﻿816 



Mr. K. K. Smith on Negative 



Using filament " A " as the hot cathode, three series of 

 observations were made, and the results of the first and 

 last are shown in Table II. Following the 2nd series, the 

 cathode had been heated to 2700° K. for twenty minutes. 

 After this the currents were as large, at the same tempera- 

 tures, as the currents which had been observed in the sealed 

 lamps. The pressure was " *' during the whole time. 

 This experiment shows that the thermionic emission from 

 tungsten is not a secondary effect arising from the presence 

 of g;is or condensable vapour, but must come from the metal 

 itself. 



Table II. 



Current. 





Temperature. 





(Amps, per sq. cm.) 

 1-12x10-5 



" A," series 1. 

 1715° K. 

 1755 

 1795 

 1840 

 1895 



" A," series 3. 

 1540 

 1575 

 1615 

 1650 

 1695 



" B," series 1. 

 1695 

 1740 

 1785 

 1835 

 1890 



225 „ 



4-50 „ 



9-0 „ 



18-0 





Immediately after the 3rd series with filament "A" as 

 cathode, filament "B" was made the cathode, and "A" the 

 anode. The results of the first series of observations under 

 these conditions are also shown in Table II. It will be seen 

 that the emission under the same vacuum conditions depended 

 upon which filament was used as the cathode. As stated 

 above, both filaments had been glowed in series, and other- 

 wise they had been treated alike, except that "A" had been 

 heated to 2700° K. for twenty minutes. Above 1900° K. 

 the currents were limited on account of the small dimensions 

 of the anode, and after a certain value was reached the 

 currents could not be increased, with constant voltage, no 

 matter how much the temperature was raised. 



The liquid air was then removed from the gauge and 

 furnace U-tubes, the trap T being closed. The pressure 

 increased from " " to 0*25 micron, and the resistance of 

 the glowing filament, " B," began to increase slowly. This 

 was caused by the oxidation of the filament by the water 

 vapour released from the U-tubes. The resulting ionization 

 in the gas neutralized the space charge effect, and the current 

 (T = 2180° K.) began to increase. Series 4, taken at once, 

 showed that below 1950° K. the currents had been decreased 



