The 1955 Cesium Atomic Clock at the National Physical Laboratory, UK.
It kept time to a second in 300 years.
The U.S. Naval Observatory operates about 70 such cesium clocks, as well as other precision clocks like hydrogen masers, in 18 vaults whose temperature and, usually, humidity are closely controlled in order to minimize perturbations by their environment. The time measurements are made by devices called time-interval counters that compare each clock's time against that of one "Master Clock," whose frequency is steered to match its time to the average of the other clocks. This time is the Observatory's measure of the atomic time called Coordinated Universal Time (UTC). Some cesium clocks are transported to remote locations in order to synchronize other clocks.
Most of the Observatory's cesium clocks are model HP5071A, made by Agilent Technologies, Inc. of Santa Clara, California. With an improved cesium tube and new microprocessor-controlled servo loops, the 5071A vastly outperforms the earlier 5061 cesium frequency standards. The Naval Observatory 5071A's feature HP's optional high-performance cesium beam tube, with accuracy 1 part in 10E12, frequency stability 8 parts in E-14, and a time domain stability of < 2E-14 with an averaging time of 5 days. Other companies that produce cesium clocks include Frequency and Time Systems, Inc. of Beverly, MA and Frequency Electronics, Inc. of Uniondale, NY.
AtomClock: Atomic Clock Time Synchronization software
The U.S. Naval Observatory operates about 70 such cesium clocks, as well as other precision clocks like hydrogen masers, in 18 vaults whose temperature and, usually, humidity are closely controlled in order to minimize perturbations by their environment. The time measurements are made by devices called time-interval counters that compare each clock's time against that of one "Master Clock," whose frequency is steered to match its time to the average of the other clocks. This time is the Observatory's measure of the atomic time called Coordinated Universal Time (UTC). Some cesium clocks are transported to remote locations in order to synchronize other clocks.
Most of the Observatory's cesium clocks are model HP5071A, made by Agilent Technologies, Inc. of Santa Clara, California. With an improved cesium tube and new microprocessor-controlled servo loops, the 5071A vastly outperforms the earlier 5061 cesium frequency standards. The Naval Observatory 5071A's feature HP's optional high-performance cesium beam tube, with accuracy 1 part in 10E12, frequency stability 8 parts in E-14, and a time domain stability of < 2E-14 with an averaging time of 5 days. Other companies that produce cesium clocks include Frequency and Time Systems, Inc. of Beverly, MA and Frequency Electronics, Inc. of Uniondale, NY.