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	Online Conversion - The SI Standard
		The SI is not static, it is a living set of standards where units are created and definitions are modified with international agreement.
		It is administered by the standards organisation: the Bureau International des Poids et Mesures (International Bureau of Weights and Measures).
		The swift worldwide adoption of the metric system as a tool of economy and everyday commerce was based mainly on the lack of customary systems in many countries to adequately describe some concepts, or as a result of an attempt to standardize the many regional variations in the customary system.
	www.onlineconversion.com /article_SI_standard.htm (2094 words)

	[No title]
		The largest shift in the frequency of any caesium standard is due to the magnetic field in the interaction region.
		Caesium atoms are the basis of atomic clocks – on which global time standards depend – and they...
		In the chip-scale clock, the caesium atoms are probed with light from an infrared laser which generates two electromagnetic fields.
	www.lycos.com /info/caesium--caesium-atoms.html (592 words)

	Caesium Occurrence Alkali metal State of matter K Thermal conductivity NA yellow gallium temperature organic compound (Site not responding. Last check: 2007-11-03)
		Caesium (cesium in the USA) pronounced /'siːziəm/; is a chemical element in the periodic table that has the symbol Cs and atomic number 55.
		Caesium is also the most electropositive and most alkaline chemical element and also has the least ionization potential of all the elements, except for francium.
		The primary compounds of caesium are its chloride and its nitrate.
	en.powerwissen.com /t6YDFYRLI4A%2BrqwovjMV0g%3D%3D_Caesium.html (956 words)

	Atomic clock
		The most accurate atomic clocks are moderated by precise astronomical measurements and the insertion and removal of leap seconds at the very end of June and December.
		Frequency reference masers use glowing chambers of ionized gas, most often caesium, because that is now how the standard second is defined.
		This definition makes the caesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements (see Cesium standard).
	www.ebroadcast.com.au /lookup/encyclopedia/at/Atomic_clock.html (911 words)

	Primary time standard - Wikipedia, the free encyclopedia
		, frequency standards) are caesium standards and hydrogen masers.
		Note 2: The international second is based on the microwave frequency (9,192,631,770 Hz) associated with the atomic resonance of the hyperfine ground state levels of the cesium-133 atom in a magnetically neutral environment.
		Realizable cesium frequency standards use a strong electromagnet to deliberately introduce a magnetic field which overwhelms that of the Earth.
	www.wikipedia.org /wiki/Primary_time_standard (163 words)

	Atomic clock - Wikipedia, the free encyclopedia
		Frequency reference masers use glowing chambers of ionized gas, most often caesium, because caesium is the element used in the official international definition of the second.
		This definition makes the caesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements (see caesium standard).
		When the radio frequency matches the hyperfine transition frequency of caesium, the caesium atoms absorb the radio waves and emit light.
	en.wikipedia.org /wiki/Atomic_clock (1112 words)

	[No title] (Site not responding. Last check: 2007-11-03)
		Novel methods of cooling and manipulating the atoms are presently being implemented in a second fountain standard constructed at NPL, and an improvement of about a factor of three in accuracy is expected.
		Caesium is not made by the same method as sodium as might have been expected.
		This is because the caesium metal, once formed by electrolysis of liquid caesium chloride (CsCl), is too soluble in the molten salt.
	lycos.cs.cmu.edu /info/caesium--methods.html (392 words)

	Caesium
		Caesium (also spelled cesium) is a chemical element in the periodic table that has the symbol Cs and atomic number 55.
		Caesium is also the most electropositive and most alkaline chemical element and also has the least ionization potential of all the elements.
		Caesium is most notably used in atomic clocks, which are accurate to 5 seconds in 300 years.
	www.websters-online-dictionary.org /Ca/Caesium.html (1409 words)

	S - Glossary of chemical terms
		Standard temperature (273.15 K) and pressure (101 325 Pa) is the standard conditions used as a basis for calculations involving quantities that vary with temperature and pressure.
		Standard electrode potential (E°) (standard reduction potentialis) are defined by measuring the potential relative to a standard hydrogen electrode using 1 mol solution at 25 °C. The convention is to designate the cell so that the oxidized form is written first.
		Standard hydrogen electrode is a system in which hydrogen ion and gaseous hydrogen are present in their standard states.
	www.ktf-split.hr /periodni/en/abc/s.html (1490 words)

	Physikalisch-Technische Bundesanstalt (PTB)
		Caesium clocks have been commercially available since the end of the 1950s.
		In addition, thermal effects or aging change the spectrum and the intensity of the Rb lamp as well as the composition of the gas in both the filter cell and in the absorption cell.
		This limits the achievable long-term frequency stability and prevents the use of uncalibrated Rb clocks for the realisation of the unit of time; it is thus called a secondary frequency standard.
	www.ptb.de /en/org/4/44/441/info2_e.htm (1110 words)

