PHYSICS 367 



as one would expect from the values of atom dimensions and 

 distances apart calculated on other grounds. It is interesting 

 to note also that it gives an explanation of a remark of 

 Richardson's that the values of (f> for some elements — such as 

 sodium, potassium, and carbon — are inversely proportional 

 to the cube roots of their atomic volumes. 



In the latest paper, Phys. Review, September last, Millikan 

 carries the matter to a further stage, drawing a distinction 

 between what he calls " intrinsic " and " spurious " contact 

 E.M.F.s. He first of all splits the w term in the photo-electric 

 equation into two parts — Wj^ required to detach an electron 

 from its parent atom, and w^ to remove it through the surface 

 of the metal ; for it has been found that insulators exhibit the 

 photo-electric effect as well as conductors. Hence, using V^ 

 and V,„^ for the stopping potentials of each of the metals faced 

 successively by a heavily oxidised surface of copper to which 

 this stopping potential is applied, and K and K^ for the contact 

 E.M.F.s between each metal and the oxidised surface, we have 

 from (3) 



6(V^ +K.) =hv - {w^ + ze;,) 

 6(VJ + K^) =hv- {w^^ + O 



.-., 6 (V,, - V„.i + K - KO = K'-eyi) + {w^' - w^). (5) 

 But in this analysis w^ and w^ are clearly the analogues of 

 w and w^ in Richardson's analysis, i.e. the pure surface work 

 terms. Hence {w./ — ^^^2)/^ is the contact E.M.F. between the 

 metals and so is equal to K — K^ Hence by (5) 



^ (V^ - V^O = ^1' - ^1 (6). 



Now Millikan has found that the stopping potentials for 

 sodium, potassium, and lithium surfaces freshly cut and tested 

 at once facing the oxidised copper surface are accurately the 

 same ; i.e. V„, = V,„^ (if we generalise this result) and so by (6) 



w-^^ — Wi =0. 



This is a most important result in connection with the 

 Quantum hypothesis. It involves one of two alternatives : 



(i) Either Wx= w-^, i.e. the work required to detach an 

 electron from an atom is the same for all atoms, which is violently 

 at variance with all our knowledge on this matter, or — 



(2) In the above analysis hv — w^ has to be replaced by 

 hv, i.e. the photo-electron which escapes with the maximum 

 velocity from a metal under the influence of light with a 

 frequency v has exactly the energy hv, when it arrives at the 

 inner surface of the metal on its way out. This implies that 

 the incident light energy is not absorbed in whole quanta at all, 

 but that absorption continues within the atom, until the electron 



