482 BELL SYSTEM TECHNICAL JOURNAL 



Since the extraction-energy of each named class of electrons increases 

 along the periodic table, the lines designated as 4, 5 and 6 in the elec- 

 tronic spectrum of silver reappear in that of tin, displaced in the direc- 

 tion of diminishing electron-speeds, that is, to the left. But, as to the 

 lines 1, 2, and 3, both the extraction-energies of the electrons and the 

 frequencies of the rays responsible for these alter as one passes from 

 silver to tin, and the net result of the double alteration is that the lines 

 are displaced to the right. 



The energy-values of the various states of ionization of an atom — 

 or, in terms of the customary picture, the extraction-energies of the 

 various classes of electrons within the atom, — may be determined with 

 a certain degree of precision from experiments such as these. However, 

 as in the case of the measurement of charge-to-mass ratios for individ- 

 ual ions by the methods of Aston and Dempster, there is little incentive 

 to develop the accuracy of the method to the highest possible extent; 

 for most of the energy-values in question can be determined with very 

 great accuracy in another way, which we will now examine. 



Absorption of Radiation Through Ionization 



When a beam of radiation of frequency v is transmitted through a 

 layer of matter, from the atoms of which it extracts electrons with an 

 expenditure of energy Af at each extraction, we should expect to find it 

 correspondingly reduced in intensity when it emerges from the layer. 



This effect is strikingly conspicuous with radiation high enough in 

 frequency to detach the tightly-bound electrons of massive atoms. 

 Let a narrow beam of "heterogeneous" radiation, containing all fre- 

 quencies throughout the widest possible range, fall from an X-ray tube 

 through slits and diaphragms upon a thin layer of such atoms; let the 

 transmitted rays be dispersed by some appropriate spectroscope, and 

 fall finally upon a photographic plate on which their spectrum — in the 

 ordinary sense of the word, not in the sense of "electronic spectrum" — 

 is outspread. 



In Fig. 10 there are three such spectra, of heterogeneous beams 

 which have passed through layers of cadmium, antimony, and barium 

 respectively. The frequency increases from right to left. The darken- 

 ing at any point is a measure of the intensity with which the X-rays 

 acted at that point. 



Below a certain frequency identical for all three elements, the pho- 

 tographic films have evidently been little affected; as soon as this 

 critical frequency is exceeded, the effect suddenly becomes enormous. 

 This critical frequency is the one for which the quantum-energy just 



