The adiabatic theorem and Berry’s phase

A study is presented of Berry’s observation that when a quantum‐mechanical system is transported on a closed adiabatic journey, a topological phase arises in addition to the usual dynamical phase expected from the adiabatic theorem. Consequences are explored in the case of a simple magnetic moment–magnetic field interaction and are shown to lead, among other things, to Dirac’s famous relation between electric and magnetic charges.