﻿Thermionic Currents from Tungsten. 815 



voltage. The measurements were repeated immediately 

 afterward, and it was found that the current had been 

 reduced one half. At the same time the pressure had in- 

 creased from 0*010 to 0*014 micron, owing, no doubt, to the 

 evolution of gas from the anode. 



The gas pressure, however, was evidently not an important 

 factor in determining the thermionic current. For example, 

 the same pressure (0*001 micron) was indicated in series 1 

 and 3, although the currents were considerably larger in the 

 latter case. On the other hand, series 3, 8, and 9 agreed 

 quite well, although the corresponding pressures were 0*001, 

 0*010, and 0*100 micron respectively. Repeated heating 

 did not increase the emission beyond what is shown for this 

 lamp in fig. 4. In view of these facts it was believed that 

 the great reduction in current, as compared with the other 

 lamps, must be caused by condensable vapour, probably 

 water vapour. The lamp had been heated in the furnace to 

 500° C, or more, but there was the possibility that water 

 vapour had entered afterwards from below the furnace, even 

 though liquid air had been around the U-tube all the time. 

 This was the first lamp which had not been sealed shortly 

 after the furnace was opened. A slight change in the 

 apparatus was therefore made before the next lamp was set 

 up. The glass tube leading to the lamp was bent into a U 

 inside the furnace, so that liquid air could be applied close 

 to the lamp. Any vapour which might arise from the un- 

 healed tubing below the furnace would then be condensed 

 before it could enter the lamp. 



Lamp 6. 



This lamp contained two parallel filaments, 1*7 cm. apart. 

 One filament " A " was 8*5 cm. in length, and the other 

 "B" 7*2 cm. It was then unnecessary to use a cylindrical 

 gauze anode, and thus one possible source of gas was 

 removed. All the platinum and copper connecting wires 

 were completely covered with melted glass before they were 

 sealed in the lamp, so that the only surfaces exposed on the 

 inside of the lamp were of tungsten and glass. At the 

 same time that the lamp was being heated in the furnace, 

 the two filaments were glowed in series for six hours at 

 temperatures varying between 2000 and 2500° K. As soon 

 as possible after the hot furnace was opened, liquid air was 

 placed around the new U-tube next to the lamp. Liquid 

 air had previously been placed around the other U-tube and 

 the charcoal tube as usual. 



