Contemporary Advances in Physics, XXIX 

 The Nucleus, Fourth Part 



By KARL K. DARROW 



The earlier parts of this series have dealt with the charge, the mass, the 

 stability or instability, and the liability to transmutation, of the atom- 

 nucleus; this one deals with the two remaining properties which are ascribed 

 to nuclei, to wit, magnetic moment and angular momentum. Since these 

 exhibit themselves chiefly by influencing the spectra of the atoms to which 

 the nuclei belong, the bulk of the article is concerned with various laws of 

 atomic spectra; advantage is taken of the opportunity to describe some 

 features of the electron-systems surrounding the nuclei, and to explain how 

 the concept of the spinning electron enters into atomic physics. There 

 follows an account of various experiments in which streams of atoms are 

 deflected by inhomogeneous magnetic fields, and the laws of the deflections 

 indicate the magnetic moment or the angular momentum of the nucleus or 

 both. Finally there is a summary and tabulation of existing knowledge of 

 these quantities. 



The Nucleus as a Quantized Vector 



UNDER this somewhat forbidding title I propose to discuss some 

 phenomena — mostly spectroscopic, but in certain cases magnetic 

 or even chemical — which are interpreted by supposing that the nuclei 

 of atoms are endowed with two vectorial qualities, magnetic moment 

 and angular momentum. One may say that these nuclei are to be 

 visualized, no longer simply as particles possessed of mass and charge 

 alone, but as bodies — ^usually, as congeries of particles both charged 

 and uncharged — which are in incessant rotation: the spinning of the 

 mass constitutes their angular momentum, the spinning of the charge 

 a perpetual circular current-flow which is equivalent to a magnet. 

 Why, then, should I not have entitled this section "The Spins and 

 Magnetic Moments of Nuclei"? Chiefly because it might have 

 suggested, at the very outset, that nuclear spins and magnetic moments 

 are observed as clearly and measured as directly, as are nuclear masses 

 and charges: which in the main is not so. With only a couple of ex- 

 ceptions (for magnetic moment) they are deduced from phenomena 

 which certainly carry no obvious sign of their character. Indeed, 

 what is called the nuclear angular momentum or the spin is distin- 

 guished by a feature, which is altogether strange and foreign to angular 

 momentum of ordinary wheels and gyroscopes and the other spinning 

 things of daily life ; and it is by virtue of this foreign and paradoxical 

 feature — and not any of the familiar qualities of spinning things — 



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