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BELL SYSTEM TECIISICAL JOURKAL 



cated by the arrow and the lettering. Such a symbol as "100 10" 

 indicates that the corresponding curve was taken down with the 

 target at 100 volts and the collector at 10 volts (the filament always 

 being at zero potential). The reason for this has been explained 

 above; the family of curves in Fig. 12 illustrates the point. 



These arc examples of the curves from which the arrangement of 

 the atom-electrons is to be inferred. The sinuous and serrated curves 

 for platinum, entirely different from the smoothly rq^nded curves 



FiR. 13 



derived from niuaiion i7), sureK- nwc ihcir shape to the numerous 

 levels among w iiich, as was show n in ihc foregoing pages, the electrons 

 of massive and electron-rich atoms are distributed; the platinum 

 atom, with its seventy-eight electrons, ranks among the most com- 

 plicated of all. The magnesium atoms, with their thirteen electrons 

 apiece, are simpler and yield curves which are simpler, but not nl 

 the type of equation (7). 



To interpret these curves Davisson has calculated llie tlislriljutioii- 

 in-angle function for electrons deflected by an idealized "limited- 

 field" atom-model, in which there is a concentrated charge -fis at 

 the centre and a charge —E uniformly spread over a spherical sur- 

 face of radius R. This uniformly-charged sphere is a sort of first- 

 approximation substitute for a spherical surface on which sever.ii 

 electrons are arranged. It is not implied that the magnesium atom 

 has all its electrons at the same distance from the nucleus, which 

 would be most improbable, as its X-ray .spectrum shows at least 

 two distinct levels; we can suppose that n out of the 12 electrons 

 are so close to the nucleus that together with it, they practically 



