A proton in a static external magnetic field may assume only two possible orientations corresponding to the energies of jiHo. The low energy orientation corresponds to that state in which the nuclear magnetic moment is aligned parallel to the external magnetic field, and the high energy orientation corresponds to that state in which the nuclear magnetic moment is aligned antiparallel (opposed) to the external magnetic field. It is possible to the induce transitions between these two orientations. The frequency v of electromagnetic radiation necessary for such a transition is given by