88 BELL SYSTEM TECHNICAL JOURNAL 



of the electrons (see Fig. 5). The slower the electron, the more its 

 traiectory is curved; if the beam comprises particles of more than a 

 single speed, it is spread into a fan, and a photographic plate placed 

 across the path of the fan records the "magnetic spectrum" of the 

 electron-beam. If the beam comprises several groups of electrons, 

 each with its own sharplv marked and definite speed, each group falls 

 upon a distinct part of the plate; if the slit limiting the beam is long 

 and narrow, the groups form long and narrow discolored bands upon the 

 plate, and these bands or "lines" constitute an electronic line- 

 spectrum. The appearance of lines in a magnetic spectrum is taken as 

 practically convincing evidence that the electrons in question issue 

 from the circumnuclear electron-families of the atoms, not from their 

 nuclei. 



For this view there is direct evidence of a very convincing character: 

 namely, that beta-ray spectra containing the same lines can be elicited 

 from ordinary stable elements not undergoing transmutation, by the 

 simple process of playing gamma-rays upon them from the radioactive 

 substance in question. Take a sample of the substance, and envelop 



Fig. 6. Part of the beta ray spectrum of radium B 



("After Ellis and Skinner, Proc. Roy. Soc. Range 0.037 to 0.054 millions of 

 equivalent volts.) 



it in a metal sheath thick enough to stop all of the electrons or alpha- 

 particles issuing from it. Some of the gamma-rays will pass through 

 the sheath, for generally some of them (not necessarily all) are more 

 penetrating than any other radiations which the substance can emit. 

 Let these fall upon another piece of metal nearby; apply a magnetic 

 field to the electrons expelled from this metal, or indeed to the electrons 

 which the gamma-rays expel through the outer surface of the sheath 

 enclosing their source; photograph the resulting spectrum. If the 

 atomic number of the irradiated metal does not depart too far from 

 that of the radioactive source — if for instance the irradiated metal is 

 uranium or lead or platinum or tungsten — the spectrum of the electrons 

 expelled from it will resemble the natural beta-ray spectrum of the 

 source, closely enough so that strong lines of the one spectrum can 

 obviously be identified with corresponding strong lines of the other. 

 Corresponding lines in the two spectra may or may not coincide with 



