1.
Speed of light
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The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 299792458 metres per second, it is exact because the unit of length, the metre, is defined from this constant, according to special relativity, c is the maximum speed at which all matter and hence information in the universe can travel. It is the speed at which all particles and changes of the associated fields travel in vacuum. Such particles and waves travel at c regardless of the motion of the source or the reference frame of the observer. In the theory of relativity, c interrelates space and time, the speed at which light propagates through transparent materials, such as glass or air, is less than c, similarly, the speed of radio waves in wire cables is slower than c. The ratio between c and the speed v at which light travels in a material is called the index n of the material. In communicating with distant space probes, it can take minutes to hours for a message to get from Earth to the spacecraft, the light seen from stars left them many years ago, allowing the study of the history of the universe by looking at distant objects. The finite speed of light limits the theoretical maximum speed of computers. The speed of light can be used time of flight measurements to measure large distances to high precision. Ole Rømer first demonstrated in 1676 that light travels at a speed by studying the apparent motion of Jupiters moon Io. In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave, in 1905, Albert Einstein postulated that the speed of light c with respect to any inertial frame is a constant and is independent of the motion of the light source. He explored the consequences of that postulate by deriving the theory of relativity and in doing so showed that the parameter c had relevance outside of the context of light and electromagnetism. After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299792458 m/s with a measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units as the distance travelled by light in vacuum in 1/299792458 of a second, as a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre. The speed of light in vacuum is usually denoted by a lowercase c, historically, the symbol V was used as an alternative symbol for the speed of light, introduced by James Clerk Maxwell in 1865. In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch had used c for a different constant later shown to equal √2 times the speed of light in vacuum, in 1894, Paul Drude redefined c with its modern meaning. Einstein used V in his original German-language papers on special relativity in 1905, but in 1907 he switched to c, sometimes c is used for the speed of waves in any material medium, and c0 for the speed of light in vacuum. This article uses c exclusively for the speed of light in vacuum, since 1983, the metre has been defined in the International System of Units as the distance light travels in vacuum in 1⁄299792458 of a second
2.
Ampere
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The ampere, often shortened to amp, is a unit of electric current. In the International System of Units the ampere is one of the seven SI base units and it is named after André-Marie Ampère, French mathematician and physicist, considered the father of electrodynamics. SI defines the ampere in terms of base units by measuring the electromagnetic force between electrical conductors carrying electric current. The ampere was then defined as one coulomb of charge per second, in SI, the unit of charge, the coulomb, is defined as the charge carried by one ampere during one second. In the future, the SI definition may shift back to charge as the base unit, ampères force law states that there is an attractive or repulsive force between two parallel wires carrying an electric current. This force is used in the definition of the ampere. The SI unit of charge, the coulomb, is the quantity of electricity carried in 1 second by a current of 1 ampere, conversely, a current of one ampere is one coulomb of charge going past a given point per second,1 A =1 C s. In general, charge Q is determined by steady current I flowing for a time t as Q = It, constant, instantaneous and average current are expressed in amperes and the charge accumulated, or passed through a circuit over a period of time is expressed in coulombs. The relation of the ampere to the coulomb is the same as that of the watt to the joule, the ampere was originally defined as one tenth of the unit of electric current in the centimetre–gram–second system of units. That unit, now known as the abampere, was defined as the amount of current that generates a force of two dynes per centimetre of length between two wires one centimetre apart. The size of the unit was chosen so that the derived from it in the MKSA system would be conveniently sized. The international ampere was a realization of the ampere, defined as the current that would deposit 0.001118 grams of silver per second from a silver nitrate solution. Later, more accurate measurements revealed that this current is 0.99985 A, at present, techniques to establish the realization of an ampere have a relative uncertainty of approximately a few parts in 107, and involve realizations of the watt, the ohm and the volt. Rather than a definition in terms of the force between two current-carrying wires, it has proposed that the ampere should be defined in terms of the rate of flow of elementary charges. Since a coulomb is equal to 6. 2415093×1018 elementary charges. The proposed change would define 1 A as being the current in the direction of flow of a number of elementary charges per second. In 2005, the International Committee for Weights and Measures agreed to study the proposed change, the new definition was discussed at the 25th General Conference on Weights and Measures in 2014 but for the time being was not adopted. The current drawn by typical constant-voltage energy distribution systems is usually dictated by the power consumed by the system, for this reason the examples given below are grouped by voltage level
3.
