Enhanced Spectrum of Magnesium in Low Voltage Arc. 1003 



which neglects the modifying effects arising in the mutual 

 repulsion o£ the electrons forming the atom. 



The highest convergence frequency in the arc spectrum 

 (nuclear charge le) of an alkali earth is 1/5 S, corresponding 

 by the quantum relation Jiv = eV to the amount of work 

 required to remove one valence electron from the neutral 

 atom. Similarly, the highest convergence frequency of the 

 simply enhanced spectrum is 1'5©, which thus corresponds 

 to the amount of work required to remove the second valence 

 electron after the first has been ejected. 



The enhanced spectrum of magnesium has been studied by 

 several investigators, recently by Fowler * and by Fuesf. 

 The value of 1/5© is v = 121,270 (wave number), corre- 

 sponding to a potential difference of 14*97 volts. Hence it 

 would appear that the simply enhanced spectrum of magnesium 

 could be excited in a 15 volt arc. In fact, the enhanced 

 spectrum of any element, if arising from a two-fold ioni- 

 zation, could be produced at less than 50 volts, since it 

 requires the energy equivalent to only 54 volts to remove 

 the second electron from the helium atom. The removal of 

 the second electron from atoms of other elements requires 

 less work on account of the repulsive force due to the 

 remaining electrons. This evident conclusion is rather 

 striking in view of the fact that enhanced spectra have been 

 in general studied with a capacity discharge at ten to twenty 

 thousand volts — a voltage representing an amount of work 

 sufficient to completely disrupt atoms of low atomic number. 



The present experiment was undertaken with the object 

 of determining approximately the value of the critical voltage 

 necessary to excite the enhanced doublets of magnesium. 

 Fig. 1 illustrates the apparatus employed. The arc was 

 formed in a 3-electrode tube having a tungsten hot-wire 

 cathode, a spiral grid mounted as closely as possible to the 

 cathode and a concentric cylindrical plate, 6 cm. in diameter, 

 in metallic contact with the grid. The accelerating field 

 was applied between the cathode and grid. These electrodes 

 were mounted within a heated and evacuated (0*0001 mm. 

 Hg) pyrex glass tube containing the boiling metal vapour. 

 By properly regulating the temperature (400° to 500° 0.) a 

 pressure range may be chosen for which relatively few 

 collisions of electrons and atoms occur in the short space in 

 which the accelerating field is applied, so that the electrons 

 passing through the grid attain the velocity corresponding 



* Fowler, Phil. Trans, ccxiv. p. 265 (1914). 



t Fues, Ann: d. Phys. lxiii. p. 1 (1920). We follow here the notation 

 used by Fues, Paschen, et al. 



3U 2 



