﻿•'814 Mr. K. K. Smith on Negative 



had been plotted. This conclusion was confirmed in every 

 one of the subsequent experiments. Under some conditions, 

 as in the last experiment with Lamp 1, and in some others 

 to be described later, it is possible that the current may show 

 a decrease, but no emission less than the initial emission has 

 been observed. 



The first observations on the two lamps (3 & 4) are in 

 good agreement at low temperatures, but at a certain point 

 the 1st series (Lamp 4) shows that the current is increasing 

 much more slowly with the temperature. The heating of 

 the filament at very high temperatures, with the thermionic 

 circuit broken, was followed (series 2) by much larger 

 currents than before. At 2000° K. curve 2 bends sharply 

 to the left, and then again to the right, while curve 3 is 

 regular. Heating the filament would tend to remove im- 

 purities and absorbed gases from the filament. It is known, 

 for example, that the oxide volatilizes in a vacuum without 

 the evolution of gas. On the hypothesis that initially there 

 existed a surface layer which hindered the emission of 

 •electrons, we should expect to find larger currents in the 

 second series. The falling off in the rate of increase wdth 

 the temperature, which occurred at about 2000° in the first 

 two series (Lamp 4), is caused by the bombardment of the 

 anode. At low temperatures, and hence with smaller 

 currents, the latter effect is not apparent. Heating the 

 filament to a high temperature without the tbermionic 

 current does not bombard the anode and free it from gas, 

 although it does rid the filament to a large extent of what- 

 ever is hindering the normal emission. Heating the filament, 

 and allowing the thermionic current to flow, cleans both 

 anode and cathode. 



Lamp 5. 



This lamp was like the preceding ones, except that the 

 filament was 6 cm. long. The observations were extended 

 over three days, and during this time there was always a 

 supply of liquid air about U and C. Out of ten series of 

 observations the largest currents are plotted in fig. 4. The 

 smallest currents were found when the filament was first 

 heated, and this agrees with the results of the other experi- 

 ments. With this lamp complete saturation was obtained, 

 whereas the results already given for Lamp 2 show that the 

 currents always increased somewhat with the voltage. When 

 voltages as high as 600 volts were first applied the results 

 were as follows : at 1800° K. the current was saturated at 

 from 200 to 400 volts, and then decreased with increasing 