Candela
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The candela is the SI base unit of luminous intensity, that is, luminous power per unit solid angle emitted by a point light source in a particular direction. A common candle emits light with an intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the word candela means candle in Latin. Like most other SI base units, the candela has an operational definition—it is defined by a description of a process that will produce one candela of luminous intensity. The definition describes how to produce a source that emits one candela. Such a source could then be used to calibrate instruments designed to measure luminous intensity, the candela is sometimes still called by the old name candle, such as in foot-candle and the modern definition of candlepower. The frequency chosen is in the spectrum near green, corresponding to a wavelength of about 555 nanometres. The human eye is most sensitive to frequency, when adapted for bright conditions. At other frequencies, more radiant intensity is required to achieve the same luminous intensity, if more than one wavelength is present, one must sum or integrate over the spectrum of wavelengths present to get the total luminous intensity. A common candle emits light with roughly 1 cd luminous intensity. A25 W compact fluorescent light bulb puts out around 1700 lumens, if light is radiated equally in all directions. Focused into a 20° beam, the light bulb would have an intensity of around 18,000 cd. The luminous intensity of light-emitting diodes is measured in millicandelas, or thousandths of a candela, indicator LEDs are typically in the 50 mcd range, ultra-bright LEDs can reach 15,000 mcd, or higher. Prior to 1948, various standards for luminous intensity were in use in a number of countries and these were typically based on the brightness of the flame from a standard candle of defined composition, or the brightness of an incandescent filament of specific design. One of the best-known of these was the English standard of candlepower, one candlepower was the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. Germany, Austria and Scandinavia used the Hefnerkerze, a based on the output of a Hefner lamp. It became clear that a unit was needed. Jules Violle had proposed a standard based on the emitted by 1 cm2 of platinum at its melting point
4.
Energy
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In physics, energy is the property that must be transferred to an object in order to perform work on – or to heat – the object, and can be converted in form, but not created or destroyed. The SI unit of energy is the joule, which is the transferred to an object by the mechanical work of moving it a distance of 1 metre against a force of 1 newton. Mass and energy are closely related, for example, with a sensitive enough scale, one could measure an increase in mass after heating an object. Living organisms require available energy to stay alive, such as the humans get from food. Civilisation gets the energy it needs from energy resources such as fuels, nuclear fuel. The processes of Earths climate and ecosystem are driven by the radiant energy Earth receives from the sun, the total energy of a system can be subdivided and classified in various ways. It may also be convenient to distinguish gravitational energy, thermal energy, several types of energy, electric energy. Many of these overlap, for instance, thermal energy usually consists partly of kinetic. Some types of energy are a mix of both potential and kinetic energy. An example is energy which is the sum of kinetic. Whenever physical scientists discover that a phenomenon appears to violate the law of energy conservation. Heat and work are special cases in that they are not properties of systems, in general we cannot measure how much heat or work are present in an object, but rather only how much energy is transferred among objects in certain ways during the occurrence of a given process. Heat and work are measured as positive or negative depending on which side of the transfer we view them from, the distinctions between different kinds of energy is not always clear-cut. In contrast to the definition, energeia was a qualitative philosophical concept, broad enough to include ideas such as happiness. The modern analog of this property, kinetic energy, differs from vis viva only by a factor of two, in 1807, Thomas Young was possibly the first to use the term energy instead of vis viva, in its modern sense. Gustave-Gaspard Coriolis described kinetic energy in 1829 in its modern sense, the law of conservation of energy was also first postulated in the early 19th century, and applies to any isolated system. It was argued for years whether heat was a physical substance, dubbed the caloric, or merely a physical quantity. In 1845 James Prescott Joule discovered the link between mechanical work and the generation of heat and these developments led to the theory of conservation of energy, formalized largely by William Thomson as the field of thermodynamics
5.
Length
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In geometric measurements, length is the most extended dimension of an object. In the International System of Quantities, length is any quantity with dimension distance, in other contexts length is the measured dimension of an object. For example, it is possible to cut a length of a wire which is shorter than wire thickness. Length may be distinguished from height, which is vertical extent, and width or breadth, length is a measure of one dimension, whereas area is a measure of two dimensions and volume is a measure of three dimensions. In most systems of measurement, the unit of length is a base unit, measurement has been important ever since humans settled from nomadic lifestyles and started using building materials, occupying land and trading with neighbours. As society has become more technologically oriented, much higher accuracies of measurement are required in a diverse set of fields. One of the oldest units of measurement used in the ancient world was the cubit which was the length of the arm from the tip of the finger to the elbow. This could then be subdivided into shorter units like the foot, hand or finger, the cubit could vary considerably due to the different sizes of people. After Albert Einsteins special relativity, length can no longer be thought of being constant in all reference frames. Thus a ruler that is one meter long in one frame of reference will not be one meter long in a frame that is travelling at a velocity relative to the first frame. This means length of an object is variable depending on the observer, in the physical sciences and engineering, when one speaks of units of length, the word length is synonymous with distance. There are several units that are used to measure length, in the International System of Units, the basic unit of length is the metre and is now defined in terms of the speed of light. The centimetre and the kilometre, derived from the metre, are commonly used units. In U. S. customary units, English or Imperial system of units, commonly used units of length are the inch, the foot, the yard, and the mile. Units used to denote distances in the vastness of space, as in astronomy, are longer than those typically used on Earth and include the astronomical unit, the light-year. Dimension Distance Orders of magnitude Reciprocal length Smoot Unit of length
6.
