  When a magnetic field is applied to a ferromagnetic body it starts to spin
(Einstein-de Haas effect). This demonstrates the intimate connection between
the electron's magnetic moment $\mu_B=e\hbar/2m_ec$, associated with its spin
angular momentum $S=\hbar/2$, and ferromagnetism. When a magnetic field is
applied to a superconducting body it also starts to spin (gyromagnetic effect),
and when a normal metal in a magnetic field becomes superconducting and expels
the magnetic field (Meissner effect) the body also starts to spin. Yet
according to the conventional theory of superconductivity the electron's spin
only role is to label states, and the electron's magnetic moment plays no role
in superconductivity. Instead, within the unconventional theory of hole
superconductivity, the electron's spin and associated magnetic moment play a
fundamental role in superconductivity. Just like in ferromagnets the
magnetization of superconductors is predicted to result from an aggregation of
magnetic moments with angular momenta $\hbar/2$. This gives rise to a "Spin
Meissner effect", the existence of a spin current in the ground state of
superconductors. The theory explains how a superconducting body starts spinning
when it expels magnetic fields, it provides a dynamical explanation for the
Meissner effect, and it explains how supercurrents stop without dissipation,
all of which we argue the conventional theory fails to explain. Essential
elements of the theory of hole superconductivity are that superconductivity is
driven by lowering of kinetic energy, which we have also proposed is true for
ferromagnets], that the normal state charge carriers in superconducting
materials are holes, and that the spin-orbit interaction plays a key role in
superconductivity. The theory is proposed to apply to all superconductors.
