The measurement of the atomic transition in an atomic fountain uses the Ramsey method. In broad strokes, the Ramsey method involves exposing a cloud of atoms to a brief radiofrequency (rf) electromagnetic field; waiting a time T ; briefly exposing the cloud to the rf field again; and then measuring what fraction of the atoms in the cloud have been driven from the initial state to final state. When the frequency of the rf field is resonant with the atomic transition, atoms are detected in the final state. The microwave frequency is swept across the atomic transition over many repeated measurements.