The Effect of Materials on Time
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Figure. NIST-7, the U.S. primary atomic clock. This standard contributes significantly to the accuracy of the rate for international time and lends long-term stability to NIST's time scale. This increased stability is needed for large network applications and for tests of scientific theories. The standard also serves as a reference in evaluating the performance of advanced commercial frequency standards. This cesium beam frequency standard is based on optical state-preparation and statedetection methods as opposed to the magnetic methods used in earlier generations of atomic standards. In this process, atoms are forced by laser radiation into the desired atomic state and then state-detected using a similar laser method. The laser system is on the small table beside the standard. NIST-7 is the world's most accurate atomic standard. It realizes the definition of the second with a fractional uncertainty of 1 x 10~u.
because good timekeeping was desperately needed for navigation. World trade was increasing dramatically, and navigation errors were causing growing numbers of mishaps with great losses of life and property. Mariners knew how to determine latitude by observing the elevation of certain celestial bodies, but a means of shipboard timekeeping was needed before such observations could be used to determine longitude. Several governments focused on this issue and offered prizes for solutions. In 1714, Great Britain passed "an act for providing a publick reward for such person or persons as shall discover the longitude at sea." Some 10 years later, John Harrison, a self-taught British clockmaker in his early 30s, learned of this offer. He dedicated the remainder of his life to the pursuit of the £20,000 prize, a huge sum in those days. He ultimately succeeded and managed to collect most of the prize money several years before his death in 1776 at the age of 83. The stories of his long development effort and his difficulty in extracting the prize are well-recounted in two articles.1'4 Harrison began with a plan to adapt the pendulum-clock concept to a rolling platform, but his prize-winning chronometer was ultimately a well-made, balance-wheel clock. For nearly two centuries following this development, mechanical marine chronometers played a key role in navigation. The innovations incorporated in Harrison's various clocks provide good examples of early attention given to the materials of clock construction. Satellite Navigation With the dawn of the satellite age in the 20th century, new opportunities for more accurate navigation arose. Several concepts evolved, but the most successful was the Global Position System (GPS), a Department of Defense (DOD) constellation of orbiting satellites, each of which broadcasts accurate time signals and accurate satellite-position information. With the reception of four such signals traveling at the speed of light from different satellites, the navigator can now determine longitude, latitude, and altitude within a few meters, anywhere on the earth. In reality, DOD has added an en
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