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BELL SYSTEM TECHNICAL JOURNAL 



and Foote and Mohler, who were the first to apply this method to 

 ionization by Hght, perceived the effect at pressures of mercury vapor 

 as low as .002 mm. Hg. It does not permit of absolute measurements; 

 but one may use it to make accurate measurements of the relative 

 ionizing-power of light of any number of wavelengths, and then stand- 

 ardize them en bloc by a determination at a single wavelength with the 

 absolute method. 



3200 



3000 



2800 2600 

 WAVE LENGTH 



2400 



2200 



Fig. 1 — Ionization by light plotted as function of wavelength for caesium (Critical 

 wavelength: 3184A). (F. L. Mohler, C. Boeckner). 



I now reproduce two of the most recently published curves of ioniza- 

 tion vs. wavelength: Fig. 1 for caesium, from F. L. Mohler and C. 

 Boeckner;^ Fig. 2 for rubidium, from E. O. Lawrence and N. E. 

 Edlefsen.^ It is the downward trend of these curves from the limiting 

 wavelength towards shorter waves which interests us now. Ionization 

 by light of a given intensity is most abundant when the quanta have 

 just the energy required to detach the electron, and no more. The 

 more the energy of the photon exceeds the strictly necessary value, 

 the less it is likely to be captured and have its energy spent for ioni- 

 zation. 



The various theories, except for one, predict a steady downward 



trend; one in particular, that of R. Becker, supplies the broken curve 



5 Bur. Stand. Journ. Res., 3, pp. 303-314 (1929). 

 ^Phys. Rev., (2) 34, pp. 233-242 (1929). 



