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Second
The SECOND (symbol: S) (abbreviated S or SEC) is the base unit of time in the International System of Units
International System of Units
/ Système International d'Unités (SI). It is qualitatively defined as the second division of the hour by sixty, the first division by sixty being the minute . The SI definition of 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 caesium 133 atom". Seconds may be measured using a mechanical, electrical or an atomic clock . SI prefixes are combined with the word second to denote subdivisions of the second, e.g., the millisecond (one thousandth of a second), the microsecond (one millionth of a second), and the nanosecond (one billionth of a second). Though SI prefixes may also be used to form multiples of the second such as kilosecond (one thousand seconds), such units are rarely used in practice. The more common larger non-SI units of time are not formed by powers of ten; instead, the second is multiplied by 60 to form a minute, which is multiplied by 60 to form an hour , which is multiplied by 24 to form a day . The second is also the base unit of time in other systems of measurement : the centimetre–gram–second , metre–kilogram–second , metre–tonne–second , and foot–pound–second systems of units
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Escapement
An ESCAPEMENT is a device in mechanical watches and clocks that transfers energy to the timekeeping element (the "impulse action") and allows the number of its oscillations to be counted (the "locking action"). The impulse action transfers energy to the clock's timekeeping element (usually a pendulum or balance wheel ) to replace the energy lost to friction during its cycle and keep the timekeeper oscillating. The escapement is driven by force from a coiled spring or a suspended weight, transmitted through the timepiece's gear train. Each swing of the pendulum or balance wheel releases a tooth of the escapement's _escape wheel_ gear, allowing the clock's gear train to advance or "escape" by a fixed amount. This regular periodic advancement moves the clock's hands forward at a steady rate. At the same time the tooth gives the timekeeping element a push, before another tooth catches on the escapement's pallet, returning the escapement to its "locked" state. The sudden stopping of the escapement's tooth is what generates the characteristic "ticking" sound heard in operating mechanical clocks and watches. Escapements are used elsewhere as well. Manual typewriters used escapements to step the carriage as each letter (or space) was typed
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SI Base Unit
The International System of Units
International System of Units
(SI) defines seven units of measure as a basic set from which all other SI units can be derived . The SI BASE UNITS and their physical quantities are the metre for measurement of length , the kilogram for mass , the second for time , the ampere for electric current , the kelvin for temperature , the candela for luminous intensity , and the mole for amount of substance . The SI base units form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology. The names and symbols of SI base units are written in lowercase, except the symbols of those named after a person, which are written with an initial capital letter. For example, the metre (US English: meter) has the symbol m, but the kelvin has symbol K, because it is named after Lord Kelvin
Kelvin
and the ampere with symbol A is named after André-Marie Ampère
André-Marie Ampère
. Other units, such as the litre (US English: liter), are formally not part of the SI, but are accepted for use with SI
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Time
_TIME_ is an American weekly news magazine published in New York City . It was founded in 1923 and for decades was dominated by Henry Luce
Henry Luce
, who built a highly profitable stable of magazines. A European edition (_ Time
Time
Europe_, formerly known as _ Time
Time
Atlantic_) is published in London and also covers the Middle East, Africa and, since 2003, Latin America. An Asian edition (_ Time
Time
Asia_) is based in Hong Kong
Hong Kong
. The South Pacific edition, which covers Australia, New Zealand and the Pacific Islands , is based in Sydney
Sydney
, Australia. In December 2008, _Time_ discontinued publishing a Canadian advertiser edition. _Time_ has the world's largest circulation for a weekly news magazine, and has a readership of 26 million, 20 million of which are based in the United States. In mid-2016, its circulation was 3,032,581, having fallen from 3.3 million in 2012. Richard Stengel was the managing editor from May 2006 to October 2013, when he joined the U.S. State Department . Nancy Gibbs has been the managing editor since October 2013
