Enhanced Spectrum of Magnesium in Loiv Voltage Arc. 1011 



important one in arc characteristics will appear at once. 

 Referring to Table I., in the 7-volt arc (below the ionization 

 potential) we find the following arc-lines : — 



Line. Intensity. Notation. 



2852 10 1-5S-2P 



3337 1- 2p x -2'5s 



3829-32-38 2 2p-U 



4571 5 l-5S-2jo 2 



5167-73-83 1- 2p-l'5s 



The two lines 2852 and 4571 are accounted for by the two 

 -resonance potentials, but the presence of the other arc lines 

 requires further explanation. It will be noted that these 

 lines are terms of series converging at 2p, the first ring out 

 from the stable 1*5 S ring, as shown in fig. 2. We have 

 found that such lines are greatly intensified by employing a 

 high current density. This fact was demonstrated in the 

 photographs published by Foote and Meggers * for the 

 csesium arc. Also in recent unpublished work of the present 

 writers with potassium, some of the strongest lines on the 

 plate, of series converging at 2p, with a current of 1000 

 milliamperes, were very faint in a similar exposure at 50 

 milliamperes, although the exposure times were so adjusted 

 that other corresponding lines showed about the same 

 intensity. This indicates that the electron normally in the 

 1*5 S ring of the magnesium atom may be ejected to the 2p 2 

 ring, either by direct impact or by absorption of radiation of 

 the frequency 1*5 S — 2p 2 , and before it has had time to return 

 to the 1*5 S ring again it collides with a second electron 

 which drives it still farther away from the nucleus of the 

 atom. It is then in a position to fall to the 2p 2 ring, pro- 

 ducing a line in a series converging at 2p 2 ,.and finally may 

 fall to 1*5 S, producing the line X4571. This explanation 

 appears more probable than the assumption that displace- 

 ments to any ring directly from the 1*5 S ring may take 

 place on electronic impact of the corresponding energy 

 value, since we have not been able to observe more than two 

 resonance potentials for any of the alkali earths (viz, 

 corresponding to electron displacements from 1*5 S to 2 P 

 and 2p 2 respectively). 



Certain lines of the enhanced spectrum may be excited 



* Foote and Meggers, Bureau of Standards Scientific Taper No. 38G. 



