Theory and Low Voltage Arcs, in Ccesium Vapour, 95 



being started, the arc operated with eitherelectrode as cathode, 

 and that no rectification of alternating current was observ- 

 able. This phenomenon is well known in certain types of 

 mercury arc. With the outer cylinder as a cathode a quiet 

 dazzlinff-glow discharge is obtained, while with the small hot 

 cathode the arc forms streams of brilliant discharge which 

 flicker back and forth along the tube. 



A much discussed question as to the ratio of intensities of 

 the components of a doublet when the exciting voltage is 

 varied may be answered by referring to fig. 4. In absolute 

 measure the ratio of intensities A8521/X8943 is constant and 

 equal to 1*5 within the errors of observation. This value 

 was further confirmed by observations on a 120-volt arc. 



Although the caesium employed was made from csesium 

 chloride of supposedly high purity, it contained traces of both 

 sodium and potassium. All exposures above 2*2 volts applied 

 potential show the presence of the first doublet of the prin- 

 cipal series l'os — 2y>i of both these metals, the ionization 

 potentials of which are 5*1 volts and 4*3 volts respectively. 

 Accordingly, in the light of the more extensive work described 

 with csesium, we may conclude that the doublets l'5s — % r p\ are 

 the single-line spectra of sodium and potassium. 



IV. Summary. 

 Theoretical. 



On the basis of several possible theories of atomic structure, 

 it is shown that the normal operation of an arc below ioniza- 

 tion might result in the excitation of a single-line spectrum, 

 a single-series spectrum, or a group spectrum consisting of 

 certain lines of different series. This latter conclusion fol- 

 lows from an extension of Bohr's theory. Thus, if inelastic 

 electronic-atomic impact occurred resulting in the ejection 

 of an electron to the j>th ring, the electron in returning to 

 the nth. ring or equilibrium may produce any combination 

 of lines represented by inter-orbit transitions within this 

 range, the single-line spectrum being a special case where 

 jp=n4 1. 



A simple explanation is offered of fluorescence phenomena 

 in vapours of the alkali metals. 



A mechanism of absorption of radiation is described, and 

 the theory proposed by K. (Jompton, that the ionization of an 

 atom below the ionization potential may be explained by 

 absorption of radiation arising in other atoms from electronic- 

 atomic impact of insufficient energy to ionize, is further 



