ALEXANDER TSIARAS / SCIENCE PHOTO LIBRARY ALEXANDER TSIARAS / SCIENCE PHOTO LIBRARY
Jim Gray, keeper of the American NBS-4 atomic clock. Gray is seen wearing a variety of clocks and watches. The NBS-4 is the long cylindrical object on the bench. It is a caesium clock, in which atoms of vapourised caesium-133 pass back and forth between magnets at each end. The caesium-133 atoms oscillate between two energy levels as they go. Counting these oscillations is the basis of the standard second (1 second = 9,192,631,770 oscillations). This accurate time keeping is passed to Paris, where signals from this and other atomic clocks are averaged to produce the world-wide Universal Time Coordinate (UTC).
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