CONTEMPORARY ADVANCES IN PHYSICS 



87 



simply mean that an alpha-particle, or a gamma-ray quantum, or a 

 different beta-particle coming out of one atom-nucleus operated on its 

 way out the expulsion of that electron from the outer electron-family 

 of that atom or some other. To take one instance only: radium C 

 and radioactinium both emit beta-rays and alpha-rays together, but 

 in the former case there is as we have seen a dual transmutation, in the 

 latter the beta-rays appear to be electrons torn out of the electron- 

 shells surrounding the atom-nuclei as the alpha-particles pass by on 

 their way out. Electrons have the same charge and the same mass, 

 whatever their origin ; although it is essential to distinguish how they 

 originate in all these cases of beta-ray-emitting substances, there is no 

 way to make the distinction except by performing experiments on 

 distribution-in-speed of the beta-rays and invoking various theories, 

 not always of the highest order of reliability, to interpret the results. 

 This is the reason why, as Meitner says, the beta-rays actually emitted 

 from self-transmuting nuclei "are the least clarified point in the entire 

 problem of the radioactive transformations." 



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Fig. 5. Apparatus for photographing beta-ray spectra 



(After C. D. ElHs and H. W. B. Skinner. Source at R, magnetic field and photo- 

 graphic plate perpendicular to plane of paper, which the plate intersects along PiPo.) 



In attacking the beta-rays the first thing to do, and indeed the only 

 thing which can be done by experiment, is to determine their distri- 

 bution-in-speed — the function which gives the relative number of 

 electrons issuing from the substance with speeds comprised between 

 any preassigned limits. The process consists in isolating, by a proper 

 system of narrow perforations and slits, a narrow beam or pencil of elec- 

 trons, and applying to this pencil a magnetic field which bends the paths 



