TABLE 688.— CONTACT (VOLTA) POTENTIALS 637 



There has been considerable controversy over the reality and nature of the contact 

 differences of potential between two metals. At present, owing to the studies of Langmuir, 

 there is a decided tendency to believe that this Volta difference of potential is an intrinsic 

 property of metals closely allied to the phenomena given in Tables 684 to 688 and that 

 the discrepancies among different observers have been caused by the same disturbing 

 surface conditions. The values are for freshly cut surfaces in vacuo. Freshly cut sur- 

 faces are more electropositive and grow more electronegative with age. That the observed 

 initial velocities of emission of electrons from freshly cut surfaces are nearly the same for 

 all metals suggests that the more electropositive a metal is the greater the actual velocity 



of emission of electrons from its surface. 



Ag Cu Fe Brass Sn Zn Al Mg 



Contact potential with Ag .05 .19 .21 .27 .59 .99 1.42 



Relative photosensitiveness 50 60 65 45 70 80 500 1000 



Pt Fe Cu Au Ag Al Mg Zn Pb Sn 



SiO, +2.22 +1.99 +1.60 +1.60 +1.42 +.93 +.93 +.45 +.16 - .30 



Glass +1.15 +1.15 + .58 + .58 + .58 +.14 +.14 —.29 —.60 —1.14 



Cu Cr Ta Mo Ni 



W +.08 +.11 —.38 —.21 —.17 



From the equation w = RT log (Na/N b ), where w is the work necessary per gram- 

 molecule when electrons pass through a surface barrier separating concentrations Na and 

 Nb of electrons, it can be shown that the Volta potential difference between two metals 

 should be 



ih — Vi = —{w2 — Wi + RT \og(N A /N B )} = Er^ 1 — 02 — 0i 



r r 



(see Table 686 for significance of symbols), since the number of free electrons in different 

 metals per unit volume is so nearly the same that RT log (Na/Nb) may be neglected. 

 The contact potentials may thus be calculated from photoelectric phenomena. They are in- 

 dependent of the temperature. The following table gives a summary of values of </> in volts 

 obtained from the various phenomena where an electron is torn from the attraction of 

 some surface. In the case of ionization potentials the work necessary to take an electron 

 from an atom of metal vapor is only approximately equal to that needed to separate it 

 from a solid metal surface. 



TABLE 689.— ELECTRODE POTENTIALS 



It should not be assumed that all the emf of an electrolytic cell is contact emf. Its emf 

 varies with the electrolyte, whereas the contact emf is an intrinsic property of a metal. 

 There must be an emf between the two electrodes of such a cell dependent upon the con- 

 centration of the electrolyte used. The following table gives in its first line the electrode 

 potential eh of the corresponding metals (in solutions of their salts containing normal ion 

 concentration) on assumption of no contact emf at the junction of the metals. The second 

 line, <p — en — 3.7 volts, gives an idea of the electrode potentials (arbitrary zero) exclusive 

 of contact emf. 



Metal Ag Cu Bi Sn Fe Zn 



ex +.80 +.34 +.20 —.10 —.43 —.76 



<t> — e h — 3.7 -.40 +.04 +.20 -.20 —.43 -.46 



SMITHSONIAN PHYSICAL TABLES 



