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"exec" writes:

Arthur T Knackerbracket has found the following story:

The invention of the atomic clock fundamentally altered the way that time is measured and kept. The clock helped redefine the duration of a single second, and its groundbreaking accuracy contributed to technologies we rely on today, including cellphones and GPS receivers.

Building on the accomplishments of previous researchers, Harold Lyons and his colleagues at the U.S. National Bureau of Standards (now the National Institute of Standards and Technology), in Washington, D.C., began working in 1947 on developing an atomic clock and demonstrated it to the public two years later. Its design was based on atomic physics. The clock kept time by tracking the microwave signals that electrons in atoms emit when they change energy levels.

This month the atomic clock received an IEEE Milestone. Administered by the IEEE History Center and supported by donors, the milestone program recognizes outstanding technical developments around the world.

For thousands of years the reference for timekeeping was the Earth's rotation rate—which was limited in accuracy. In the 1920s the quartz crystal oscillator circuit was invented. It kept time according to the mechanical resonance of vibrating crystals of piezoelectric material—which created electrical signals with a precise frequency. The circuits were accurate enough to measure and record variations in the Earth's rotation, but they were still limited in performance and sensitive to environmental changes.

Physicist James Clerk Maxwell was perhaps the first to recognize that atoms could be used to keep time. In 1879 he wrote to electricity pioneer William Thomson, suggesting that the "period of vibration of a piece of quartz crystal" would be a better absolute standard of time than the mean solar second (based on the Earth's rotation) but would still depend "essentially on one particular piece of matter" and therefore would be "liable to accidents." Maxwell theorized that atoms would work even better as a natural standard of time. Thomson wrote in the second edition of the Elements of Natural Philosophy, published in 1879, that hydrogen atoms, sodium atoms, and others were "absolutely alike in every physical property" and "probably remain the same so long as the particle itself exists."

-- submitted from IRC

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