Theory and Low Voltage Arcs in Ccesium Vapour, 87 



work on single-line spectra the observations would not have 

 shown the presence of other lines had they existed. Thus 

 the observed single-line spectrum of mercury is \2537, but 

 the second line of the series A,143(j might be present, and, 

 lying in the extreme ultra-violet, it could not have been 

 detected by the methods employed *. 



II. Experimental. 



The present work was undertaken with the object of 

 obtaining information as to the existence of single-line 

 Fig-. 2. spectra, single-series spectra, and group 

 spectra. Caesium appeared to be a de- 

 sirable material, since all of its spectral 

 lines, except certain combination lines in 

 the far infra-red, may be photographed 

 by properly sensitized plates. The appa- 

 ratus is shown in fig. 2. The ionization 

 chamber consisted of a pyrex glass tube 

 having an optically ground pyrex plate 

 fused to one end, through which the arc 

 was viewed. The anode was a nickel 

 cylinder, and the cathode a lime-coated 

 platinum cylinder about 5 mm. in dia- 

 meter, heated by a small heating-coil 

 inside. The cathode was thus made an 

 equipotential surfnce, and because of its 

 large area the current was high (150 milli- 

 amperes at six volts), even though the 

 temperature was maintained at 500° C. 

 It is quite desirable to operate the cathode 

 considerably below a red heat, as otherwise, 

 on long exposure, stray reflexion of the 

 emitted light and heat may obscure the 

 caesium lines in the range X 6500 to X 9000. 

 All lead-wire seals were tungsten in 

 Corning Q 702 P glass. The ionization 

 tube was evacuated and maintained at 

 about 00001 mm. of mercury gas pres- 

 sure, and heated to 200° C, thus giving 

 a considerable vapour - pressure. The 

 radiation emitted near the anode where 

 the electrons had gained their maximum 

 velocity was photographed. 



V 



Vn 



V 



Ionization tube. 



* Cf. McLennan, Proc. Rov, Soc. Lond. xcii. p. 305 (1916) ; xci. 

 p. 485 (1915). 



