Fr.: horloge astronomique
A precise pendulum clock with separate dials for seconds, minutes, and hours. It was originally used by astronomers to calculate astronomical time.
Fr.: horloge atomique
A modern clock, in which the characteristic frequencies of certain atoms (most commonly chosen cesium 133) are utilized for precision time measurement. → atomic fountain clock.
atomic fountain clock
sâ'at-e favvâre-ye atomi
Fr.: horloge à fontaine atomique
An → atomic clock based on the principle of the → atomic fountain. A ball of atoms, usually → cesium (133Cs), created by the → laser cooling technique, is trapped in the intersection region of six laser beams. The ball is thrown upward by a laser beam and passes twice through a cavity where the atoms interact with the → microwave radiation generated by an → oscillator. The ball reaches the summit of its trajectory (about 1 m above the cooling zone) and then due to gravity falls through the same microwave cavity. The microwave radiation causes the electrons of the cesium atoms to move between two specific → energy states as they pass through the cavity. The clock is based on a → hyperfine transition (9.192631770 GHz) between two energy states in the electronic → ground state of the atom. The upper hyperfine state can in principle radiate to the lower state by → spontaneous emission, but the process takes a very long time -- thousands of years. Selection and detection of the hyperfine state is performed via → optical pumping and laser induced resonance fluorescence. In a carefully controlled setup, a relative uncertainty of 10 -16 can be reached for the cesium clock. This means an accuracy of 1 sec every 300 million years. This fluorescence is measured by a detector. The entire process is repeated until the maximum fluorescence of the cesium atoms is determined. This determination is used to lock the oscillator to the atomic frequency of cesium, which is used to define the SI → second. The first atomic fountain for metrological use was developed at the Paris Observatory (A. Clairon et al. 1996, Proc. 5th Symp. Frequency Standards and Metrology, p. 45).
Fr.: horloge à cesium
M.E. clokke "clock with bells," from O.Fr. cloque "bell" (Fr. cloche, Du. klok, Ger. Glocke), M.L. clocca "bell," of Celtic origin.
Sâat from Ar.
Successive risings and lowerings of voltage on the electrodes of a CCD in order to move the electrons from one pixel to the next.
Fr.: dans le sens des aiguilles d'une montre
In the same direction as the rotating hands of a clock when viewed from in front.
From → clock + wise "way, manner," O.E. wise (adj.), from wis, from P.Gmc. *wisaz (cf. Du. wijs, Ger. weise "wise"), PIE base *weid-/*wid- "to see, to know;" cf. Av vaeda "I know," Skt. veda "I know," Gk. oida "I know".
Sâ'atsu, from sâ'at, → clock, + su "direction," Mid.Pers. sôg, sôk "side, direction".
Fr.: en sens inverse des aiguilles d'une montre
In a direction opposite to the rotating hands of a clock.
From counter- "contrary; opposite; opposing," + → clockwise.
Pâdsâ'atsu, from pâd-, → counter-, + sâ'atsu, → clockwise.
An ancient form of clock, used by several civilizations, consisting of a water container with a small hole from which the water slowly dripped. Time was reckoned by the level of the water remaining in the container.
Pang "a copper bason with a small hole in the bottom, for water in which it is placed to flow through, used for measuring time" used in Iran.