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CV. The Excitation of the Enhanced Spectrum of Magnesium 

 in a Loiu Voltage Arc. By Paul D. Foote, Ph.D., 

 W. F. Meggers; Ph.D., and F. L. Mohler, Ph.D.* 



[Plate XXVII.] 



Synopsis. 



A vacuum arc is described in which the electronic-atomic collisions 

 take place at any desired definite velocity of the electrons. The neutral 

 magnesium atom absorbs quanta of the folloioing values, which may be 

 emitted as equivalent quanta of radiation : 46*9 volts, L-radiation ; 22-8 

 volts (probably), producing double ionization and resulting in the simple 

 enhanced and the arc spectra ; 7*61 volts, producing simple ionization 

 and resulting in the arc spectrum; 4*33 volts, resulting in the two-line 

 spectrum ; 2*70 volts resulting in the single-line spectrum. The simply 

 ionized magnesium atom absorbs quanta as follows : 14"97 volts, producing 

 double ionization and resulting in the simple enhanced spectrum ; 

 4*4 volts resulting in the single-line enhanced spectrum. With high 

 current density other quanta may be absorbed corresponding to funda- 

 mental lines of the subordinate series. 



SOMMERFELDf has shown there is evidence that 

 enhanced spectra arise from the removal of more than 

 one electron from the normal atom. In the case of the 

 alkali earths the simplest type of enhanced spectra would be 

 produced by the removal, of both valence electrons followed 

 by the return of one of these to its equilibrium position 

 in successive interorbital transitions J. We have to deal 

 accordingly with a core of net charge +2e. The outer 

 structure of the atom is then similar to the simply ionized 

 element of the next lower atomic number, with the exception 

 that the net nuclear charge is *2e instead of e. Hence the 

 enhanced spectra should be doublets, characteristic of the 

 "arc" spectrum of the alkali metals, but with 4 N instead 

 of N appearing in the simple spectral series formulae. Also 

 the separation of the doublets in the enhanced spectra of an 

 alkali earth should be approximately four times that for the 

 corresponding alkali. Sommerfeld (loc. cit.) has pointed out 

 that these conditions are approximately confirmed by existing 

 data. An exact confirmation would not be expected on the 

 basis of these deductions made from the simplest theory 



* Communicated by the Authors, and published by permission of the 

 Director of the Bureau of Standards. 



t Sommerfeld, Atombau mid Spectrallinien, i-nd edition (1920). 



X More generally interpreted as transitions between quasi -stationary 

 states of different quantum number. 



