NIST-7
Encyclopedia
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...
used by the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
from 1993 to 1999. The caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
beam clock served as the nation's primary time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
and frequency standard during that time period, but it has since been replaced with the more accurate NIST-F1
NIST-F1
NIST-F1 is a cesium fountain clock or atomic clock in the National Institute of Standards and Technology in Boulder, Colorado, and serves as the United States' primary time and frequency standard...
, a caesium fountain atomic clock that neither gains nor loses one second in 60 million years.