636 



TABLE 685.— ELECTRON EMISSION (/ = amp/cm 2 ) AND (W = watts/cm 2 ) FOR A 



NUMBER OF MATERIALS 218 



The table gives emission data for a range of temperature, for the most frequently used metals 

 and for thoriated tungsten (ThW). Values of A and b used in calculation of / (amp/cm 2 ) are 

 those given in Table 684. For ThW, the values used are A = 3.0 and <j> = 2.72, b = 3.15X10*. 



219 Dushman, Saul, The scientific foundations of vacuum technique, John Wiley & Sons, Inc., 1949. Reprinted 

 by permission. 



* Layer of thorium on tungsten. 



TABLE 686.— PHOTOELECTRIC EFFECT 



A negative charged body loses its charge under the influence of ultraviolet radiation 

 because of the escape of negative electrons freed by the absorption of the energy of the 

 radiation. The radiation must have a wavelength shorter than some limiting value Ao 

 characteristic of the metal. The emission of these electrons, unlike that from hot bodies, 

 is independent of the temperature. The relation between the maximum velocity v of the 

 expelled electron and the frequency v of the radiation is (i)mt/ 2 = hv — P (Einstein's equa- 

 tion) where h is Planck's constant (6.62 X 10" 27 erg sec), hv, the energy of a "quanta," P, 

 the work which must be done by the electron in overcoming surface forces, (i)nw 2 is the 

 maximum kinetic energy the electron may have after escape. Richardson identifies the P 

 of Einstein's formula with the </>, of electron emission of Table 683. The minimum fre- 

 quency v (corresponding to maximum wavelength X ) at which the photoelectric effect 

 can be observed is determined by hv = P. P applies to a single electron, whereas w applies 

 to 96,500 coulombs (6.02 X 10 23 electrons) ; therefore iu=NP = .00399^ ergs. <t> = (12.4 

 X 10" 5 )X„ volts. 



TABLE 687.— THE ELECTRON AFFINITY OF THE ELEMENTS, IN VOLTS 



Photo- 

 electric 

 and Photo- 

 Contact Thermionic contact electric Miscel- 

 Metal (Henning) (Langmuir) (Millikan) (Richardson) laneous 



Tungsten — 4.52 — — — 



Platinum — — — 4.3 4.45 



Tantalum — 4.31 — — — 



Molybdenum — 4.31 — — — 



Carbon — 4.14 — — — 



Silver 4.05 — — — — 



Copper (4.0) — — 4.1 — 



Bismuth — — — 3.7 — 



Tin 3.78 — _3.5 — 



Iron 3.86 3.2: — — — 



Zinc 3.46 — — 3.4 — 



Thorium — 3.36 — — — 



Aluminum 3.06 — — 2.8 — 



Magnesium 2.63 — — 3.2 — 



Titanium — 2.4? — — — 



Lithium — — 2.35 — — 



Sodium — — 1.82 2.1 — 



SMITHSONIAN PHYSICAL TABLES 



