SDMi: C().\ii:Mi'(iK.ii<y .mr.ixcis i\ rinsics~r mj 



is 'mpiTri-plibk'; if one wen- lo irradialc an inc'an<lfsti.-ni iiin^sii.-ii 

 tilaiiK'nt tlu- I'xtra current of pliotoelcctrons would lie too small to 

 notice. If we assume outrislit that P does not \ary greatly frtiin 

 riK)m-tem|H'r,iture up to the tem()eratures of incandescence, and 

 therefore compare photoelectric data upon cool metals with thermionic 

 data upon the same nu-tals when hot, we fmd that iheie is a fairly 

 gtKKl a^jreement. X'alues of the thermionic constant /) between l and ."» 

 \()lts corropond to photoelectric sensiti\eness commencing between 

 :M(H) and 2.."i(M) Angstrom units, and this correctly describes the 

 beh.uinr of several of the hea\>- high-melting-point metals: i)hoto- 

 electric sensiti\eness extending well up into the \isible spectrimi, 

 such as the alkali metals dispUu', corresponds to values of P e of the 

 order of '2 volts and lower, and such \alues arc indicated by the 

 thermionic experiments made upon sodium and potassium by Richard- 

 s*)n under the inevitably bad conditions. 



Contact-potential-differcnce, one of the longest known ot all elec- 

 trical phenomena — \'olta discovered it — agrees admirabh' with this 

 interpretation of the photoelectric constant P. Imagine that we have 

 pieces of two metals, potassium and silver for example, which arc 

 drawn out and welded together at one end, and at their other ends 

 are spread out into plates and face one another across a vacuous 

 space. We know that the opposing faces behave as if they were at 

 ^ssentially different potentials, the potent iaI-difTerencc V between 

 them being characteristic of the two metals and independent of the 

 size or separation of the opposing faces. Yet this potential-difference 

 r is not equal to, is indeed usually much greater than the potential- 

 ilifference between the interiors of the metal across the welded joint, 

 which is deduced from the Peltier effect. The only way to resoKe 

 the contradiction is to assume that it is the region just outside the 

 poiassiuni which differs by V from the region just outside the silver; 

 the metals themselves are at nearly the same potential, but there is 

 a double-layer at the surface of each which establishes a fixed potential- 

 drop between it and the vacuum. Representing b>- Pi e and by P2 f 

 the voltage-drops at these two double-layers, b\- M the potential- 

 (lifTerence between the interiors of the metals as inferred from the 

 IVltier effect, by V the potential-difference between the regions just 

 outside the metals which we iflentif\- with the contact potential 

 difference, we find 



/>„ e-P, e= V+.\f (i:i) 



in which .1/ is so small compared with the other terms thai hence- 

 forth we will leave it out. 



