Mr Aston, Distribution of intensity 317 



The distribution of intensity along the positive ray parabolas of 

 atoms and molecules of hydrogen and its possible explanation. By 

 F. W. Aston, M.A., Trinity College (D.Sc, Birmingham). Clerk- 

 Maxwell Student of the University of Cambridge. 



[Read 19 May 1919.] 



No one working with positive rays analysed by Sir J. J. 

 Thomson's method can fail to notice the very remarkable intensity 

 variation along the molecular and atomic parabolas described by 

 him under the term ' beading.' It will be sufficient for the reader 

 to refer to Plate III of his monograph on the subject {Rays of 

 positive electric, p. 52) to realise how striking these can be. 

 Beadings at points corresponding to energy greater than the normal 

 have been quite satisfactorily accounted for by multiple charges 

 {I.e., p. 46), but the ones with which this paper is concerned have 

 a smaller energy than the normal, actually half, and fractional 

 charges are presumably impossible. Nevertheless they seem 

 capable of a simple explanation and an opportunity of putting 

 this to the test occurred recently while making some experiments 

 to determine the best form and position of the cathode pre- 

 liminary to the design of an apparatus to carry the analysis to 

 higher degrees of precision. 



The observations were made with an apparatus essentially of 

 the form now well known {I.e., p. 20) the discharge tube being 

 arranged to be removable with the minimum trouble to change 

 or move the cathode. As no camera suitable for photographic 

 recording was immediately available or necessary a willemite 

 screen and visual observation was employed. This form has many 

 obvious disadvantages and in addition, owing to the enormous 

 difEerence in sensitivity between the parabolas of hydrogen and 

 those due to heavier elements the latter can only be seen with 

 difficulty. It has however one notable advantage, namely that 

 sudden and even momentary changes in intensity can be observed 

 and correlated in time with changes in the discharge or in the 

 intensity of other lines. As no accurate measurements were 

 intended a large canal ray tube was employed so that the H^ and 

 H^ parabolas could be easily seen even with the less effective types 

 of cathode. 



It was soon realised that the appearance on the screen was in 

 general the sum of two superposed effects which could be only 

 unravelled like the writings on a palimpsest by eliminating one of 

 them. This by good fortune it was found possible to do under 

 certain conditions. For the sake of clearness it is proposed to 



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