November iS, 1922] 



NA TURE 



1 



the forward edge of a rain area, and to stop short 

 of actual thunderstorm formation, while still being 

 sufficiently marked for the necessary- readjustment 

 of charge to originate electro-magnetic waves. The 

 difficulty of picturing readjustments propagating 

 radiation of such energy content as to produce 

 audible atmospherics at distances of more than iooo 

 kilometres, without producing visible lightning or 

 audible thunder at ground stations near the source, 

 is considerable, but not so great as the difficulty 



of picturing sufficient " full scale " lightning dis- 

 charges, or other known phenomena, to account for 

 the reception of atmospherics at an annual average 

 rate of more than one per second at a station^in 

 these latitudes. 



The writer desires to acknowledge his indebtedness 

 to the Meteorological Office and to the Radio Research 

 Board, for providing the facilities for carrying out 

 this work, and for granting permission to publish 

 the results. 



X-Ray Electrons. 



AMONG the items of the programme of section 

 A of the British Association at Hull this year, 

 there was one of outstanding interest consisting of 

 the description of some very beautiful experiments 

 which apparently constitute still another triumph 

 for the quantum theory and the atomic theory of 

 Bohr. Both M. le Due de Broglie and Prof. R. 

 Whiddington, who described the experiments, have 

 recently been working on the same subject, namely, 

 the properties of the electrons ejected from metallic 

 atoms by the incidence of X-rays ; and their results 

 are in general agreement. The method of procedure 

 has been to allow a beam of characteristic X-rays 

 of known frequency, for example from a tungsten 

 anticathode, to fall upon a prepared metallic surface, 

 sav of silver. The electrons which, as a consequence, 

 emerge from the silver do not all possess, as Barkla 

 at first supposed, equal amounts of energy. They 

 thus have different velocities, and, by the well-known 

 method of the application of a suitable magnetic 

 field, the original mixed bundle of electrons can be 

 differentially deflected, and spread out into a 

 " magnetic spectrum." A focussing device is em- 

 ployed whereby the electrons of the same speed are 

 concentrated upon the same part of the photo- 

 graphic plate, so that each line in the spectrum 

 corresponds to a group of electron's having a definite 

 velocity. There is a certain amount of general 

 fogging of the plate, but the comparatively sharp 

 lines superimposed are unmistakable. Several actual 

 plates were shown both by M. de Broglie and Prof. 

 Whiddington. 



The interpretation of these spectra, which are of 

 somewhat simple appearance, proves to be most 

 important in relation to current theories of quanta 

 and atomic structure. In the first place, the pheno- 

 menon obeys the general law of photo-electric effects, 

 in that the velocity, and therefore the energy, of 

 the electrons expelled depends only on the frequency, 

 and not on the intensity, of the exciting X-radiation. 



Of still greater importance is the bearing of the 

 experimental results on Bohr's theory of atomic 

 constitution. As is well known, this theory involves 

 that the electrons, in number N, which surround 

 the nucleus of an atom of atomic number N, are 

 distributed in a certain number of regions, or layers, 

 each characterised by the work which it is necessary 

 to expend in order to remove an electron from the 

 region under consideration, and bring it to the 

 exterior of the atom. If we denote by the letters 

 A", L, M, etc., the levels of these regions, we can 

 attribute to them energies of extraction HY, It'/, 

 Wir, etc. The fundamental principle underlying the 

 production of the magnetic spectra above mentioned 

 will be made clear by quoting from M. de Broglie's 

 remarks : 



" What appears to happen is that if radiation of 

 frequency » strikes one of these electrons, situated, 

 for example, in the region A", it communicates energy 

 equal to hv in order to extract the electron from the 



atom ; it is clear that the corpuscle, once removed 

 from the atomic edifice, will possess a resultant 

 energy equal to hv - Wk." 



In this, of course, h is Planck's constant, and the 

 resultant energy of the electron, which proves to 

 have the value specified, is that which is measured 

 experimentally by means of the magnetic deflection. 

 For truly monochromatic X-radiation, the magnetic 

 spectrum would thus consist of a few lines, corre- 

 sponding to the various different regions in the atom 

 from which electrons may be ejected, i.e. to the various 

 possible values of IF. Unless hv is greater than W 

 the radiation is incapable of extracting electrons 

 from the atomic region in question. This proves to 

 be true experimentally ; unless an anticathode is 

 used for which the frequency of the characteristic 

 radiation is sufficiently large in relation to at least 

 some of the energies of electron extraction for the 

 irradiated metal, no magnetic spectrum appears. 

 With a Coolidge tube as the source of X-rays it has 

 not been possible to make hv large enough to extract 

 the more deep-seated electrons in metallic atoms of 

 high atomic number ; but the employment of 7-rays, 

 with their much greater frequency, has enabled 

 Ellis to extend the process to these regions, and 

 to prove in this case also the validity of the general 

 relation. 



The lines in the magnetic spectra are usually 

 composite. This arises from the fact that the X-rays 

 used are seldom monochromatic, the characteristic 

 radiation from the anticathode having several com- 

 ponents. Again quoting M. de Broglie : 



" Each line of the spectrum of the incident X-rays 

 re-echoes on each level of the illuminated atom in 

 such a way that we obtain at once an analysis both 

 of the spectral lines of the illuminating beam and of 

 the Bohr levels of the illuminated atom." 



The method, as M. de Broglie pointed out, serves 

 for measuring, without the intervention of a crystal, 

 the frequency and wave-length of X-radiation. It 

 thus furnishes a means of checking the magnitudes 

 of the crystal spacings which form the basis of X-ray 

 analysis. 



The papers of M. de Broglie and Prof. Whiddington 

 evoked great interest in the Section. There was 

 some discussion, particularly with reference to the 

 general fogging of the magnetic spectrum plates, 

 which seemed to point to some of the ejected electrons 

 having all sorts of emergent velocities. Prof. 

 Lindeman suggested the possibility of having to 

 assume that in the atom there were numerous electron 

 levels, instead of the comparatively small number 

 assumed b\ r Bohr. Sir Ernest Rutherford, however, 

 was satisfied that no such explanation was needed, 

 for the reason that the fogging was inevitable, owing 

 partly to the general radiation from the anticathode, 

 and partly to the fact that some of the ejected 

 electrons would lose random amounts of energy from 

 various causes along their paths to the photographic 

 plate. A. O. Raxkine. 



NO. 2768, VOL. IIO] 



