404 



Table 619. 

 CONTACT (VOLTA) POTENTIALS. 



There has been considerable controversy over the reality and nature of the contact differences of potential betwe* 

 two metals. At present, due to the studies of Langmuir, there is a decided tendency to believe that this Volta diflfe 

 ence of potential is an intrinsic property of metals closely allied to the phenomena just given in Tables 516 to 518 ar 

 that the discrepancies among different observers have been caused by the same disturbing surface conditions. T] 

 following values of the contact potentials with silver and the relative photo-sensitiveness of a few of the metals a 

 from Henning, Phys. Rev. 4, 228, 1914. The values are for freshly cut surfaces in vacuo. Freshly cut surfaces a 

 more electro-positive and grow more electro-negative with age. That the observed initial velocities of emission • 

 electrons from freslily cut surfaces are nearly the same for all metals suggests that the more electro-positive a metal 

 the greater the actual velocity of emission of electrons from its surface. 



Contact fjotential with Ag. . . 

 Relative photo-sensitiveness . 



Mg 

 1.42 

 1000 



From the equation w = RT logiN j^/N g) , where w is the work necessary per gram-molecule when electrons pa| 

 through a surface barrier separating concentrations iV^ and Njf of electrons, it can be shown (Langmuir, Tr. Aj 

 Eletroch. Soc. 29, 142, 1916, et seq.) that the Volta potential difference between two metals should be 



Vi — i'2= p [wi — u'l + RT \os(Nji/Njg)} = -^, — - = <l>2 — <f>i 



(see Table 517 for significance of symbols), since the number of free electrons in different metals per unit volume is 

 nearly the same that KT log (N j^^/N g) may be neglected. The contact potentials may thus be calculated from phol 

 electric phenomena (see Table 517 for references). They are independent of the temperature. The following tab 

 gives a summary of values of 4> in volts obtained from the various phenomena where an electron is torn from the attr£ 

 tion of some surface. In the case of ionization potentials the work necessary to take an electron from an atom of mei 

 vapor is only approximately equal to that needed to separate it from a solid metal surface. 



(a) The Electron Affinity of the Elements, in Volts. 



Metal. 



Tungsten. . . . 

 Platmum. . . . 

 Tantalum. . . . 

 Molybdenum, 



Carbon 



Silver 



Copper 



Bismuth 



Tin 



Iron 



Zinc 



Thorium 



Aluminum. . . 

 Magnesium. . 

 Titanium. . . . 



Lithium 



Sodium 



Contact. 

 (Henning. 



Thermionic 



(Langmuir.) 



2.4? 



Photo- 

 electric 



and 

 contact. 



(Millikan.) 



Photo- 

 electric. 

 (Richardson) 



4.1 

 3-7 

 3-5 



3-4 



2.8 



Miscel- 

 laneous. 



(6) It should not be assumed that all the emf of an electrolytic cell is contact emf. Its emf varies with the ek 

 trolyte, whereas the contact emf is an intrinsic property of a metal. There must be an emf between the two electrci 

 of such a cell dependent upon the concentration of the electrolyte used. The following table gives in its first line I 

 electrode potential c^ of the corresponding metals (in solutions of their salts containing normal ion concentration)! 

 assumption of no contact emf at the junction of the metals. The second line, </> — e^ — 3.7 volts, gives an idea of 

 electrode potentials (arbitrary zero) e.xclusive of contact emf. 



Smithsonian Tables. 



