Contemporary Advances in Physics, XXIX, The Nucleus, Fourth Part

NDER 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 s i m p l y as p a r t i c l e s p o s s e s s e d of m a s s a n d 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 exceptions (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 distinguished 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-- 285 286 BELL SYSTEM TECHNICAL JOURNAL that the concept of "spin" is a valuable addition to atom-models.