TABLES 516-518. 43 



Note: The phenomena of Electron Emission, Photo-electric Effect and Contact (Volta) Potential treated in the 

 subsequent tables are extremely sensitive to surface conditions of the metal. The most consistent observations have 

 been made in high vacua with freshly cut metal surfaces. 



TABLE 516. Electron Emission from Hot Metals. 



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



The number n reaching the surface with velocities above this critical velocity = N(RT/2irM)le~KT where N ** 

 number of electrons in each cm 3 of metal, R the gas constant (83.15 X io erg-dyne), T the absolute temperature, M 

 the atomic weight of electron (.000546, O = 16), v> the work done when a "gram-molecule" of electrons (6.06 X io 

 electrons or 96,500 coulombs) escape. It seems very 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 conductor, then the saturation current has been found to follow the equation 



assuming N and w constant with the temperature; this is equivalent to the equation for n just given and is known as 

 Richardson's equation. In the following table due to Langmuir (Tr. Am. Electroch. Soc. 29, 125, 1916) 12000 = satura- 

 tion current per cm 2 for T = 2000 K; < = w/F - Rb/F = work done when electrons escape from metal in terms of 

 equivalent potential difference in volts; F = Faraday constant = 96,500 coulombs. 



* Best determined value of table, pressure less than io~ 7 mm Hg. 

 TABLE 517. Photo-electric Effect. 



t Schlichter, 1915. 



A negatively charged body loses its charge under the influence of ultra-violet light because of the escape of nega- 

 tive electrons freed by the absorption of the energy of the light. The light must have a wave-length shorter than some 

 limiting value Xo characteristic of the metal. The emission of these electrons, unlike that from hot bodies, is independ- 

 ent of the temperature. The relation between the maximum velocity v of the expelled electron and the frequency v 

 of the light is (i/2)mv 2 = hv P (Einstein's equation) where h is Planck's constant (6.58 X lo" 27 erg. sec.); hv some- 

 times taken as the energy of a "quanta," P, the work which must be done by the electron in overcoming surface forces. 

 (i/2)mv z is the maximum kinetic energy the electron may have after escape. Richardson identifies the P of Einstein's 

 formula with the w of electron emission of the preceding table. The minimum frequency v& (corresponding to maxi- 

 mum wave-length Xo) at which the photo-electric effect can be observed is determined by hv = P. P applies to a 

 single electron, whereas w applies to one coulomb (6.062 X io 23 electrons); therefore w = NP = .oo399J'o ergs. < = 

 (12.4 X io- 5 )Xo volts. See Millikan, Pr. Nat. Acad. 2, 78, 1916; Phys. Rev. 7, 355, 1916; 4, 73, 1914; Hennings, 

 Phys. Rev. 4, 228, 1914. 



TABLE 518. Ionizing Potentials and Single-line Spectra. 



When electrons are accelerated through gases or vapors, especially those with small electron affinity (inert gases, 

 metallic vapors) at well-defined potentials a large transfer of energy takes place between the moving electrons and 

 the gas atoms. There appear to be two types of inelastic encounters under such circumstances: the first accompanied 

 by the emission of a radiation of a single line at a potential called the resonance potential and satisfying the relation 

 hv = eV where V is the potential fall, v the frequency and h Planck's constant; the second ionizes the gas (ionization 

 potential), exciting the radiation of a composite spectrum. The latter potential satisfies a relation hv = eV except 

 that v is now the limiting frequency of a series of lines. The following table was communicated by Tate and Foote 

 (see Phil. Mag. 36, 64, 1918). 



* Computed from relation Ve = hv or V = I2334/X volts; X in Angstrom units, 

 t Computed from h = o.53o8X7io- M t Limit of principal series. 



Limit of principal series of single lines, i.sS. I Short wave-length line of first doublet of principal 



1 Combination series line i.s5 - 2p* ** First line principal senes single lines 1.55 - zP. 



SMITHSONIAN TABLES. 



