This replica in the NIST in Your Community exhibit shows how NIST's fountain clock (NIST-F1) uses atoms to determine the duration of one second. One of the two most accurate clocks in the world, NIST-F1 subdivides the second into more than 9 billion equal parts. It will neither gain nor lose a second in nearly 20 million years.

Who needs a clock this good? NIST's ultra-accurate time services help navigate spacecraft through the solar system, synchronize telecommunications and computer operations, and measure fundamental processes like the speed of chemical reactions.