3 64 



PHENOMENA, ATOMS, AND MOLECULES 



Fig. 23 gives some typical data with the filament during accumulation 

 at 300°, at 970° and looi °. 



0. 



Fig. 23. Oa vs. time at 300°, 970° and looi^K, for ^o = 6.20 X 10^^. Data obtained 

 by direct flashing method for use in measuring a^. 



Each experimental point shown for curves I, II and III corresponds to 

 a separate run in which the caesium was allowed to accumulate for the 

 time t after cleaning the filament by flashing. The straight line dr^wn 

 through the origin has been drawn with a slope equal to ao\ia as determined 

 by this stead}- ion current method. The experimentally determined points 

 are seen to lie quite accurately on these lines at sufficiently low values of 6. 

 We shall see that at 970 and 1001° the deviations from the straight line 

 at the higher ^'s in curves II and III agree with those calculated from 

 the known evaporation rates of these films at those high temperatures. 



The data obtained with the filament at 300° K. during the accumulation 

 time, curve I, show that the observed points lie quite accurately on the 

 straight line through the origin up to values of 6 of about 0.07. The 

 deviations at higher 6 are due to evaporation of Cs as atoms during the 

 flashing. This is shown by the fact that the curve remains entirely un- 

 changed if Ti is varied from 300 to 800° K., but the deviations do depend 

 slightly on the flashing temperature and on the rapidity with which the 

 temperature is raised. The direct flashing method is thus only applicable 

 for values of 6 up to about 0.08. 



The fact that the experimental points lie on the straight lines of slope 

 ao\ia for sufficiently small 6 proves that ttav^a^ equals ao\iai- We conclude 

 that for temperatures up to about 800° and for values of 6 up to 0.07, aa 

 is constant and equal to ao within the experimental error of less than 

 I percent. At 970° the same conclusion may be drawn up to ^ = 0.02. 



(2) Two filament method. After allowing caesium to accumulate on 

 filament A at temperature Ti up to a definite value of <3a = OLa^l^J, Oa is 

 measured by the two filament method. If Q represents the quantity of 



