PHOTONS AND ELECTRONS 11 



two plane parallel electrodes enclosed in a tube, one of them being the 

 illuminated target, the other the so-called "collector." If the potential 

 difference V between collector and target is positive — collector more 

 positive than target — we ofcserve this photoelectric current im- If V 

 is negative and its absolute value is large, there is no current at all; for 

 all of the electrons which emerge from the target are driven back into it 

 by the adverse field, and the net effect is the same as though they had 

 never come out. If V is negative and its absolute value is small, there is 

 some current, i, thovigh not so great as im. To speak more precisely: 

 if F is varied continuously from large positive to large negative values, 

 i remains at first constant and equal to im, then drops gradually to zero; 

 it becomes zero at a certain negative value of V, which we will call 

 Fo. 



These facts are interpreted by supposing that the light ejects electrons 

 from the metal, and the electrons escape in various directions with 

 various amounts of kinetic energy, these amounts ranging from zero up to 

 a maximum value E^^. When V > 0, or even (in an ideal arrangement 

 with infinitely extensive plates and no space charge) when V = 0, all 

 of these particles attain the collector. When, however, V is made 

 progressively smaller than zero, the adverse field prevents first some 

 and finally all of them from reaching the collector — first the slowest 

 and the most oblique, then the faster and the less oblique, and finally 

 the fastest which shoot out normally from the target, are held up and 

 driven back whence they came. The particular value Fo of adverse 

 voltage just defined is the one which just suffices to hold up these par- 

 ticular electrons of kinetic energy E^^^ and velocity directed normally 

 to the plates; and it is related to E^^ by the equation,* 



F _ ^Z? fQ\ 



^ma. 300 ^^^ 



Let us now suppose that (a) the emerging electrons were originally 

 among the "free" electrons which circulate within the metal and are 

 responsible for its high electrical conductivity; (h) the kinetic energy of 

 these free electrons, so long as they do not absorb any Hght, never exceeds 

 a certain maximum value eWi (I choose this odd notation because it is 

 used in the theory) ; (c) each of them which escapes does so by virtue of 

 ha\ang absorbed the entire energy hc/\ or hv of Si photon; (d) in escaping 

 each must sacrifice not less than a certain minimum amount eWa of its 

 kinetic energy in separating itself from the metal. 



^ The kinetic energy of a particle of charge e which is just able to overcome an 

 adverse potential difference V is equal to eV, provided that all three quantities E, 

 e, and V are expressed in c.g.s. units. The factor 300 in equation (9) is introduced 

 when E and e are expressed in c.g.s. units, but V is expressed in volts. Note that V 

 always includes contact potential difference. 



