t 353 ] 



XXXI. A New Cadmium Vapour Arc Lain]?. 

 By Frederick Bates *. 



r | 1 HE necessity for increasing the intensity as well as the 

 -I number of monochromatic light sources has frequently 

 been emphasized during the past few years. Unfortunately 

 but little has been accomplished toward attaining this 

 objective. In 1906, the writer f directed attention to the 

 importance of this subject and suggested that the so-called 

 yellow-green line (X = 5461 A) of incandescent mercury 

 vapour be adopted as the source for standardization purposes 

 in polarimetric work. The quartz-mercury vapour-lamp 

 was a great advance in that it provided not only the yellow- 

 green line, -but several additional lines of lesser intensity. 

 The best available methods of optical purification are such 

 that a monochromatic source is of little A^alue unless the line 

 is sufficiently removed from its immediate neighbours that 

 nearly complete separation by spectrum filtration is possible. 

 If sufficient light to satisfy modern practical and research 

 needs is to be obtained from any such source it is necessary 

 to use a relatively wide slit, with a consequent probable 

 inclusion of other wave-lengths in the immediate vicinity of 

 the one desired. 



When the most intense ol all known light sources, 

 namely, the direct radiation of the sun, is utilized, the 

 necessary slit width, while less than that for any other 

 known source, must still be such as to include a relatively 

 large number of wave-lengths. The resultant wave-length 

 or so-called optical centre of gravity of such a group of 

 waves can be considered as a monochromatic light source in 

 only a very restricted sense, and finds effective application 

 in but few fields of work. It is especially unsuited to the 

 ^tudy of phenomena which change rapidly with change of 

 wave-length. The necessity for obtaining additional intense 

 light sources is consequently imperative. 



Among the possible sources which have been suggested is 

 that of the rotating arc with cadmium-silver alloy electrodes. 

 This source gives a number of fairly intense lines sufficiently 

 isolated from each other and fairly well distributed through- 

 out the spectrum. The writer has carried out many experi- 

 ments with tliis source, using an improved rotating arc. It 



* Communicated by the Author. 



t Bulletin of the Bureau of Standards, vol. ii. p. 2-'S ( .>. 



Phil. Mai). S. 6. Vol. 3D. No. 281. March 1920. 2 A 



