﻿806 Mr. K. K. Smith on Negative 



Experimental Results. 

 Lamp 1. 



Lamp 1 contained a filament 5 cm. long, and a cylindrical 

 anode. After the lamp had been heated in the furnace for 

 9 hours, liquid air was applied at U and C, and the filament 

 was glowed for an hour and a half at various temperatures 

 above 2500° K. The thermionic currents varied from 3 to 

 40 milliamperes. The trap T was closed at intervals, and 

 the increase in pressure during five minutes was noted. In 

 the first period the pressure rose from 0*005 to 0*050 micron ; 

 in the last period the increase was from *' " to 0*003 micron. 

 The furnace (still above room temperature) was then opened, 

 and the lamp was sealed immediately. The pressure indi- 

 cated was " 0." 



Professor Richardson measured five series of thermionic 

 currents greater than 1 microampere, the potential difference 

 between the anode and the cathode being 120 volts. The 

 results are shown in fig. 2, in which log i —1/2 log T 

 is plotted against 10000/T. Series 1A, 2 A, and 3 A were 

 taken with decreasing temperatures, the others with increasing 

 temperatures. It will be noticed that the points 1'or the 3rd 

 series are shifted to the left of the preceding ones. At first 

 sight, this would seem to indicate less current at a given 

 temperature as the heating was continued. Jt is believed* 

 however, that this is not to be ascribed primarily to a change 

 in the emitting power of the surface, but rather to a change 

 in the resistance of the filament, owing to excessive heating. 

 After the first series of readings, the filament was heated for 

 over an hour at a temperature above 2500° K. The ther- 

 mionic current was turned off to prevent unequal beating, 

 which is likely to burn the filament out at one end. Between 

 the 2nd and 3rd series the filament was heated to a still 

 higher temperature for 20 minutes, and the resistance in- 

 creased from 1625 to 1G70 units, or about 2'8 per cent. The 

 same thermionic current w ? as obtained with R=960 after 

 overheating as with R = 945 just before. These resistance 

 changes as the filament evaporates cause the temperatures 

 to be overestimated, and are sufficient to account for the 

 above variations in thermionic current. 



In series 4, thermionic currents (not shown in fig. 2) up 

 to 320 milliamps. were obtained, or 8 times as much as had 

 been obtained before the lamp was sealed off. Immediately 

 after this large current was obtained, it was found that at 



