EVAPORATION OF ATOMS 



357 



no attempt was made to correct for the influence of the second filament 

 held at the potential of the first in these experiments. The second filament, 

 acting as a grid, would decrease the observed emissions and hence the 

 slopes as found. The theoretical ratio of ion to electron slopes is 62.6 and 

 the observed value was 63.6. 



Fig. 21. Electron currents limited by space charge from clean tungsten. 



An equation, similar to Eq. (21), for the rate of positive ion evapora- 

 tion at very low ^'s may be developed 



& = 61.31-23,699/r, 

 &i = 6.80-99,900/r. 



(22) 



This equation gives Vp within one percent up to approximately 6 = 0.02. 

 The limit of application is near 6 — 0.03 where the error is about 15 per- 

 cent. By diflferentiation an expression giving the position of the maximum 



in the Vp vs. 9 curve is obtained 



^n,ax = r/(99,900-6.8r) (23) 



at 500° K., 1000° K., and 2000° K., ^^ax is 0.00513, 0.01075 and 0.0232. 



