International System of UnitsThe International System of Units, internationally known by the abbreviation SI (from its official French name, Système international d'unités), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce.
SI base unitThe SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all other SI units can be derived.
Planck constantThe Planck constant, or Planck's constant, denoted by h{\displaystyle h}, is a fundamental physical constant of foundational importance in quantum mechanics: a photon's energy is equal to its frequency multiplied by the Planck constant, and a particle's momentum is equal to the wavenumber of the associated matter wave (the reciprocal of its wavelength) multiplied by the Planck constant.
Caesium standardThe caesium standard is a primary frequency standard in which the photon absorption by transitions between the two hyperfineground states of caesium-133atoms is used to control the output frequency. The first caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK and promoted worldwide by Gernot M. R. Winkler of the United States Naval Observatory.
Speed of lightThe speed of light is the speed of electromagnetic waves. Light travels at slower speed inside materials like glass or water; its highest speed is in a vacuum. The speed of light in vacuum is a universal physical constant denoted c (in ISO and IEC standards c 0 {\displaystyle c_{0}} ), exactly equal to 299792458 m⋅s−1.Metric prefixA metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decimal. Each prefix has a unique symbol that is prepended to any 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.
GramThe gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one thousandth of a kilogram. Originally defined in 1795 as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre [1 cm], and at the temperature of meltingice", the defining temperature (0 °C) was later changed to the temperature of maximum density of water (approximately 4 °C).
MassMass is an intrinsic physical quantity of a body which in modern physics is generally defined as the strength of an object's gravitational attraction to other bodies. The unit of mass in the International System of Units (SI) is the kilogram (kg). Mass can also be understood as a measure of the energy content of a body (see E=mc²).
Clipping (morphology)In linguistics, clipping, also called truncation or shortening, is word formation by removing some segments of an existing word to create a diminutive word or a clipped compound. Clipping differs from abbreviation, which is based on a shortening of the written, rather than the spoken, form of an existing word or phrase.
Isotopes of caesiumStandard atomic weightAr°(Cs)132.90545196±0.00000006132.91±0.01 (abridged)Caesium (55Cs) has 41 known isotopes, ranging in mass number from 112 to 152. Only one isotope, Cs, is stable. The longest-lived radioisotopes are Cs with a half-life of 1.33 million years, Cs with a half-life of 30.04 years and Cs with a half-life of 2.0650 years. All other isotopes have half-lives less than 2 weeks, most under an hour.
Kilo-Look up in Wiktionary, the free dictionary.Base unit of measurementA base unit of measurement (also referred to as a base unit or fundamental unit) is a unit of measurement adopted for a base quantity. A base quantity is one of a conventionally chosen subset of physical quantities, where no quantity in the subset can be expressed in terms of the others. The SI base units, or Systéme International d'unités, consists of the metre, kilogram, second, ampere, kelvin, mole and candela.
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