﻿Thermionic Currents from Tungsten. 809 



current to flow unlimited by any space charge effect *. This 

 is proved by the fact that the currents were practically 

 saturated at 120 volts. Even with a blue glow in the lamp, 

 the points at the upper end of curve 5 fall far to the left of 

 the straight line. Hence this deviation cannot be the result 

 of a space charge effect, but is undoubtedly caused by the 

 large thermionic currents, which were of the order of mag- 

 nitude of the heating current +. 



Lamp 2. 



Lamp 2 contained a filament 8*5 cm. in length, and a 

 cylindrical anode of copper gauze. It was heated for 14 

 hours in all, and liquid air was applied at C, but not at U 

 (at first). The filament was glowed for about 3J hours, the 

 thermionic current during the last hour being 10 milli- 

 amperes. In the first 5 minute test with the trap T closed, 

 the increase of pressure was from 0*001 to 0*050 micron ; 

 in the last 30 minutes the increase, if any, was not more 

 than 0*0001 micron from " 0." Liquid air was then applied 

 to U to condense the mercury vapour, but no change in the 

 thermionic current could be detected. The lamp was then 

 sealed, and the thermionic currents were measured from 

 1050° K. up. The results (plotted in fig. 3) show that 

 within the limits of experimental error, the equation 



i b 

 i = aTe-T. 



is satisfied throughout. (In all the figures the current is 

 expressed m amperes per sq. cm.) 



In order to determine how the thermionic current depended 

 upon the voltage, tests were made at the following tempera- 

 tures : 1070, 1335, 1465, 1590, 1710, and 1825° K. To 

 reduce the results to the same scale, the current with 200 

 volts has been taken as the unit measure in each case, and 



* Langmuir, loc. cit. 



t Since these experiments were completed, Dr. Dushman has published 

 (Phys. Rev. iv. p. 121, 1914) the results of some experiments in which 

 he observed the decrease in the thermionic current caused by bombard- 

 ment of the anode. The temperatures were above r2000° X. and the 

 maximum currents obtained appear to be about the same as those 

 observed by Dr. Langmuir. The values of the currents given for the 

 €oolidge X-ray tube (Phys. Rev. ii. p. 409, 1913) are larger, but not so 

 large as the writer's. 



