EVAPORATION OF ATOMS 



365 



^.s 



4.0 



3.0 



2,0 



= /.o 



O 



O too 200 300 400 SOO 600 '7O0 <500 SOO 



t Cs£C^NOS) 



1000 1100 laoo 



Fig. 24. Oa vs. time at various filament temperatures and Cs pressures (m.o) as 

 indicated. Data obtained by two filament method for use in measuring ao. 



electricity corresponding to the observed ballistic kick produced when A is 

 flashed, we have 



Q = ai,vlJLatSAfoioe, 



(37) 



where Sa is the apparent surface area of the filament A within the cylinder 

 Co, and e is the electron charge. By dividing Q by 5"^/^, we thus obtain 

 values of a^vaoyiat. 



The experimental points shown in Fig. 24 represent data obtained by 

 this method for a series of relatively low filament temperatures, vis., 300, 

 4^7' 53O) 590 srid 704° K. and with three different caesium pressures 

 which gave, by the steady ion current method, values of ao\ia of 1.20 X 10^^, 

 2.1 X 10^^ and 6.2 X 10^^ atoms cm"^ sec."^. 



The straight lines marked I, II and III which pass through the origin 

 have been drawn with slopes ao[ia as determined by the steady ion current 

 method. It is seen that the observed points, for sufficiently low 6, lie within 

 the experimental error on the straight lines. Since the ordinates of the ex- 

 perimental points are aayao\iJ, while those of the straight lines are ao[iat, 

 this agreement proves that ttav = i and therefore aa = i for values of 6 up 



