﻿Sodiuyn-Potassium Vapour Arc Lam/). 945 



lines are broadened and show much reversal. Modern prac- 

 tical and research needs require intense radiation and a mono- 

 chromatic source. The chief line must be sufficiently removed 

 from its immediate neighbours, so that if a relatively wide 

 slit is used, other wave-lengths in the immediate vicinity of 

 the one desired are excluded. The quartz-mercury vapour 

 lamp provides such a source ; it is easy to construct and 

 work, and does not require continuous pumping to keep it 

 exhausted while running. A sodium vapour arc lamp, 

 working on the same principle, has been designed and 

 constructed by Rayleigh *, but it is more difficult to work 

 than the mercury lamp. Iron electrodes are unsuitable, 

 since they fuse and drop off after the lamp has been in use 

 for an hour or two. Tungsten, which seems to withstand 

 the action of sodium vapour, is used instead of iron. This 



Fig. 1. 





nffc? 



lamp requires an applied potential difference of 200 volts 

 when working, although the actual drop of potential across 

 the arc is Aery much less. The author f has used, previously, 

 a sodium vapour electric discharge-tube which gives intense 

 sodium radiation, but requires continuous heating while the 

 electric discharge is passing. The sodium-potassium vapour 

 arc lamp described in the present work needs no applied 

 heat, can be worked with a small applied potential difference, 

 and requires no attention while it is running. 



The form of lamp is shown in fig. 1. It is made of quartz, 

 the bulbs A, B being about 3 cm. in diameter, and joined by 

 a piece of quartz tubing C of internal bore 5 mm. and length 



* Hon. R. J. Strutt, Proe. Roy. Soc, A. xcvi. (1919). 

 t Proc. Phys. Soc. xxxiii. pt. ii. (1921). 



