490 BELL SYSTEM TECHSICAL JOURSAL 



charge +'2e, the residuum is found to be a nucleus of an element two 

 steps farther down in the procession of elements. Thus in passing 

 from one element to the next above it, the nuclear charge is found to 

 be augmented by e. This law is deduced from numerous observations 

 on the elements beyond the eighty-first. 



Moseley's law. The square root of the fre(iuenc\- o( the A'a-ra\' 

 (a prominent and easily-identified member of the X-ray spectrum) 

 increases by' a constant amount in passing from one element to the 

 ne.xt above it. This law is valid from the twelfth to the ninety-second 

 element m the periodic table. The same law go\erns, though not 

 with such entire accurac\', the other ideniiriablc members of the 

 X-ray spectrum. 



Apart from all interpretation, Moseley's law means that there is a 

 certain important measurable quantity which i.s very characteristic 

 of the elements and increases uniformly and steadily from one to the 

 ne.\t, over almost the entire procession. The mere existence of such a 

 quantity inspires confidence that there is a true physical seriation 

 of the elements, but by interpretation a great deal more can be added. 

 ■ Bohr's theory of the atoms of hydrogen and ionized helium lead to 

 this result: when a single electron forms an atomic system with a 

 nucleus of charge Ne, one of the frequencies which this system can 

 radiate — and the frequency which, on the whole, it would oftenest 

 and most intensel\- radiate -is t-qual lo 



v=~ RN-, R = 2Tr-ni e\ li\ ( 1 ) 



4 



This is verified for h\(lrogi-n and ionized helium, each of these atoms 

 consisting of a nucleus and a single electron. No other such atom has 

 yet been isolated and made to radiate. Biii we might imagine that 

 in a massive atom containing many electrons, one lies deep down 

 beneath the others, and revolves by itself in the field of the nucleus, 

 undisturbed by the rest. In this case there would be an X-ray fre- 

 fiuenc>- emitted by the atom, given b>- (1). The difference between 

 the values of the sc|uare rool of tlii> frc(|ui-n(\ for conseculixe ele- 

 ments would be constant ami r(ni,il id 



^=>/! 



R. (2) 



Now the oi)ser\ed constant difference betweiMi tile \aiues ol the s(]uare 

 root of the Ka frequency for consecutive elements docs conform to 

 (2). But the actual value of the frequency does not conform to (1) 



