﻿Sodium-Potassium Vapour Ave Lamp. 947 



very faint compared with the sodium lines under all condi- 

 tions, and they became weaker as the temperature increased. 

 The relative brightness of the sodium and potassium lines 

 differed in various parts of the lamp, and the subordinate 

 series lines of sodium varied in intensity compared with the 

 D lines, although the latter were always the brightest. At 

 the cathode the potassium lines were very weak, while the 

 subordinate series lines of sodium were strong (PL Y. fig. I.) 

 At C the radiation was very intense (PI. V. tig. II.), while at 

 the anode the potassium lines were brighter than they were 

 at other parts of the lamp (PL V. fig. in.). The spectrograms 

 were photographed with a constant deviation type of spectro- 

 meter, Wratten panchromatic plates being used. The sub- 

 ordinate series lines of sodium became faint when the current 

 was reduced, and when the lamp was heated externally 

 (PL V. fig. iv.). The electric discharge gave a radiation 

 consisting almost entirely of the D lines (PL Y. fig. v.). 



The intensity of the spectrum lines emitted by a mixture 

 of vapours when subject to electrical stimulus depends on the 

 ionization and resonance potentials of the various vapours, 

 and also on the partial vapour pressures. On the Bohr 

 theory the spectrum lines have their origin in the movements 

 of an electron within the atom when it moves from one 

 temporary orbit to another. In the case of sodium the 

 innermost orbit is that represented by the limit of the 

 principal series —that is, by the term 1*5 S. The second orbit 

 is represented by the term 2p, and the frequency of the 

 resonance line is that of the first principal line 1*5 S— 2/>. 

 The theoretical value of the resonance potential of sodium 

 vapour is 2*10 volts, and electrons of this energy produce 

 the D lines. Electrons having a velocity corresponding to 

 about 5*13 volts are able to ionize sodium vapour and cause 

 it to emit all the lines, including those of the subordinate 

 series. The resonance potential of potassium vapour is 1*60 

 volts, while the ionization potential is 4*33 volts. In a mixture 

 of sodium and potassium vapours, as the accelerating potential 

 is increased, the 7699, 7665 doublet of potassium should 

 appear first, then the 5896, 5890 doublet of sodium, followed 

 by the subordinate series of potassium and sodium respectively. 

 The doublet 7699, 7665 being near the limit of the visible 

 spectrum would be faint, and so it is to be expected that the 

 D lines will, under all conditions, be the brightest lines in 

 the spectra. 



When the current density is increased, the subordinate 

 series lines increase in luminosity. Sodium vapour having 

 only one resonance potential, the elevation in energy of the 



