This chapter provides the formalism of quantum mechanics to the description of measurements. It describes how an atomic system behaves in a slowly varying applied field. In the Stern-Gerlach (S-G) experiment an atomic or molecular beam is passed through an inhomogeneous magnetic field. The field causes a spatial separation of systems having different values of the magnetic moment and allows one to determine the moment by measuring the deflection of the emergent beams. The S-G magnets and the recombining fields merely force the wave function to undergo a sequence of contortions, which are then carefully undone; the experiment is vacuous, and is not a measurement. The mathematical formalism of quantum mechanics also allows to compute the beam trajectory. If one accepts J. von Neumann's formulation of the quantum mechanics, one is led to far reaching and Dot entirely palatable conclusions.
Bohr’s correspondence principle is that associated with each observable in classical mechanics is an observable in quantum mechanics that follows certain quantization axioms which can be used to explain the energy spectra of atoms and quantum oscillators. In Schrodinger’s wave mechanics, the state of the system at any time is described by a density operator in a Hilbert space. The chapter describes how the Pauli equation can be used to calculate the average electric dipole moment and average magnetic dipole moment of an atom in an external electromagnetic field. This calculation enables to calculate the electric polarization field and the magnetization field of a material.