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Quantity |
Time |
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Unit |
second |
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Symbol |
s |
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Definition |
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. This definition refers to a cesium atom in its ground state at a temperature of 0 K. |
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Comments |
| The definition of the second based on the cesium atom dates from 1967, although the last clarifying sentence was added in 1997. |
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| Prior to this the second was considered to be a fraction of the tropical year (1960), and before that a fraction of the solar day. Such definitions were abandoned in favour of definitions based on molecular transitions because it is easier to measure these to a much higher degree of accuracy. |
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| Many cesium clocks are currently in use; the best can produce an SI second to an accuracy of 2 parts in 1015 . It is necessary to use special relativistic corrections for the movement of the cesium atom within the experiment. The effects of general relativity, that is the non-uniform nature of the gravitational field, may be ignored in small enough experimental set-ups. |
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Such cesium clocks act as sources for synchronising clocks in metrology laboratories around the world. This synchronisation can now be carried out via the Global Positioning System whose satellites carry cesium clocks. In this way the discrepancy between two ground-based clocks can be calculated by the GPS, and once corrections have been made to allow for the different travel times of the data, the clocks can be synchronised. Again, corrections must be made for the actions of relativity.
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References |
(A) The International System of Units , Bureau International des Poids et Mesures, 7 th edition, 1998 |
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