92 BELL SYSTEM TECHNICAL JOURNAL 



This is why we know in some cases, and suppose in the others, that 

 when the electrons issuing from a radioactive substance constitute the 

 Hnes of a Hne-spectrum they are not themselves coming from the nuclei; 

 they are merely the signs of gamma-rays coming from the nuclei. 



This discovery disposes of one potential objection to the displace- 

 ment-law of Fajans and Soddy. Radioactinium (for instance) is a 

 substance which emits alpha-rays and passes over into a substance 

 two steps farther down the procession of the elements, as the displace- 

 ment-law requires; but it also emits beta-rays, and since no alternative 

 product one step farther up the procession has been discovered, the 

 displacement-law would be gravely threatened if it were necessary to 

 suppose that these come from the nuclei. There is no such necessity; 

 and since the beta-rays display a line-spectrum, it is intrinsically all 

 the more likely that they come from the circumnuclear electron-shells. 



En revanche the character of the thus-far-analyzed beta-ray line- 

 spectra makes it all the more difficult to understand what becomes of 

 the electrons which must truly be emitted from the nuclei, in the trans- 

 mutations in which the daughter-substance is displaced one step up 

 the procession from its parent. When a substance is undergoing a 

 transmutation of the other kind, the alpha-particles which its atoms 

 emit all have very nearly the same speed. One would certainly expect 

 that the electrons emitted from the nuclei of all atoms of radium B at 

 their instants of transmutation emerge with the same speed. If so, 

 they should compose a sharp line in the beta-ray spectrum of radium B. 

 Now there are certainly some lines in this particularly rich spectrum 

 which have not yet been definitely and exactly explained by the theory 

 which I described before; but it appears that none of them is very 

 prominent, and most of the experts refuse to admit that any one of 

 them is composed of electrons coming forth direct and unretarded from 

 the nucleus. There are other substances which display beta-ray 

 spectra comprising but a few lines, one of which some physicists believe 

 to contain the nuclear electrons. 



If the nuclear electrons are not to be assigned to the lines, there 

 remains but one alternative; they must be identified with the electrons 

 composing the continuous beta-ray spectrum which underlies the lines 

 and intervenes between them. The best way to study this spectrum 

 is to dispense with the photographic plate, and set a Faraday-chamber 

 to receive the electrons, with its aperture somewhere in the plane which 

 the plate formerly occupied ; if then the magnetic field is continuously 

 varied, the spectrum slides across the aperture, and at each particular 

 value of the fieldstrength the electrons of a particular limited speed- 

 range pass into the chamber and are counted (more precisely, the total 

 charge which they bear is measured, which comes to the same thing). 



