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BELL SYSTEM TECHNICAL JOURNAL 



radiation-frcciiu'iuics, tlu' (lilTcrence between these is not so radical as 

 once it seemed, and we ma\- without hesitation call them by some such 

 name as electron ic spectra. 



Each line in such a sped rum is produced by electrons of a definite 

 extraction-energy, extracted by radiation of a definite frequency. Con- 

 tinuing with the policy of referring to electrons with a definite extrac- 

 tion-energy as being definitely individualized within the atom, I will 

 designate the electrons of greatest extraction-energy for any particular 

 kind of atom as the K electrons; those of next greatest extraction- 

 energy as the L electrons, and then the Mand iV electrons in due order. 

 (Later it will be necessary- to subdivide these classes, but for the mo- 

 ment this may be avoided.) At other times I shall speak of these elec- 



Fig. 9 



trons as belonging to the K level, the L level, and so forth; still other 

 terms in use are the K shell and the L shell, or the K ring and the L ring. 

 In a given electronic spectrum we may expect to find a set of lines due 

 to K, L, M and other electrons, for each frequency represented in the 

 incident radiation; unless there are some of these frequencies for which 

 the quantum energy hv is less than the extraction-energies of some of 

 the electron-groups, in which case there will be no corresponding lines. 



An ideally simple electronic spectrum would be produced by a single 

 radiation-frequency; l)ut this is impracticable, for even if one were to 

 eliminate from the stream of X-rays proceeding out of an X-ray tube 

 all but one frequency, the irradiated atoms would themselves supply 

 others. 1^ As in the mass-spectra upon Aston's plates, this unavoidable 

 complexity is actually an advantage; it helps in identifying the severed 

 lines. 



In Figs. 8 and 9, photographs taken in the manner already men- 

 tioned, there appears the electronic spectrum due to siK-er atoms irra- 



"5 These are in fact especially efficient in ejecting electrons, as they originate 

 within the atom-layer itself. 



