We concisely review the equivalence among the dynamics of spin, a spinning top, and an electron subjected to a magnetic monopole. The equivalence becomes clearer when the finite inertia of spin is taken into account. We describe how to calculate spin inertia, considering metallic ferromagnets as an example, where conduction electrons flow among localized magnetic moments. The presence of conduction electrons effectively changes the dynamics of the localized magnetic moments; in particular, the electrons add a finite inertial term to the magnetic moments. We also introduce the interesting history of monopole harmonics, which is a generalized concept of spherical harmonics describing an electron wave function subject to a monopole magnetic field. It is named by Wu and Yang in 1976, but the function itself appeared in much older literature, such as the Landau- Lifshitz's textbook on quantum mechanics. It appears in quantization of the motion of spinning tops or diatomic molecules, which was already considered in as early as 1926, soon after Schrodinger published his pioneering paper on quantum wave mechanics.
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