Measurements with separated oscillatory fields and hydrogen masers

Descriptions are given of the methods of separated and successive oscillatory fields and of the atomic hydrogen maser. Their applications to precision spectroscopy of atoms and molecules are discussed along with specific examples of fundamental measurements best made with such devices. The applications of these methods to atomic clocks are also discussed. Cesium separated oscillatory fields atomic beam clocks are accurate to 10-13 and hydrogen masers are stable for several hours to 10-15. A discussion is given of fundamental experiments that require highly stable clocks, such as long base line radioastronomy, precise measurements of pulsar frequencies and stabilities, tests of the theories of relativity and accurate navigation both on the earth and in outer space