	NPL Time Metrology
		At room temperature, caesium atoms are equally divided between two internal atomic states - the hyperfine levels of the ground state of the atom referred to in the definition of the second - which differ slightly in energy.
		It is these primary standards which provide steering corrections to the world’s atomic timescale, UTC, to keep the duration of the scale interval as close as possible to the defined value for the second.
		In addition, a limiting factor in the performance of a beam standard is the slight difference in the phase of the microwaves between the two arms of the U-shaped microwave cavity (known as a Ramsey cavity).
	www.npl.co.uk /time/background/csfreq_standard.html (3358 words)

	SI - Wikipedia, the free encyclopedia
		The SI is not static, it is a living set of standards where units are created and definitions are modified with international agreement.
		It is administered by the standards organisation: the Bureau International des Poids et Mesures (International Bureau of Weights and Measures).
		The NIST recommends that "L" be used instead, a usage which is common in the U.S., Canada and Australia, and has been accepted as an alternative by the CGPM.
	en.wikipedia.org /wiki/SI (1582 words)

	NPL: Wavelength Standards Research, Optical Frequency Metrology
		definition of the metre, optical frequency standards have been used as one of the principal means of realising the metre for practical purposes.
		However, the second, and hence the unit of frequency, the Hertz, are defined in terms of a microwave frequency standard, the caesium primary frequency standard or "atomic clock".
		The caesium frequency lies in the microwave region of the electromagnetic spectrum, at 9.192 631 770 GHz, whereas radiation in the optical region of the electromagnetic spectrum, including visible light, is at frequencies of 100s of THz.
	www.npl.co.uk /length/wss/research/opt_freq_metrology.html (337 words)

	Atomic clock - Speaking clock (Site not responding. Last check: 2007-11-03)
		This definition makes the caesium atoms are able to absorb the radio frequency matches the hyperfine transition frequency of caesium, electrons in the official international definition of the second.
		Atomic systems under consideration for use in optical frequency standards or clocks are installed at some long-wave and medium-wave broadcasting stations to deliver a very precise carrier frequency, which also has its usage as standard frequency.
		Rubidium clocks are used in the caesium atoms are able to absorb the radio waves and move between two energy states.
	speaking-clock.mantleclock.org /atomic-clock (982 words)

	RadioClock-Homepage - Detail information about the time
		According to this definition, the unit of time is realised with caesium atomic clocks which are manufactured by industry or constructed and operated by research laboratories to meet highest accuracy demands.
		For the caesium atom in particular, the constancy in time of fcs much better than that of, for example, the oscillation period of a pendulum, the oscillation frequency of a quartz or the rotation period of the earth.
		Caesium atoms are evaporated in the vacuum chamber of an atomic clock.
	www.heret.de /radioclock/ptb.htm (2346 words)

	Optical Frequency Standards and Metrology Applications
		Optical frequency standards are already used for the practical realisation of the metre in a laboratory environment.
		However optical frequency standards offer the prospect of stabilities and reproducibilities which surpass that of the caesium primary standard and can be compared to this standard with high accuracy, pointing to the potential for an optical re-definition of the second in the longer term.
		Frequency standards are an enabling technology for satellite navigation systems, whilst at the same time satellite systems are the dominant mechanism for distributing standards of time and frequency worldwide.
	www.npl.co.uk /optical_frequency_standards/applications (521 words)

	6moons audio reviews: Esoteric G25U Reclocker/Upsampler
		In this way, the ultra-precise quantum-mechanical properties of the atomic transition frequency of the Caesium can be used to tune the microwave oscillator to the same frequency (except for a small amount of experimental error).
		Rubidium clocks are prized for their low cost, small size (commercial standards are as small as 400 cubic centimeters) and short-term stability.
		A Rubidium standard is a frequency standard whereby a specified hyperfine transition of electrons in Rubidium-87 atoms is used to control the output frequency.
	www.6moons.com /audioreviews/esoteric2/g25u_2.html (1960 words)

	Atomic clock at AllExperts
		Frequency reference masers use glowing chambers of ionized gas, most often caesium, because caesium is the element used in the official international definition of the second.
		This definition makes the caesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements (see caesium standard).
		When the radio frequency matches the hyperfine transition frequency of caesium, electrons in the caesium atoms are able to absorb the radio waves and move between two energy states.
	en.allexperts.com /e/a/at/atomic_clock.htm (1112 words)

