In 1949 I was looking for a way to measure nuclear magnetic moments by the molecular‐beam resonance method, but to do it more accurately than was possible with the arrangement developed by I. I. Rabi and his colleagues at Columbia University. The method I found was that of separated oscillatory fields, in which the single oscillating magnetic field in the center of a Rabi device is replaced by two oscillating fields at the entrance and exit, respectively, of the space in which the nuclear magnetic moments are to be investigated. During the 1950's this method became extensively used in the original form. In the same period more general applications of the method arose, and the principal extensions included:
▸ Use of relative phase shifts between the two oscillatory fields
▸ Extension generally to other resonance and spectroscopic devices, such as masers, which depend on either absorption or stimulated emission
▸ Separation of oscillatory fields in time instead of space
▸ Use of more than two successive oscillatory fields
▸ General variation of amplitudes and phases of the successive applied oscillatory fields.
