Since the beginning of
time, man has been fascinated with the measurement of time and has devised more accurate
machines to trap and measure time. Today, time is precisely controlled in the United
States by the most accurate clock in North America, the atomic clock of the US National
Institute of Standards and Technology, Time and Frequency Division in Boulder, Colorado.
A Team of atomic physicists continually measures every second of every day to an accuracy
of ten billionths of a second per day. These physicists have created an international
standard, measuring a second as 9,192,631,770 vibrations of a cesium 133 atom in a vacuum.
This atomic clock regulates the WWVB radio transmitter located in Fort Collins, Colorado,
where the exact time signal is continuously broadcast throughout the United States at 60
kHz to take advantage of stable longwave radio paths found in that frequency range. Radio
waves at these low frequencies use the earth and the ionosphere as a wave-guide and follow
the curvature of the earth for long distances.
The built-in antenna system will receive the WWVB signal anywhere in North America where
long-wave reception is undisturbed. A microprocessor activates the receiver and process
the time signal from Fort Collins overnight. Even adjustment for daylight savings time,
and leap second are automatic.
To learn more about Radio Controlled Clocks, visit the NIST Physics Laboratory Time and Frequency Web Site.
