490 BELL SYSTEM TECHNICAL JOURNAL 



that ^"^"^ions appear in argon bombarded by 45- volt electrons: do 

 45 equivalent volts constitute the amount of energy necessary to 

 remove two electrons at once from a normal atom, or the amount 

 necessary to remove one electron from an atom which a prior electron- 

 impact has ionized? The question is not different in principle from 

 one arising in measurements of the first ionizing-potential, whether 

 the first appearance of ions signifies simply that atoms are being ionized 

 in two stages; but apparently it is harder to settle by direct evidence. 



Analysis of the spectra of the ion and of the atom whenever prac- 

 ticable, discloses definitely the energy-differences between the state 

 of double ionization, the state of single ionization, and the normal 

 state of the neutral atom. Thus with helium the first of these is 

 greater than the second by 54 and then the third by 79 equivalent 

 volts. Similar calculations for magnesium show that the first is 

 greater than the second by 15 equivalent volts; as the spectrum of 

 the ion il/g"^ in Foote's just-cited experiments appeared at about that 

 energy of the bombarding electrons, the atoms in the vapor must have 

 been ionized by two successive impacts. 



In the course of R. A. Millikan's observations upon droplets of oil 

 floating in ionized gases, he found that they never captured charges 

 amounting to 2e or a greater multiple of e, except in the solitary 

 instance of helium traversed by alpha-particles; in this case about 

 one out of every six positive charges captured was a double electron- 

 charge 2e. He concluded that not more than one electron was ever 

 detached from an atom in a single operation, except that among 

 encounters of alpha-particles with helium atoms about one-sixth 

 caused both of the electrons of the struck atom to be torn away." 



Detachments of two or more tightly-bound electrons from a massive 

 atom, whether effected in one operation or in several, might be revealed 

 by additional absorption-edges in the spectrum of an X-ray beam 

 after passing through matter; certain delicate features in X-ray 

 spectra have in fact been explained in this manner. 



Thermal Ionization ^* 



In addition to all the information about ionization by particular 

 agents such as electrons of specified speeds and radiation of specified 

 frequencies, there is reason for making certain assertions about ioniza- 



^' The percentage may well have been much greater, since many of the ions left 

 behind after the passage of the alpha-particle were probably produced by secondary, 

 not primary ionization (R. H. fowler). 



28 General references: ¥.. A. Milne, Proc. Pliys. Soc. London 36, pp. 94-113, and 

 literature there cited; A. .\. xNoyes, H. A. Wilson, Pro. Nat. Acad. Sci. 8, pp. 303-307 

 (1922). 



