NIST-F1 Cesium Clock
NIST-F1 is referred to as a fountain clock because it uses a fountain-like movement of atoms to obtain its improved reckoning of time.
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"NIST-F1 Cesium Fountain Clock."
http://www.nist.gov/public_affairs/releases/n99-22.cfm
http://www.nist.gov/public_affairs/releases/n99-22.cfm
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