642 BELL SYSTEM TECHNICAL JOURNAL 



light of frequency v falls upon a metal, electrons emerge from it with 

 \el(jcities which are distributed in a manner quite dislimt from 

 Maxwell's distribution and have nothing to do with the temperature 

 of the metal. The kinetic energies of some of the electrons attain a 



certain upper limit IT,,,, but iioiie siirj^.tsses it; 11',,, is a linear fiiiirtidU 

 of J' i;i\ i-ii liy \\\v (•(|ii,iti<)ii 



\\;„=hi'-P, (12) 



// being IManck's constant, P a positive constant characteristic of the 

 metal. This is an exceedingly strong intimation that each t)f the 

 emerging e'ectrons, while still inside the metal, suddenly absorbed 

 a quantum of energy hv from the light and departed with it, giving 

 up a fixed quantity P in passing through the surface. (Those which 

 issue with energies clearly less than Wm can be supposed to ha\e 

 started distinctly beneath the surface and to have lost additional 

 energy in struggling through the metal to it). Translating /' into 

 potential-drop, we see that it represents the potential-difference or 

 the "strength" of the surface double layer. It may be determined 

 by measuring Wm for light of \-arious frequencies, plotting it against 

 frequency, and extrapolating the resulting straight line to its inter- 

 section with the axis of frequencies. Or it may, in princijile, be 

 determined by plotting the photoelectric current as a function of 

 frequency, and extrapolating the curve to its intersectit)n with the 

 axis of frequencies, where no electrons escape and the photosensitive- 

 ness ceases; but cur\es are not so easy to extrapolate as straight lines, 

 and there are some anomalous results which are still unexplained. 



It would seem an easy matter to measure the strength of the double- 

 layer by both photoelectric and thermionic methods upon a single 

 substance. But it is rather difficult; for one rea.son, the substances 

 for which the photoelectric currents are easy to produce and measure 

 are precisely the metals upon which good thermionic measurements 

 are next to imi)ossible. and \ i( -e \ersa. Thi' best photdeit'Ctric meas- 

 urements have been iii.icii- upon tiie .ilk.ili nu-t.il-. which .ire \ei\ 

 sensitive to visible ligiii ; i)iit they cannot he t<irnK-ii inin \\ire>, .md 

 \-olatilize furiously when heated enough to produce an important 

 thermionic effect, tilling the evacuated tube with dense vai)ors which 

 ruin the accuracy of the measurements. The best thermionic lueasure- 

 ments have been made upon platinum and tungsten, which are not 

 sensiti\-c at all to visible light, and begin to be sensitive far out in 

 the uUra\iolet where experiments with radiation are difficult. I'ur- 

 thermore there is the capital difficulty that the photoelectric me.ismc- 

 menls must be confined to temperatures where the thermionic cm rent 



