1952 -- NIST completes the first accurate measurement of the frequency of the cesium clock resonance. The apparatus for this measurement is named NBS-1.
1954 -- NBS-1 is moved to NIST's new laboratories in Boulder, Colorado.
1956 -- Commercial cesium clocks become available, costing $20,000 each.
1959 -- NBS-1 goes into regular service as NIST's primary frequency standard.
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.
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.
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.
For more information, download NIST Primary Frequency Standards and the Realization of the SI Second. A detailed overview of the NIST Primary Frequency Standards. Includes historical information, a technical description of the devices, color photographs of the standards, and an extensive list of references. Published in December 2007. 17 pages.