CONTEMPORARY ADVANCES IN PHYSICS 



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the slit 5i. The part of the beam which is not intercepted goes on into 

 the chamber C. In the notation of equation (8), the current Qe is that 

 reported by the galvanometer G2, the current Qoe is the sum of those 

 reported by G2 and the other galvanometer d; the distance x is 

 measured from the slit Si to the pair of slits S2SZ, and therefore has a 

 constant value Xo, while N is varied by admitting or withdrawing gas. 

 In certain experiments of Erode and in those of Rusch, the electron- 

 beam passes through a metal-walled channel — as though the slits Si 

 and ^2 were circular apertures, the opposite ends of a metal tube. 

 In the similar devices of Mayer and of Maxwell, the distance x is 

 varied by sliding the chamber C back and forth, thus lengthening or 

 shortening the chamber B; for which purpose, B and C are made of 

 lengths of tubing of which the latter telescopes into the former. 



Fig. 3 — Apparatus for measuring cross-section for interception, of the "Mayer " type. 



(T. J. Jones, Physical Review.) 



Comparing this now with Ramsauer's device, one sees that in either 

 apparatus, electrons which are considerably deflected in collisions fail 

 to reach the collector. However, among the various effects which were 

 listed in the last paragraph but one, which a molecule might conceiva- 

 bly produce, there are some which would cause electrons to quit the 

 beam as defined in the Ramsauer apparatus, but not as defined in 

 Mayer's,- — for instance, a corpuscle which had lost much of its speed 

 without suffering change of direction of motion would go right on into 

 C of Fig. 3, as though it had had no encounter at all, while in the 

 Ramsauer scheme it would figure among the missing. The quantity 

 a determined by a scheme of Mayer's kind is thus essentially dif- 

 ferent from the a determined by one of Ramsauer's kind ; and indeed 

 any change in detail of either, any widening of the slits for example, 

 should involve a change in the meaning of the a which the experiment 

 reveals — to each apparatus, its own cross-section of the molecule! 

 Thus by applying a retarding-potential between the slits ^2 and S3 of 

 Fig. 3, one might withhold from the collector such electrons as had 



