﻿812 



Mr. K. K. Smith on Negative 



and did not exceed 0*002 micron until the current was as 

 large as 0*130 amp. (1*95 amp. per sq. cm.). With this 

 current the pressure increased (with the trap T closed) from 

 0*002 to 0*060 micron in 6 minutes. Currents as large as 

 0*350 amp. were observed, while the pressure increased to 

 0*600 micron, and then decreased to 0400 micron. The 

 temperature was at least 3000° K., and the filament burned 

 out a few minutes later. 



The results are shown in fig. 4. Points in the middle of 

 the curve fall along a straight line from which the value 

 6=69,500 is calculated. At the lowest temperature (1950°) 

 the current is about 1/600 of the current observed with 

 Lamp 1 at the same temperature. At 2500° K. the corre- 

 sponding ratio is about 1/3, while at the highest temperature 

 the two currents are practically identical. These results will 

 be discussed after they have been compared with those of 

 the next experiment. 



Lamp 4. 

 This lamp was an exact duplicate of the preceding one. 

 After the usual treatment the filament was given its initial 

 heating (by a current) at 2100° K. This was continued for 

 3^ hours, during which time the thermionic current increased 

 from 3 to 133 microamperes. The observations of the last 

 40 minutes are shown in the following table : — 



Time. 



Pressure. 



Total Current. 



10.42 p.m. 



0006 micron. 



16 



.45 





18 



.47 





21-5 



.50 





27-5 



.52 





30-4 



.53 





42 



.54 





51 



.55 





06-5 



.57 



0001 





.58 





109 



.f>9 





114 



11.00 





116 



11.20 



00001 



133 



With a potential difference of 210 volts the 1st series of 

 observations at different temperatures was taken, after which 

 the thermionic circuit was broken, and the filament was 

 glowed for 40 minutes at temperatures varying from 2700° 

 to 3000° K. The 2nd and 3rd series were then taken in 

 close succession. During all these series the pressure was 

 " 0," according to the gauge, whether the trap T was closed 



