Section III, 1919 [19] Trans. R.S.C. 



The Use of the Jamin Interferometer for the Estimation of Small Amounts 

 of Helium or Hydrogen in Air^ 



By Professor J. C. McLennan, F.R.S., and R. T. Elworthy, B.Sc. 



(Read May Meeting, 1919.) 



Introductory 



In the determination of the permeability of balloon fabrics to 

 helium and to hydrogen a method was required for the estimation 

 of small amounts of these gas in air. 



Various types of interferometer have been employed in similar 

 work on other gases. Frenzel,^ for example, used a Rayleigh Zeiss 

 instrument for hydrogen-air mixtures, and the same instrument has 

 been adopted at the United States Bureau of Standards^ in the course 

 of work on the permeability of balloon fabrics to hydrogen. Several 

 other observers* have used the Jamin interferometer for the determin- 

 ation of refractive indices and dispersions in various gases. 



W. Burton^ in a determination of the refractive indices and the 

 dispersion in argon and helium, made use of the relation 



Np-l = (l-fat) 76- f A 



P.L 



where No = refractive index 



f — number of bands passing a standard line 

 A = wave length of standard line 

 P — change of pressure 

 L = length of tubes of interferometer. 



In Burton's work, two brass tubes of equal length, fitted with 

 worked plane-glass ends of equal thickness contained respectively 



^ Communicated by permission of the Admiralty. 



^Frenzel. Zeit fur Flugtechnik und Motor Luftschifïahrt, 5-264-1914. 



3 United States Bureau of Standards. Tech. Paper. No. 113, 1918. 



* "The Refractive Indices of gaseous nitric oxide, sulphur dioxide and sulphur 

 trioxide." Cuthbertson & Metcalf, Proc. Roy. Soc. A 80-406-1907-1908. 



"On the dispersion of gaseous mercury, sulphur, phosphorus and helium." 

 Cuthbertson and Metcalf. Proc. Roy. Soc. A 80-411-1907-1908. 



"On the refraction and dispersion of Krypton and Xenon and their relation to 

 that of HeHum and Argon." Cuthbertson. Proc. Roy. Soc. A 81-440-1908. 



* "The refractive index and dispersion of light in Argon and Helium." W. Burton. 

 Proc. Roy. Soc. A 80-390-1908. 



