The second (s) is defined by taking the fixed numerical value of the cesium frequency ∆νCs, the unperturbed ground-state hyperfine transition frequency of the cesium-133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to s−1.
The number of periods or cycles per second is called frequency. The SI unit for frequency is the hertz (Hz). One hertz is the same as one cycle per second. Standard frequencies and the correct time are broadcast by radio stations WWV and WWVB in Colorado, and WWVH in Hawaii. NIST delivers digital timing signals by telephone and through the internet.
Official U.S. Government time is provided by NIST and USNO. NIST also offers an Internet Time Service (ITS) and an Automated Computer Time Service (ACTS) that allow setting of computer and other clocks through the Internet or over standard commercial telephone lines. Free software for using these services on several types of popular computers can be downloaded there. Information about these services can be found on the Time and Frequency Division Web site.
This comic book-style video animation series has been developed to help middle school students learn about the 7 SI base measurement units. By reading the vibrations of her laser-cooled cesium atoms, Professor Second can synchronize any frequency and correct any clock. A second is the time it takes an excited cesium atom to vibrate 9,192,631,770 times.atomic clocks and other time Frequently Asked Questions (FAQs).PDF (Click link to download the attached file)Timeline (pun intended)!