Enhanced Spectrum of Magnesium in Low Voltage Arc. 1005 



spark-lines of the alkali earths, although the majority of 

 electrons do not attain a velocity much exceeding that 

 corresponding to the simple ionization potential. 



Plate XXVII.* illustrates typical spectra of magnesium 

 obtained for electron velocities of 3*2, 6' 5, 10, 20, 30, and 

 60 volts. It should be emphasized that in the reproduction 

 of such photographs much of the detail of the original 

 negatives is obscured. However, the general effect of 

 increased electron velocity is readily noted. An analysis 

 of some of the plates measured is given in Table I. These 

 spectrograms were made with Seed's 30 plates, using a large 

 Hilger quartz spectrograph, Type C. The lines were 

 carefully measured and reduced to wave-lengths by the 

 method of the Hartman-Cornu interpolation formula, using 

 Fowler's measurements as standards. The arc lines of 

 magnesium were given by Fowler f on the Rowland scale 

 of wave-lengths and a table of enhanced lines in international 

 Angstrom units was published later J. The arc lines in 

 Table 1. are Fowler's values corrected to the international 

 scale, except for the lines belonging to the last three triplets 

 of the diffuse and of the sharp series which are here listed 

 for the first time. The intensities represent careful visual 

 estimates of the relative photographic effect of the different 

 lines. 



* Explanation of PL XXVII. — The voltages represent the maximum 

 values of applied potentials, since the accelerating' potential was con- 

 nected to the negative hot-wire terminal. The following legend gives 

 other data : — 



No. Volts. Milliamperes. Time of exposure. 



24 3-2 1 15 hours 



22 6-5 5-6 2 hours 



13 10 300 10 minutes 



16 20 200 45 minutes 



14 30 200 30 minutes 



15 60 400 15 minutes 



Iii the first two exposures the plates were so badly fogged around 

 A 4600 by the continuous spectrum from the hotwire that in making 

 prints for this reproduction it was necessary to over-expose. In this 

 way it was possible to reduce in the print the intensity of the continuous 

 spectrum, but the brightness of the emission-lines of magnesium are also 

 reduced somewhat by this procedure. For the same reason the repro- 

 duction does not show the fainter lines in spectra obtained at the higher 

 voltages. Table I., representing a careful analysis of our best spectro- 

 grams, should be regarded as supplementing and correcting these repro- 

 ductions, which, at best, are difficult to make and are not wholly 

 satisfactory. 



f Fowler, Proc. Roy. Soc. Lond. lxxxix. p. 133 (1918). 



t Fowler, Phil. Trans, ccxiv. p. 205 (1914). 



