Density of the vapour in the mercury arc and the relative intensities of the radiated spectral lines, with special reference to the forbidden line 2270

Abstract

The mercury arc line (1 1 S 0 — 2 3 P 2 ) is ruled out by the selection principle as the transition 2 3 P 2 → 1 1 S 0 involves a change of the inner quantum number by 2. Takamine and Fukuda, however, have detected this line in the end-on radiation from a mercury vapour lamp of “the branched arc” type devised by Dr. Metcalfe and the author for the study of selective absorption in luminous mercury vapour. Foote, Takamine and Chenault have obtained the same result using an arc discharge'in mercury vapour with a hot cathode. Takamine has recently studied the intensity variations of this line with change of current density, using a modified form of “the branched arc.” The interesting fact brought out by the above investigations is that the intensity of the line increases rapidly with current density up to about 0·25 amps, cm. -2 , and then falls off rapidly. In addition, Takamine apparently finds that the intensity variations of the line in question run parallel to those of the band at 2345 attributed to HgH. An increase in the current density in a mercury arc is usually accompanied by an increase in the density of the vapour through which the discharge passes, and one is led to expect a modification in the character of the spectrum by an increase in the concentration of normal atoms resulting from an increase of the density of the vapour, analogous to the effect of a foreign gas like helium or argon on fluorescence in mercury vapour. The present investigation was undertaken with the object of studying (1) the intensity variation of the line 1 1 S 0 — 2 3 P 2 as compared with the intensities of neighbouring lines when, keeping the length of the arc, the current density in it and the voltage drop between the electrodes unaltered, the density of the vapour is varied; (2) the change in its absolute intensity when the current density is altered, the density of the vapour remaining unchanged, and (3) the influence of lowering the density of the vapour on the relative and absolute intensities of the lines belonging to a series in the arc spectrum, other conditions of excitation remaining unaltered.