644 BELL SYSTEM TECHXICAL JOURNAL 



Xow imagine that liglu of a high freqiienc>' v,. falls upon ihe potas- 

 sium; it elicits electrons of which the maxinuun energy- at emergence 

 is }iVo — P\; these highest-speed electrons arrive at the sih'er plate 

 with energy {hvo — P\^e—V), having had to overcome the additional 

 potential-drop 1' in passing from the region just outside the potassium 

 to the region just outsiflc the siKer. (The reader can make the changes 

 in language recjuired if 1' happens to be of the sign corresponding to 

 a potential-rise), l-rom (13) we see that this energy of arri\al is 

 equal to {hvo — Pi, e) — an expression from which P\. the onl\- ciuantity 

 characterizing the irradiated metal, has fallen out! Therefore the 

 electrons arrive at the silver plate wMth the same maximum speed, 

 whether the irradiated metal be potassium, sodium, silver, or an\- 

 other metal! (unless we hit upon a metal for which hv,,<Pi c, in which 

 case we shall never get any at all). 



This experiment is usually performed by putting a battery between 

 the silver and the irradiated metal, and adjusting its E.M.F. until 

 the fastest electrons are just turned back before reaching the siKer; 

 this is known as "determining the stopping potential." If our inter- 

 pretation of contact-potential-difference is correct, the stopping- 

 potential must be independent of the irradiated metal, and depend 

 only on the material of which ilie collecting-electrode is made: furtluT. 

 the diflerencc between ilu- sio])ping-potentials observed with iwn 

 different metals as collecting-electrodes should be equal lo ilieir 

 contact potential difference. These predictions ha\e been xcrilied 

 in se\'eral sets of experiments, notably by Richardson and C'onipton. 

 Millikan developed the interesting theoretical consequences which 

 they suggest. There should be similar relations in\olving tlurmionii- 

 currents; obserxations confirming them ha\e been made, bui luii so 

 extensi\ely published; they are more difficult to make with accurac\- 

 because the thermionic electrons have no definite recognizable maxi- 

 mimi \elocii\. 



We seem to li,i\i- niaisii.iik'd a forniidaiilc .unount of e\ ideiue in 

 faxor of till' electron-theory of conduction witli tile associated idea 

 ol the suit. ICC double-la\'er. \'el it would be niisleadiiig nol to poiiil 

 out that an ec|uati(jn quite as satisfactor\' as (S) in representing the 

 thermionic current as function of temperature can be deduced b>- 

 reasoning in an entirely different fashion from entirely different 

 postulates. This, the lhermod\iiamical method of speculating about 

 the ihermionic etTecl, was originated !>> II. .\. Wilsiiii; ii consi-is 

 essentially in assuming a thoroughgoing .iii,l1(ii;\ bciwccn the nuillnw 

 of electrons from a hot met.il .ind the evaporation of molecules from 

 a solid or a liquid. We know ili.ii if an evacuated chamber is partly 



