Return Time Standards
1945 Isidor Rabi, a physics professor at Columbia University, suggests a clock could be made from a technique he developed in the 1930's called atomic beam magnetic resonance.
1949 Using Rabi's technique, NIST (then the National Bureau of Standards) announces the world's first atomic clock using the ammonia molecule as the source of vibrations.
1952 NIST announces the first atomic clock using cesium atoms as the vibration source. This clock is named NBS-1.
1954 NBS-1 is moved to NIST's new laboratories in Boulder, Colo.
1955 The National Physical Laboratory in England builds the first cesium-beam clock used as a calibration source.
1958 Commercial cesium clocks become available, costing $20,000 each.
1960 NBS-2 is inaugurated in Boulder; it can run for long periods unattended and is used to calibrate secondary standards.
1963 The search for a clock with improved accuracy and stability results in NBS-3.
1967 The 13th General Conference on Weights and Measures defines the second on the basis of vibrations of the cesium atom; the world's timekeeping system no longer has an astronomical basis.
1968 NBS-4, the world's most stable cesium clock, is completed. This clock was used into the 1990s as part of the NIST time system.
1972 NBS-5, an advanced cesium beam device, is completed and serves as the primary standard.
1975 NBS-6 begins operation; an outgrowth of NBS-5, it is one of the world's most accurate atomic clocks, neither gaining nor losing one second in 300,000 years.
1989 The Nobel Prize in Physics is awarded to three researchers -- Norman Ramsey of Harvard University, Hans Dehmelt of the University of Washington and Wolfgang Paul of the University of Bonn -- for their work in the development of atomic clocks. NIST's work is cited as advancing their earlier research.
1993 NIST-7 comes on line; eventually, it achieves an uncertainty of 5 parts in 10 to the 15th power, or 20 times more...
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