Atomic Standards of Length and Time
|Title||Atomic Standards of Length and Time|
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|No. of pages||23|
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The units of measurement used in science for evaluating the fundamental quantities length and time are the metre and the second. Although originally related to a particular dimension of the Earth, the metre has since 1889 been defined as the distance, under specified conditions, between two parallel lines engraved on a platinum-iridium bar (the International Prototype Metre) which is carefully preserved in a vault at the International Bureau of Weights and Measures at Sevres in France. Two astronomical units of time are now recognised one, the mean solar second, which is related to the diurnal period of rotation of the Earth, and the other, more precisely defined and designated in 1956 as the second,* which is related to the period of revolution of the Earth about the Sun as represented by the duration of a particular tropical year. During the next few years there is every prospect that new definitions of the metre and the second will become adopted which will be expressed in terms of certain fundamental characteristics of the atom...