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Commercial rubidium clocks are less accurate than caesium atomic clocks, which serve as primary frequency standards, so a rubidium clock is usually used as a secondary frequency standard. Commercial rubidium frequency standards operate by disciplining a crystal oscillator to the rubidium hyperfine transition of 6.8 GHz (6 834 682 610.904 Hz).
Rubidium standard clocks are prized for their low cost, small size (commercial standards are as small as 1.7 × 10 5 mm 3) [33] and short-term stability. They are used in many commercial, portable and aerospace applications. Modern rubidium standard tubes last more than ten years, and can cost as little as US$50.
A common size for cells inside cordless tool battery packs. This size is also used in radio-controlled scale vehicle battery packs and some Soviet multimeters. 1 ⁄ 2-, 4 ⁄ 5 - and 5 ⁄ 4-sub-C sizes (differing in length) are also available. Soviet 332 type can be replaced with R10 (#4, 927, BF, U8) or 1.5 V elements from 3 V 2xLR10 packs ...
A crystal oscillator depends for its frequency on its physical dimensions, which vary with fabrication and environmental conditions. A rubidium standard is a secondary standard even though it uses atomic transitions, because it takes the form of a gas cell through which an optical signal is passed. The gas cell has inherent inaccuracies because ...
A standard clock comprises a frequency standard, a device to count off the cycles of the oscillation emitted by the frequency standard, and a means of displaying or outputting the result. Frequency standards in a network or facility are sometimes administratively designated as primary or secondary .
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The first clock known to strike regularly on the hour, a clock with a verge and foliot mechanism, is recorded in Milan in 1336. [96] By 1341, clocks driven by weights were familiar enough to be able to be adapted for grain mills, [97] and by 1344 the clock in London's Old St Paul's Cathedral had been replaced by one with an escapement. [98]
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