on the 



experiments, seems to fill the whole bulb. This luminosity 

 appears quite gradually as the temperature of the cathode is 

 slowly raised, so that it is very difficult to say when it first 

 begins. It is not accompanied by any sudden increase in the 

 thermionic emission, showing that the increase of temperature 

 of the cathode produced by the luminous discharge is general, 

 and is not confined to a few points of its surface. Further 

 ■experience of working with Nernst filaments has made it 

 possible to control the heating of a filament even at tempe- 

 ratures as low as 1100° C, and a comparison of the electron 

 emission from lime heated upon a Nernst filament with that 

 from lime heated upon platinum has now been made. The 

 filament used was covered with pure lime, prepared from 

 marble, in the manner described in the paper already re- 

 ferred to. It was heated by an alternating current, and the 

 thermionic emission was measured in a vacuum obtained 

 by the use of charcoal cooled in liquid air. During the 

 observations there was a faintly luminous discharge through 

 the residual gas; but at the lowest temperatures the lumi- 

 nosity was very difficult to discern, and perhaps was sometimes 

 absent. 



For measuring the emission from lime heated upon 

 platinum a fine platinum tube was fitted into the apparatus in 

 the place of the Nernst filament. This tube was covered with 

 lime and could be heated by an alternating current from a 

 transformer. The gas pressure was reduced as low as possible 

 by means of charcoal cooled in liquid air, and the thermionic 

 current was measured as the temperature of the lime-covered 

 tube was gradually raised. A large negative emission occurred 

 immediately the cathode became luminous. The current soon 

 became steady; there was no gradual increase as had been 

 •observed in an earlier research with a platinum tube covered 

 with the material of a Nernst filament, and the first obser- 

 vations of the emission at different temperatures were the 

 largest values obtained. 



In comparing the emission from this lime-covered platinum 

 with that from a similarly covered Nernst filament, a difficulty 

 arises on account of the unsaturated nature of the thermionic 

 current given by a lime cathode. Even at the very low 

 pressures and comparatively low temperatures used in these 

 experiments, any increase in the applied E.M.F. caused an 

 increase in the observed emission, and with the highest 

 voltages it was deemed safe to apply, there, was no sign of 

 saturation. Since a saturation current cannot be measured, 

 it is necessary, in order to compare the electron emissions in 

 the two cases, to measure the thermionic currents with the 



