On the angular momentum of a system of quantum particles

The properties of angular momentum and its connection to magnetic momentum are explored, based on a reconsideration of the Stern-Gerlach experiment and gauge invariance. A possible way to solve the so called spin crisis is proposed. The separation of angular momentum of a quantum system of particles into orbital angular momentum plus intrinsic angular momentum is reconsidered, within the limits of the Schr\"odinger theory. A proof is given that, for systems of more than two particles, unless all of them have the same mass, the possibility of having eigenvalues of the form $(n+1/2)\hbar$ is not excluded.