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International System Of Units
The INTERNATIONAL SYSTEM OF UNITS (abbreviated as SI, from the French _Système internationale (d'unités)_) is the modern form of the metric system , and is the most widely used system of measurement . It comprises a coherent system of units of measurement built on seven base units . The system also establishes a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system was published in 1960 as a result of an initiative that began in 1948. It is based on the metre–kilogram–second system of units (MKS) rather than any variant of the centimetre–gram–second system (CGS). SI is intended to be an evolving system, so prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves. The 24th and 25th General Conferences on Weights and Measures (CGPM) in 2011 and 2014, for example, discussed a proposal to change the definition of the kilogram , linking it to an invariant of nature rather than to the mass of a material artefact, thereby ensuring long-term stability. The motivation for the development of the SI was the diversity of units that had sprung up within the CGS systems and the lack of coordination between the various disciplines that used them
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Hour
An HOUR (symbol : H; also abbreviated HR.) is a unit of time conventionally reckoned as  1⁄24 of a day and scientifically reckoned as 3,599–3,601 seconds , depending on conditions. The SEASONAL, TEMPORAL, or UNEQUAL HOUR was established in the ancient Near East as  1⁄12 of the night or daytime . Such hours varied by season , latitude , and weather . It was subsequently divided into 60 minutes , each of 60 seconds . Its East Asian equivalent was the shi , which was  1⁄12 of the apparent solar day ; a similar system was eventually developed in Europe which measured its EQUAL or EQUINOCTIAL HOUR as  1⁄24 of such days measured from noon to noon. The minor variations of this unit were eventually smoothed by making it  1⁄24 of the mean solar day , based on the measure of the sun's transit along the celestial equator rather than along the ecliptic . This was finally abandoned due to the minor slowing caused by the Earth
Earth
's tidal deceleration by the Moon
Moon
. In the modern metric system , hours are an accepted unit of time equal to 3,600 seconds but an hour of Coordinated Universal Time
Time
(UTC) may incorporate a positive or negative leap second , making it last 3,599 or 3,601 seconds, in order to keep it within 0.9 seconds of universal time , which is based on measurements of the mean solar day at 0° longitude
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Minute
The MINUTE is a unit of time or of angle . As a unit of time, the minute is equal to  1⁄60 (the first sexagesimal fraction ) of an hour , or 60 seconds . In the UTC time standard , a minute on rare occasions has 61 seconds, a consequence of leap seconds (there is a provision to insert a negative leap second, which would result in a 59-second minute, but this has never happened in more than 40 years under this system). As a unit of angle, the minute of arc is equal to  1⁄60 of a degree , or 60 seconds (of arc) . Although not an SI unit for either time or angle, the minute is accepted for use with SI units for both. The SI symbols for _minute_ or _minutes_ are MIN for time measurement, and the prime symbol after a number, e.g. 5′, for angle measurement. The prime is also sometimes used informally to denote minutes of time. In contrast to the hour, the minute (and the second) does not have a clear historical background. What is traceable only is that it started being recorded in the Middle Ages due to the ability of construction of "precision" timepieces (mechanical and water clocks). However, no consistent records of the origin for the division as  1⁄60 part of the hour (and the second  1⁄60 of the minute) have ever been found, despite many speculations. Historically, the word "minute" comes from the Latin _pars minuta prima_, meaning "first small part"
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Hyperfine Structure
In atomic physics , HYPERFINE STRUCTURE refers to small shifts and splittings in the energy levels of atoms , molecules and ions , due to interaction between the state of the nucleus and the state of the electron clouds. Hyperfine structure
Hyperfine structure
