CONTEMPORARY ADVANCES IN PHYSICS 89 



one another; that depends on the kind of metal irradiated; a prom- 

 inent Hne in the spectrum of (for instance) radium B will be composed 

 of electrons having energy somewhat greater than that of the electrons 

 forming the corresponding line in the spectrum elicited from uranium, 

 somewhat less than that for the corresponding line from platinum. 

 But if the irradiated metal be isotopic with the substance into which the 

 radioactive source is being transmuted, corresponding lines will be 

 found to coincide with one another. One obtains a beta-ray spectrum 

 having many lines in common with that of radium B, by allowing the 

 gamma-rays to play upon and expel electrons from a piece of a metal 

 isotopic with radium C — that is to say, bismuth.^^ 



Whether one uses an atom-model or not, these facts suggest that 

 some at least of the electrons emerging from a radioactive substance 

 are hurled out by some sort of a secondary process operated upon the 

 already-transmuted atoms by the accompanying gamma-rays, working 

 in the same manner as they work upon atoms exposed to them outside. 

 This suggestion becomes much more precise when the atom-model is 

 invoked; for the contemporary model is designed to give a vivid ex- 

 planation of the lines in the electronic spectra elicited by X-rays and 

 gamma-rays playing upon the atoms of the stable metals. 



Every such line is composed of electrons extracted from a particular 

 group, in the circumnuclear electron-family of the atom, by radiation 

 of a particular frequency. Think of the most tightly-bound elec- 

 trons of all, the so-called i^-electrons, to be imagined as lying or 

 revolving closer than any of the others to the nucleus. Merely to ex- 

 tract one electron of this set, a definite amount of energy Wk must be 

 imparted to the atom. Conceive a beam of radiation of frequency p 

 pouring over a multitude of similar atoms; to each it communicates 

 either no energy at all, or else a definite amount of energy equal to 

 hv = 6.57- 10~-^j^. If this "quantum" unit of energy exceeds Wk, and 

 if the radiation extracts a i^-electron from an atom, the liberated elec- 

 tron will fly away with a kinetic energy equal to the excess of the im- 

 parted energy Aj^ over the extraction-energy or "binding-energy" Wk- 



(13) Kinetic Energy = T = hv — Wk- 



This equation determines the initial speed of the departing X-elec- 

 trons.'"^ 



-^Introduction, pp. 184-192; to this I refer also for reproductions of some very 

 beautiful photographs of beta-ray spectra taken by J. Danysz and M. de Broglie. 



'^ If the speed v of the electrons is inferior to 3-10' cm/sec, it is permissible to set 

 for T the familiar expression \mv^, putting for m the "rest-mass" wo = 9'10~^*g of 

 the electron. Otherwise it is necessary to take account of the dependence of the mass 

 of the electron upon its speed, preferably by using the formula derived from the 



