90 BELL SYSTEM TECHNICAL JOURNAL 



If there is but one frequency in the inflowing radiation, the spectrum 

 of the emitted electrons will contain one line composed of what were 

 formerly X-electrons. It will contain others, composed of electrons 

 which originally belonged to other and less firmly-bound sets within 

 the atoms. We distinguish, in order of decreasing binding-energy, 

 K and Lj and Ln and L„i and M^ and Mu and Mm and M„ and M^ 

 and still further classes of electrons. The electron-spectrum due to 

 radiation of a single frequency attacking atoms of a single kind com- 

 prises a line for each of these classes (apart from those, if any, for which 

 the binding-energy exceeds the quantum-energy hv so that the radia- 

 tion cannot detach them) and the speed of the electrons composing 

 each line is determined by an equation like (13), with the appropriate 

 extraction-energy PTli or Wi^u or whichever it may be inserted in 

 place of Wk- If there is more than one frequency in the incident 

 radiation, each produces its own system of lines. These statements 

 are proved, and the binding-energies are determined for all the classes 

 of electrons and most of the kinds of metallic atoms, by irradiating 

 metals with X-rays of which the frequencies are known, for they can 

 be separately measured." To ascertain the binding-energy of, let us 

 say, the L„ electrons of platinum, one has only to look into the stand- 

 ard tables. 



Now we have seen already that the physical and chemical properties 

 of each radioactive substance, so far as they are known, are almost 

 exactly like those of its stable isotope (if it has one) ; and with this rule 

 the resemblance between the beta-ray spectrum of a radioactive sub- 

 stance and the electronic spectrum which its gamma-rays elicit from 

 its stable isotope most admirably conforms. When a line in the 

 former spectrum obviously corresponds to a line in the latter, both 

 presumably are composed of electrons extracted from the same level 

 by the same radiation. The same gamma-rays are working upon 

 atoms isotopic with one another, and therefore endowed with electron- 

 Theory of Relativity, to wit : 



T = moc"- \ , - 1 \ = hv - Wk 



X-rays generated by artificial means never have frequencies so high that the electrons 

 which they expel move rapidly enough for the simple substitution T = hmv- to be 

 inadequate; but the frequencies of some of the gamma-rays are so great that the 

 electrons which they extract even from the i^-layers of massive atoms depart with 

 speeds much exceeding 3 ■ lO^ cm/sec. J. Thibaud has made direct measurements of 

 a certain gamma-ray frequency and of the speed of the electrons which it ejects from a 

 certain group of known extraction-energy, which are compatible with one another 

 and with equation (13) if the relativity-formula for T is used, but decidedly incom- 

 patible if T be set equal to Iniav^ or to the once well-known expression derived by 

 Abraham (J. Thibaud, Ueffet photo eledri que compose; Paris (Masson) 1926). 

 27 Introduction, pp. 192-195, 273-282. 



