THERMIONIC ELECTRON EMISSION 



423 



energy components normal to the surface are in the range (F„, dVn) 

 volts. It has been customary to plot iV(F„) versus F„ for equation 

 (28). At r = 0, N{V,) decreases linearly with F„ from a value of 



/ rv-vne\ 



FERMI-DIRAC N (Vn) = ^^^i ^ (^H-£~io^j 



/ e2 \!^ Vne 



CLASSICAL N(Vn)=n( ^^^^^^j fyj 



K= 5.75 VOLTS (l FREE ELECTRON PER 

 ATOM ASSUMED FOR TUNGSTEN) 



Vn IN VOLTS 

 Fig. 1 — Classical and Fermi-Dirac distributions. 



lirGemk/h^ when F„ = 0, to zero when Vn = K/e; for F„ > K/e, 

 ^(^n) — 0. For 7" > 0, the function is much the same except in the 

 neighborhood of F„ = K/e and for F„ > K/e; the curve is here 

 everywhere higher than the curve for 2" = and decreases exponen- 

 tially. Since only those electrons can escape for which Vn — Pm 

 > (3/2)ir, we are primarily interested in the exponential portion of 

 the curve. It is therefore more advantageous to plot log 7V(F„) 

 rather than 7V(F„). 



In Fig. 1 curves 1 and 2 are for equation (28) at 2" = and 

 T — 1800° K., respectively; while curve 3 is for the classical case or 

 equation (29). For curves 1 and 2, the value of K/e has been taken 

 as 5.75 volts which is the value appropriate for tungsten assuming one 



