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atomic clock (NIST-F1)

An atomic clock is the most accurate type of timepiece in the world, designed to measure time according to vibrations within atoms. NIST-F1, the United States' standard atomic clock, is said to be so accurate that it would neither gain nor lose a second in over 30 million years. Atomic clocks are used to coordinate systems that require extreme precision, such as Global Positioning System ( GPS ) navigation and the Internet. A group of atomic clocks located in a number of places throughout the world is used in conjunction to establish Coordinated Universal Time ( UTC ).

Like a regular clock, an atomic clock keeps time according to oscillation, which is a periodic variation or movement between two entities or between two states of a single entity, created by changes in energy. In a pendulum-driven clock, for example, the oscillation is the back and forth movement of the pendulum (the oscillator ). Such a clock keeps time according to the frequency of the pendulum's swing, which will be more or less accurate, depending on a number of variables. The precision of an atomic clock, on the other hand, depends upon the fact that an atom, caused to oscillate, will always vibrate at the same frequency.

In 1945, Isidor Rabi, a physics professor at Columbia University, proposed that atomic vibrations could be used to keep time, based on something he'd developed called atomic beam magnetic resonance. Four years later, the National Bureau of Standards (now the National Institute of Standards and Technology ) had developed an atomic clock that used the vibrations of ammonia molecules. NIST-F1, the United States' current standard, uses cesium atoms; it and a similar atomic clock standard in Paris are the most accurate timepieces ever made.

The first commercial cesium-based atomic clocks were manufactured by the National Company, a Massachusetts-based firm; Frequency Electronics, FTS, and Hewlett-Packard ( HP ) are among the companies producing them today. Atomic clocks have never been widely used in consumer products because they are typically large and use too much power. Recently, however, NIST developed an atomic clockwork that overcomes these problems. About the size of a grain of rice and accurate to within one second in 126 years, the new mechanism could soon be manufactured on computer chips and used in consumer market handheld devices, such as radios, GPS systems, and cellular telephones.

This was last updated in March 2011

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Just reading the English translation of Einstein's article on "On the electrodynamics of moving bodies" and noticed a comment in Section 5 - "7 Not a pendulum-clock, which is physically a system to which the Earth belongs. This
case had to be excluded". My assumption has been that all clocks measure the same equivalent time. Can anyone explain why a pendulum clock is different? Newton actually took account of pendulum clock differences at different latitudes in The Principia.
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