contrasts with mere fine structure , which results from the interaction between the magnetic moments associated with electron spin and the electrons' orbital angular momentum . Hyperfine structure, with energy shifts typically orders of magnitudes smaller than those of a fine-structure shift, results from the interactions of the nucleus (or nuclei, in molecules) with internally generated electric and magnetic fields. In atoms, hyperfine structure occurs due to the energy of the nuclear magnetic dipole moment in the magnetic field generated by the electrons, and the energy of the nuclear electric quadrupole moment in the electric field gradient due to the distribution of charge within the atom. Molecular hyperfine structure is generally dominated by these two effects, but also includes the energy associated with the interaction between the magnetic moments associated with different magnetic nuclei in a molecule, as well as between the nuclear magnetic moments and the magnetic field generated by the rotation of the molecule
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Ground State
The GROUND STATE of a quantum mechanical system is its lowest-energy state ; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state. In the quantum field theory , the ground state is usually called the vacuum state or the vacuum . If more than one ground state exists, they are said to be degenerate . Many systems have degenerate ground states. Degeneracy occurs whenever there exists a unitary operator which acts non-trivially on a ground state and commutes with the Hamiltonian of the system. According to the third law of thermodynamics , a system at absolute zero temperature exists in its ground state; thus, its entropy is determined by the degeneracy of the ground state. Many systems, such as a perfect crystal lattice , have a unique ground state and therefore have zero entropy at absolute zero. It is also possible for the highest excited state to have absolute zero temperature for systems that exhibit negative temperature . CONTENTS * 1 Ground state
Ground state
has no nodes in one-dimension * 2 Examples * 3 Notes * 4 Bibliography GROUND STATE HAS NO NODES IN ONE-DIMENSIONIn one-dimension, the ground state of the Schrödinger equation can be proven to have no nodes. Consider the average energy of a state with a node at x = 0; i.e., ψ(0)=0
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Caesium
CAESIUM or CESIUM is a chemical element with symbol CS and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28.5 °C (83.3 °F), which makes it one of only five elemental metals that are liquid at or near room temperature . Caesium has physical and chemical properties similar to those of rubidium and potassium . The most reactive of all metals, it is pyrophoric and reacts with water even at −116 °C (−177 °F). It is the least electronegative element, with a value of 0.79 on the Pauling scale . It has only one stable isotope, caesium-133 . Caesium is mined mostly from pollucite , while the radioisotopes , especially caesium-137 , a fission product , are extracted from waste produced by nuclear reactors . The German chemist Robert Bunsen and physicist Gustav Kirchhoff discovered caesium in 1860 by the newly developed method of flame spectroscopy . The first small-scale applications for caesium were as a "getter " in vacuum tubes and in photoelectric cells . In 1967, acting on Einstein\'s proof that the speed of light is the most constant dimension in the universe, the International System of Units used two specific wave counts from an emission spectrum of caesium-133 to co-define the second and the metre . Since then, caesium has been widely used in highly accurate atomic clocks
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Atomic Clock
An ATOMIC CLOCK is a clock device that uses an electron transition frequency in the microwave , optical , or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services , to control the wave frequency of television broadcasts, and in global navigation satellite systems such as GPS
GPS
. The principle of operation of an atomic clock is based on atomic physics ; it uses the microwave signal that electrons in atoms emit when they change energy levels . Early atomic clocks were based on masers at room temperature. Currently, the most accurate atomic clocks first cool the atoms to near absolute zero temperature by slowing them with lasers and probing them in atomic fountains in a microwave-filled cavity. An example of this is the NIST-F1 atomic clock, one of the national primary time and frequency standards of the United States. The accuracy of an atomic clock depends on two factors. The first factor is temperature of the sample atoms—colder atoms move much more slowly, allowing longer probe times. The second factor is the frequency and intrinsic width of the electronic transition. Higher frequencies and narrow lines increase the precision
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Si Prefix
