EVAPORATION OF ATOMS 



359 



Ve, Vp, and Vo as 6 changes. The curves are calculated for 1000° K. but their 

 general course is similar at other temperatures. 



24. 



20 



-vb 

 r^ 16 



7^ 



C5 I A 



CJ 



-J 



12 

 10 

 8 

 6 

 4. 



Fig. 22. Atom (va), ion (vp), and electron (ve) evaporation rates at 



1000° K, with zero field. 



Heats of evaporation, according to the Clapeyron equation, are given 

 by Z^o = — ^ [^ In p/d{i/T) ] and are obtained for atoms, ions and electrons 

 at any value of 6 as follows : 



^o=1.987[2.303B + r/2] calories 



2.303B + r/2 



= volts, 



11,606 



where B is Bq for atoms B^ for ions, and B^ for electrons. The term T /2. is 

 introduced when Eq. (24) is changed to a form containing the pressure (/>) 

 instead of the rate (v) in accord with Eq. (5), in order to calculate ^o as 

 given by the Clapeyron equation. T may be taken as T^ = 800°, the average 

 'filament temperature in the range (600-1200° K.) ordinarily used. T/2 

 corresponds at most to only a few hundredths of a volt. 



The work function for electrons, i.e., the exponent (Fe) in an equation 

 of the Dushman type 



is 



r,= (2.303B,-2rj. 11,606 volts. 



