Tables 667-669 r<>y 



Note: The phenomena of electron emission, photoelectric effect and contact (Volta) potential treated in the 

 following tables are extremely sensitive to surface condition of the metal. The most consistent observations 

 have been made in high vacua with freshly cut metal surfaces. (See Dushman, Rev. Mod. Phys., 2, 381, 1930.) 



TABLE 667.— Electron Emission from Hot Solid Elementary Substances 



(Most of the following is taken from Dushman, loc. cit., 1930.) 



Among the free electrons within a metal some may have velocities great enough to escape 

 the surface attraction. The number reaching the surface with velocities above this critical 

 velocity = N — (RT/2irM) in e- w,Tt where TV = no. of electrons/cm 3 of metal, R the gas 

 constant (83.14 X io 5 erg-dyne), T, the absolute temperature, M, the atomic weight of an 

 electron (.000545, O = 16), w the work done when a gram-molecule of electrons (6.06 X io 23 

 electrons or 96,500 coulombs) escape. It seems probable that this work is done against the 

 attraction of the electron's own induced image in the surface of the conductor. When a 

 sufficiently high + field is applied to escaping electrons so that none return to the condurtor, 

 then the saturation current has been_ found to follow the equation 



i = a \JT-e' h ' T (Richardson's equation) 

 assuming N and W constant with T. This is equivalent to the equation for Adjust given. 

 The equation 



I = AT 2 e- b " /r (Laue's equation) 

 is just as valid theoretically and Dushman (Phys. Rev., 21, 623, 1923) considers A should 

 be a universal constant (60.2 amp./cm 2 /deg. 2 /and b dependent upon the emitter. The data 

 is not accurate enough to distinguish between the two formulas, b or b is a measure of the 

 latent heat of evaporation of the electrons, i. e., the energy needed to get the electrons 

 through the surface. While used in C K. in the above equation, it is customary to express it 

 in volts by the relation b a k = <f>oe where k = Boltzmann's constant, e, the electronic charge, 

 and </>o is known as the work function, whence 



4>o = 8.62 X io- b b (volts) 

 The experimental values of A do not seem to be independent of the substance. 



(Above table of best authenticated values is from Dushman, loc. cit., p. 394, 1930. His table contains values 

 for C, Ca, Cs, Hf, Ni. See also I.C.T.) 



TABLE 668. — Electron Emission from Thorium-Coated Filaments (Monomolecular), / (0) 



Values given for Dushman with W filaments coated with monomolecular films of thorium. 

 6 = (bo — b w )/(bTh — b w ) where bo, bn and b w represent values of b in the emission equation 

 for partly covered, completely covered, and pure tungsten surface. 



Io and 0o refer to 1900 . 



TABLE 669. — Emission Current, 7, Emission Efficiency I/W, Diffusion, D, 7 for Zero Field, 

 Completely Activated Surface, Th on W 



Smithsonian Tables 



