﻿818 Mr. K. K. Smith on Negative 



the water vapour. The resistance of the filament remained 

 constant when liquid air was about the U-tube, otherwise it 

 increased gradually. The liquid air was removed once more, 

 and repeated measurements showed, as before, that the 

 currents increased rapidly with the voltage when liquid air 

 was around the U-tube ; if it was not, the currents were 

 saturated. After the liquid air had once been removed the 

 vacuum conditions were never so good as before, but the 

 pressure was always so low ( < 1 micron) that ionization 

 by collision had no effect. 



Lamp 7. 



This experiment was undertaken in order to measure the 

 currents before and after the lamp was sealed. The filament 

 was 2' 6 cm. long, and the anode was a cylinder of copper 

 gauze. The lamp was heated 10 hours on one day, and 

 6 hours on the next day. After the first series of observa- 

 tions had been taken, the trap T was closed, and was kept 

 closed until the lamp was sealed 6 hours later. The pressure 

 was " " before any measurements were taken, and no indi- 

 cation of pressure could be seen up to the time that the lamp 

 was sealed. After sealing, the liquid air was removed from 

 the U-tube, U, and three or four minutes later the pressure 

 in the gauge was 0'370 micron, but it did not increase 

 after that. This was the pressure of the gases that had been 

 condensed. 



The results of the typical series are shown in fig. 5. The 

 first currents were less than those that had been observed in 

 the first heating of the other filaments. Afterwards they 

 increased, as can be seen in series 4. This condition was 

 not permanent, however, for after the filament had been 

 kept at a temperature of about 2000° K. for two hours, it 

 was found that the currents were smaller than in series 4, 

 although they were still much larger than in the 1st series. 

 These results (series 6) have not been plotted, since they are 

 represented well enough by series 9. After continued 

 heating the emission was again about the same as in the 

 4th series, and the lamp was then sealed. 



A few days later measurements were taken on the sealed 

 lamp. Without any preliminary heating whatever, series 9 

 was taken with increasing temperatures, followed immedi- 

 ately by series 9 A with decreasing temperatures. It is 

 evident that a marked change has been produced by simply 

 heating the filament once. The final results have been 

 plotted, and it will be seen that the points lie quite accurately 



