Contemporary Advances in Physics, XXX, The Theory of Magnetism

A G N E T I S M is a quality which we attribute to the atom. We affirm t h a t iron, nickel, gadolinium, gaseous oxygen, and in fact all substances, are magnetic because there is magnetism in their atoms. Indeed we go even deeper, and affirm t h a t the individual electrons and the nuclei within the atoms are magnetic. Nevertheless, the atomic theory of magnetism is a really valuable theory. Perhaps t h a t "nevertheless" sounds out of place; but I assure you t h a t without it there would be a trace of paradox in the statement, which perhaps our grandfathers would have been quicker at discerning than are we. Let me explain my meaning by referring to the atomic theory, or as it is usually called the kinetic theory, of gases. Those who designed this theory succeeded in explaining the pressure, the temperature, and the viscosity of gases, without attributing a single one of those qualities to the atoms. To the atoms they assigned the properties of moment u m and velocity and kinetic energy; those other qualities which I just named were then interpreted in terms of these,--they were interpreted as what we call statistical properties of the great multitude of atoms which constitutes a gas. This was a real explanation of pressure and viscosity and temperature, in the fullest sense of the word "explanat i o n " -- o r anyhow, in the fullest sense of t h a t word which is customary in physics. B u t along with these properties of pressure and viscosity * Expanded from a lecture delivered on J a n u a r y 14, 1936, at the School of Engineering of Y a l e University, a n d still bearing obvious traces of its original form.