	Steadier lasers could make best atomic clock yet - tech - 30 November 2006 - New Scientist Tech (Site not responding. Last check: 2007-11-03)
		Nevertheless, conventional atomic clocks, which use caesium ions, could soon be replaced by those featuring arrays of strontium atoms as the gold standard for time keeping.
		Although caesium ions switch between these states â“ or resonate â“ 9,192,631,770 times each second, physicists are keen to find an even faster element, with which to create more accurate clocks.
		"Caesium will eventually go out of fashion," Ye says, who is confident that further improvements in the steadiness of his trapping laser should allow strontium to take pole position.
	www.newscientisttech.com /article.ns?id=dn10700&feedId=online-news_rss20 (411 words)

	Atomic clock Summary
		As of the late 1990s, research was under way to shift the standard of timekeeping from atomic clocks to devices that measure time by millisecond pulsars, the extremely fast speed of neutron stars.
		The National Institute of Standards and Technology based in Boulder, Colorado houses a cesium atomic clock (so-called because it uses atoms of cesium) that is accurate to one second in six million years.
		The two primary technologies under consideration for use in optical frequency standards are single ions isolated in an ion trap and neutral atoms trapped in an optical lattice.
	www.bookrags.com /Atomic_clock (1973 words)

	Frequency Standards - An Introduction
		The secondary, and tertiary standards are traceable to the primary standard.
		Cesium (caesium) Standard: The cesium standard is the primary standard of time/frequency.
		Rubidium Standard: Rubidium is known as the secondary standard for time/frequency.
	www.tutorialsweb.com /rf-measurements/frequency-standards/frequency-standards-index.htm (779 words)

	The Atomic Clock
		In spite of its shortcomings the astronomical second was the most precise of all our standards and with the help of quartz clocks was made instantly available with an accuracy to one part in a hundred million or a thousandth of a second in the length of a day.
		The UK standard frequency transmissions were controlled by the NPL and we had the confidence to express and publish their values in terms of the atomic unit.
		It may be that with the absorption of caesium as the standard interest in alternatives waned, since I have not heard of any further developments.
	www.btinternet.com /~time.lord/TheAtomicClock.htm (4023 words)

	CalendarHome.com - Atomic clock - Calendar Encyclopedia
		Today's best atomic frequency standards (or clocks) are based on more advanced physics involving cesium beams and fountains.
		This definition makes the caesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements (see Cesium standard).
		The radio frequencies are set by the clocks and are a precision standard, useful for adjusting receivers.
	encyclopedia.calendarhome.com /Atomic_clock.htm (982 words)

	Bambooweb: Atomic clock
		An atomic clock is a type of clock that uses an atomic frequency standard as its counter.
		Today's best atomic frequency standards (or clocks) are based on more advanced physics involving caesium beams and fountains.
		When the radio frequency matches the hyperfine transition frequency of caesium, the caesium atoms absorb the radio waves and emit light.
	www.bambooweb.com /articles/a/t/Atomic_clock.html (763 words)

	MP Digest (Site not responding. Last check: 2007-11-03)
		The Rubidium frequency standard essentially consists of a voltage controlled crystal oscillator, which is locked to a highly stable atomic transition in the ground state of the Rb87 atom.
		The rubidium frequency standard may be thought of as consisting of a cell containing the rubidium in its vapor state, placed into a microwave cavity resonant at the hyperfine frequency of the ground state.
		Accuracy is comparable with that of the standard caesium with an operating life approximately 5 times that of Cs.
	www.mpdigest.com /Articles/2004/Mar2004/quartlock (874 words)

	ESA - Navigation - New clocks to keep time for Galileo satellite navigation system (Site not responding. Last check: 2007-11-03)
		Based on atomic frequency standards, they are under development for Europe's Galileo satellite navigation system at the Observatoire de Neuchatel and Temex Neuchatel Time, both located in the centre of the Swiss clock industry.
		If they were left to run indefinitely, though, their accuracy would drift, so they need to be synchronised regularly with a network of even more stable ground-based reference clocks.
		The caesium frequency standard is based on atomic transitions in the caesium atom and is the standard on which universal time is based.
	www.esrin.esa.it /esaNA/ESAHLEUM5JC_index_2.html (873 words)

	Nist atomic clock - Clock (Site not responding. Last check: 2007-11-03)
		This achieves excellent short term stability to other standards, clock but lower long term accuracy equal to the internationally agreed definition of the caesium-133 atom, was built by Louis Essen in 1955 at the detector.
		National standards agencies maintain an accuracy of 10-9 seconds per day, and a feedback clock loop which is clock used to tune the microwave oscillator to the transition of the caesium-133 atom, was built in 1949 at the U. National Bureau of Standards and Technology NIST based in Boulder, Colorado.
		Frequency reference masers uses glowing chambers of ionized gas, most often caesium, because clock face caesium is the element used in many commercial, portable and grandfather clock aerospace applications.
	clock.mantleclock.org /nist-atomic-clock (728 words)

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