A METRIC PREFIX is a unit prefix that precedes a basic unit of measure to indicate a multiple or fraction of the unit. While all metric prefixes in common use today are decadic , historically there have been a number of binary metric prefixes as well. Each prefix has a unique symbol that is prepended to the unit symbol. The prefix kilo- , for example, may be added to gram to indicate multiplication by one thousand: one kilogram is equal to one thousand grams. The prefix milli- , likewise, may be added to metre to indicate division by one thousand; one millimetre is equal to one thousandth of a metre. Decimal
Decimal
multiplicative prefixes have been a feature of all forms of the metric system , with six dating back to the system's introduction in the 1790s. Metric prefixes have even been prepended to non-metric units. The SI PREFIXES are standardized for use in the International System of Units (SI) by the International Bureau of Weights and Measures (BIPM) in resolutions dating from 1960 to 1991. Since 2009, they have formed part of the International System of Quantities
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Millisecond
A MILLISECOND (from milli- and second ; symbol: MS) is a thousandth (0.001 or 10−3 or 1/1000) of a second . 10 milliseconds (a hundredth of a second) are called a CENTISECOND. 100 milliseconds (one tenth of a second) are called a DECISECOND. To help compare orders of magnitude of different times , this page lists times between 10−3 SECONDS and 100 seconds (1 milli second and one second). See also times of other orders of magnitude . CONTENTS * 1 Examples * 2 See also * 3 References * 4 External links EXAMPLES This section DOES NOT CITE ANY SOURCES . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (June 2014) (Learn how and when to remove this template message ) * 1 millisecond (1 ms) – cycle time for frequency 1 kHz ; duration of light for typical photo flash strobe; time taken for sound wave to travel ca
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Microsecond
A MICROSECOND is an SI unit of time equal to one millionth (0.000001 or 10−6 or 1/1,000,000) of a second . Its symbol is μS. One microsecond is to one second as one second is to 11.574 days. A microsecond is equal to 1000 nanoseconds or 1/1,000 milliseconds . Because the next SI prefix
SI prefix
is 1000 times larger, measurements of 10−5 and 10−4 seconds are typically expressed as tens or hundreds of microseconds. A microsecond of sound signal sample (44.1 kHz, 2 channel, 24 bit, WAV) is typically stored on 4 µm of CD , 2 bits per µs per 4 µm. CONTENTS * 1 Examples * 2 See also * 3 References * 4 External links EXAMPLES * 1 microsecond (1 μs ) – cycle time for frequency 1×106 hertz (1 MHz), the inverse unit. This corresponds to radio wavelength 300 m (AM medium wave band), as can be calculated by multiplying 1 µs by the speed of light (approximately 3.00×108 m/s) to determine the distance travelled. * 1 microsecond – the length of time of a high-speed, commercial strobe light flash (see air-gap flash ). * 1.8 microseconds – the amount of time subtracted from the Earth\'s day as a result of the 2011 Japanese earthquake . * 2 microseconds – the lifetime of a muonium particle * 2.68 microseconds – the amount of time subtracted from the Earth's day as a result of the 2004 Indian Ocean earthquake
2004 Indian Ocean earthquake

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Nanosecond
A NANOSECOND (NS) is an SI unit of time equal to one billionth of a second (10−9 or 1/1,000,000,000 s). One nanosecond is to one second as one second is to 31.71 years. The word nanosecond is formed by the prefix _nano_ and the unit _second_. Its symbol is NS. A nanosecond is equal to 1000 picoseconds or  1⁄1000 microsecond . Because the next SI unit is 1000 times larger, times of 10−8 and 10−7 seconds are typically expressed as tens or hundreds of nanoseconds. Times of this magnitude are commonly encountered in telecommunications , pulsed lasers and some areas of electronics . CONTENTS * 1 Etymology * 2 Speed of light * 3 Common measurements * 4 See also * 5 References ETYMOLOGYThe word nanosecond is formed by the prefix _nano_ and the unit _second_. Its symbol is NS. The earliest use of the term is by George Gamow . Another early reference commonly given is to Admiral Grace Hopper , who used to give out pieces of wire about a foot long to illustrate the eventual problem of building very high speed computers. If it takes light a nanosecond to go a foot (in a vacuum, slower in copper), then a computer built with parts connected by half this distance, 15 centimetres (5.9 in) of wire, would take at least a nanosecond to send data to a part and get a response. The solution, developed in Hopper's lifetime, was first the integrated circuit and later the multi-core processor
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