684 BELL SYSTEM TECHNICAL JOURNAL 



2000 for caesium. Dividing these values by iVi, we see that this means 

 that for electrons of the corresponding speed, the atom is as great an 

 obstacle as would be a sphere of cross-section 3.10"^"* cm-., of radius 

 10 Angstroms or more. Sizes such as these, when compared with the 

 (J for ionization or with the gas-kinetic cross-section of the atom, are 

 surprisingly large. 



The molecules of the common molecular gases also yield curves with 

 maxima, located however at energy-values lower than those at which 

 the peaks for the noble gases stand. Erode in 1925 discovered a minor 

 maximum for nitrogen at some 20 equivalent volts; but the really im- 

 portant ones lie much lower, those for hydrogen and nitrogen some- 

 where between 2 and 3. In Fig. 13 I show a curve for a trimolecular 

 gas, carbon dioxide; Ramsauer has lately found that to the left of the 

 point where this curve is cut off in the graph, it rises rapidly again. 

 Oxygen shows a sharp minimum near 0.25. One sees that the so-called 

 "anomaly" of argon is nothing anomalous at all; it is merely an 

 example unusually conspicuous of a feature which atoms and molecules 

 generally display. 



Observations on the Scattered Electrons Themselves 



Recall the classical experiment of Franck and Hertz — the one which 

 led to the discovery of "inelastic impacts" of electrons against atoms, 

 the discovery of the transfers of energy from electrons to atoms which 

 result in excitation. There are three electrodes in a tube: a filament, 

 the source of electrons — a grid, at a potential V volts higher than the 

 filament — a plate beyond the grid, at a potential lower than this latter 

 by a small and constant amount. The number of electrons arriving 

 at the plate is plotted as function of V, and certain "breaks" are seen 

 in the curve; these fix the location of the critical energy- values of the 

 electrons, at which various modes of excitation first become possible. 



Now this is not much different from the experimental method of 

 Lenard, of Mayer and of others, in the work which I have just been 

 citing; only, there is an important difference in aim; much more atten- 

 tion was paid by Franck and Hertz to the breaks, and the role of the 

 retarding-potential between the plate and the grid. Franck and Hertz 

 were studying electron-scattering with especial regard to the distribu- 

 tion-in-energy, and to the energy-losses, of the scattered corpuscles. 

 In other work of theirs, they had auxiliary collectors oft' to one side 

 from the grid, thus in effect studying scattering at large angles. It is 

 further and more elaborate work of this sort which we have now to 

 consider. 



