312 BELL SYSTEM TECHNICAL JOURNAL 



corpuscle; I have held to these alternative words quite long enougli, 

 I think, to bring out all of their connotations. Then the energy' / 

 is brought to unit area of the plate, in unit time, by I/hv of the quanta; 

 which also bring momentum amounting to I fc. Shall we not divide 

 up the momentum equally among the quanta as the energy is divided, 

 and say that each is endowed with the inherent energy hv and with the 

 inherent momentum hv/c ? 



The idea is a fascinating one, but not so easy to put to the trial 

 as one might at first imagine. None of the phenomena I have de- 

 scribed in the foregoing pages affords any means of testing it. In 

 studying the photoelectric effect, we concluded that each of the 

 electrons released from an illuminated sodium plate had received 

 the entire energy of a packet of radiation; but this does not imply 

 that each of them had received the momentum associated with that 

 energ)'; the momentum passed to the plate, to the framework support- 

 ing it, e\entually to the earth. The same statement holds true 

 for the release of electrons from the deep levels of heavy atoms, such 

 as de Broglie and Ellis observed. Even if the same experiments 

 should be performed on free atoms, as for example on mercury vapor, 

 no clear information could be expected; for the momentum of the 

 absorbed radiation may divide itself between the released electron 

 and the residuum of the atom, and this last is so massi\'e that the 

 speed it would thus acquire is too low to be noticed. Only one way 

 seems to be open; this is, to bring about an encounter between a 

 quantum of radiation and a free electron, so that whatever momentum 

 aiul whatever energy are transferred to the electron must remain 

 wiiii it, and cannot be passed along to more massive objects where 

 the momentum, so far as the possibility of observing it goes, is lost. 

 A priori one could not be certain that even this way is open; radiation 

 might ignore electrons which are not lightly bound to atoms. 



Arthur H. Complon, then of Washington University, is the physicist 

 whose experiments were the first that clearly and strikingly disclosed 

 such encounters between quanta of radiation and sensibly free elec- 

 trons. Others had observed the effect which reveals them, but his 

 were the first measuremenls accurate enough for inference. Unaware 

 al the moment of the meaning of his data, he realized it almost imme- 

 diately afterward, and .so established the fact and the explanation 

 both — a twofold achievement of a very unusual magnitude, whence 

 the phenomenon recei\ed the name of "Compton effect" h\ a universal 

 acceptance, and deser\edly. 



What Complon obscr\ed was not ihe pre.sence of electrons pos- 

 sessed of momentum acquired from radiation — these electrons were 



