SOME CONTEMPORARY .inf.lXCES IN I'HYSICS- VII 293 



which rt'liMso electrons when illiiininaled with hulit of wi<le C()n\eii- 

 ient ranges of fre(|Ueiiry and color. Most nielals must l)e irradiated 

 with ultraviolet light, and the cxiK'rinieiits heconie very dilViciilt if 

 they nuist be (K-rforiued with lii;ht of frequencies far from the visible 

 spectrum. The values which Millikan obtained for sodium and for 

 litiiium agree within the experimental error witii one another and 

 with the mean value /, =0.')7.1() ■' (2) 



The maximum energy of the electrons released by light of the 

 frequency v is therefore equal to a quantity hv which is the same, 

 whatever metal be illuminated by the light — a ([uantity which is 

 characteristic of the light, not of the metal — minus a cjuanlity P 

 which, there is ever\' reason to belie\e, is the cjuota of energy sur- 

 renilered by each electron in passing out across the boundar\-surface 

 of the metal. 1 1 is 05 i/ each of the released electrons had received 

 a quantity hv ol energy from the light. I will go one step further, 

 and la>' down this as a rule, with another cautioush-inserted us if to 

 guard against too suddenly daring an inno\ation: 



Photoelectric emission occurs as if the energy in the light icere concen- 

 trated in pHtckets, or units, or corpuscles of amount hv, and one whole 

 unit were delivered over to each electron. 



This is a perfectly legitimate phrasing of equation (1), bin I doul)! 

 whether anyone would e\er have emplo\'ed it, e\en with the guarded 

 and apologetic as if, but for the fact that the \alue of // given in (2) 

 agreed admirably well with the value of that constant factor involved 

 in Planck's theory, the constant to which he had given this very 

 symbol and a somewhat similar role. Deferring for a few pages one 

 other extremely relevant feature of the photoelectric effect (its "in- 

 stantaneity") I will proceed to examine these other situations. 



An effect which might well be, though it is not, called the inverse 



photoelectric effect, occurs when electrons strike violently against 



metal surfaces. Since radiation striking a metal inay elicit electrons, 



it is not surprising that electrons bombarding a metal should excite 



radiation. Electrons moving as slowly as those which ultraviolet or 



l)lue light excites from sodium do not have this power; or possibly 



they do, but the radiation they excite is generally too feeble to be 



detected. Electrons moving with speeds corresponding to kinetic 



energies of hundreds of equivalent volts, '^ and especially electrons 



'One equivalent volt of energy' = the energy- acquired by an electron in passing 

 across a potential-rise of one volt=e/300 ergs= 1.591.10 " ergs. This unit is 

 usually called simply a "volt of energy", or "volt", a bad us^ige but ineradicable. 

 .Also "sijcefi" is used interchangeably with "energy" in speaking of electrons, and 

 one finds (and, what is worse, cannot avoid) such deplorable phrases as "a speed 

 of 4.9 volts" ! ! ! 



