Tables 616-618. 4^3 



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 616. Electron Emission from Hot Metals. 



Amone 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 = iV(/?r/ 2 7rAf)5e /;/' where iV = 

 number of electrons in each cm' of metal, R the gas constant (83.13 X io« erg-dyne), T the absolute temperature, M 

 the atomic weight of electron (.000546, O = 16), w the work done when a "gram-molecule" of electrons (6.06 X lo*' 

 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 -f- field is applied to escaping 

 electrons so that none return to the conductor, then the saturation current has been found to follow the equation 



i = aVre-*/^, 

 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) i:ooq = satura- 

 tion current per cm^ for T = 2000 K°; (p — 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 10"' mm Hg. 

 TABLE 517. Photo-electric Effect. 



t Schlichter, 19x5. 



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 i> 

 of the light is (i/2)mvi = hv — P (Einstein's equation) where h is Planck's constant (6.58 X io~2' 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)mt^ is the maximum kinetic energy the electron may have after escape. Richardson identifies the P of Einstein's 

 formula with the k' of electron emission of the preceding table. The minimum frequency Vt, (corresponding to ma.xi- 

 mum wave-length Xo) at which the photo-electric effect can be obser\'ed is determined bv hv = P. P applies to a 

 single electron, whereas w applies to one coulomb (6.062 X id^ electrons); therefore w = NP = .oo399i'o ergs. <^ = 

 (12.4 X io-6)X<i volts. See Millikan, Pr. Nat. Acad. 2, 78, 1916; Phys. Rev. 7, 355, 1916; 4, 73, 1914; Hennings, 

 Phys. Rev. 4, 228, 1914. 



TABLE 618. Ionizing and Resonance Potentials of the Elements. 



(Abridged by permission from "Origin of Spectra, " Foote and Mohler, 1922) 



When electrons are accelerated through gases or vapors (especially monatomic gases of small electron affinity and 

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

 gas atoms. Below the critical value the collision is elastic. In general two types of inelastic encounters occur: the 

 first, accompanied by the emission of the radijtion of a single spectrum line at a potential called the resonance potential, 

 Vr; an outer electron of the atom then undergoes an interorbital transition; the relation \iv = eVr holds where v is the 

 frequency of the radiation and h Planck's quantum. The energy absorbed at the resonance is not enough to completely 

 eject an electron but only displaces it to an outer orbit, e.g., in the alkali group the electron is displaced from the is to 

 the 2p orbit, the first energy level outside. In returning the energy emitted has the frequency is-2p; there may be more 

 than one resonance potential due to other displacements. The second type of encounter completely removes an electron 

 and ionizes the gas (ionization potential Vi). This potential in general satisfies a relation hi' = eV except that now v 

 corresponds to the highest convergence frequency in the arc spectrum of the material (monatomic vapor), to the limit of a 

 series the first line of which corresponds to the resonance potential. In the case of the ionization potential the electron 

 may return by a variety of interorbital transitions, each resulting in an emission of a quantum ol wave-number v, subject 

 to the coiiservation of energy condition: Shc-i"]! = eViio*. With numerous atoms and electrons returning to equilib- 

 rium in different manners, there results the composite result of the emission of the complete arc spectrum. 



(For conclusion of Table, see page 442). 



Smithsonian Tablcs. 