Mass
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In physics, mass is a property of a physical body. It is the measure of a resistance to acceleration when a net force is applied. It also determines the strength of its gravitational attraction to other bodies. The basic SI unit of mass is the kilogram, Mass is not the same as weight, even though mass is often determined by measuring the objects weight using a spring scale, rather than comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity and this is because weight is a force, while mass is the property that determines the strength of this force. In Newtonian physics, mass can be generalized as the amount of matter in an object, however, at very high speeds, special relativity postulates that energy is an additional source of mass. Thus, any body having mass has an equivalent amount of energy. In addition, matter is a defined term in science. There are several distinct phenomena which can be used to measure mass, active gravitational mass measures the gravitational force exerted by an object. Passive gravitational mass measures the force exerted on an object in a known gravitational field. The mass of an object determines its acceleration in the presence of an applied force, according to Newtons second law of motion, if a body of fixed mass m is subjected to a single force F, its acceleration a is given by F/m. A bodys mass also determines the degree to which it generates or is affected by a gravitational field and this is sometimes referred to as gravitational mass. The standard International System of Units unit of mass is the kilogram, the kilogram is 1000 grams, first defined in 1795 as one cubic decimeter of water at the melting point of ice. Then in 1889, the kilogram was redefined as the mass of the prototype kilogram. As of January 2013, there are proposals for redefining the kilogram yet again. In this context, the mass has units of eV/c2, the electronvolt and its multiples, such as the MeV, are commonly used in particle physics. The atomic mass unit is 1/12 of the mass of a carbon-12 atom, the atomic mass unit is convenient for expressing the masses of atoms and molecules. Outside the SI system, other units of mass include, the slug is an Imperial unit of mass, the pound is a unit of both mass and force, used mainly in the United States
7.
Time
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Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past through the present to the future. Time is often referred to as the dimension, along with the three spatial dimensions. Time has long been an important subject of study in religion, philosophy, and science, nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective measuring systems. Two contrasting viewpoints on time divide prominent philosophers, one view is that time is part of the fundamental structure of the universe—a dimension independent of events, in which events occur in sequence. Isaac Newton subscribed to this realist view, and hence it is referred to as Newtonian time. This second view, in the tradition of Gottfried Leibniz and Immanuel Kant, holds that time is neither an event nor a thing, Time in physics is unambiguously operationally defined as what a clock reads. Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities, Time is used to define other quantities—such as velocity—so defining time in terms of such quantities would result in circularity of definition. The operational definition leaves aside the question there is something called time, apart from the counting activity just mentioned, that flows. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy. Furthermore, it may be there is a subjective component to time. Temporal measurement has occupied scientists and technologists, and was a motivation in navigation. Periodic events and periodic motion have long served as standards for units of time, examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms. Time is also of significant social importance, having economic value as well as value, due to an awareness of the limited time in each day. In day-to-day life, the clock is consulted for periods less than a day whereas the calendar is consulted for periods longer than a day, increasingly, personal electronic devices display both calendars and clocks simultaneously. The number that marks the occurrence of an event as to hour or date is obtained by counting from a fiducial epoch—a central reference point. Artifacts from the Paleolithic suggest that the moon was used to time as early as 6,000 years ago. Lunar calendars were among the first to appear, either 12 or 13 lunar months, without intercalation to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months
8.
International System of Units
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The International System of Units 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 the result of an initiative began in 1948. It is based on the system of units rather than any variant of the centimetre-gram-second system. The motivation for the development of the SI was the diversity of units that had sprung up within the CGS systems, the International System of Units has been adopted by most developed countries, however, the adoption has not been universal in all English-speaking countries. The metric system was first implemented during the French Revolution with just the metre and kilogram as standards of length, in the 1830s Carl Friedrich Gauss laid the foundations for a coherent system based on length, mass, and time. In the 1860s a group working under the auspices of the British Association for the Advancement of Science formulated the requirement for a coherent system of units with base units and derived units. Meanwhile, in 1875, the Treaty of the Metre passed responsibility for verification of the kilogram, in 1921, the Treaty was extended to include all physical quantities including electrical units originally defined in 1893. The units associated with these quantities were the metre, kilogram, second, ampere, kelvin, in 1971, a seventh base quantity, amount of substance represented by the mole, was added to the definition of SI. On 11 July 1792, the proposed the names metre, are, litre and grave for the units of length, area, capacity. The committee also proposed that multiples and submultiples of these units were to be denoted by decimal-based prefixes such as centi for a hundredth, on 10 December 1799, the law by which the metric system was to be definitively adopted in France was passed. Prior to this, the strength of the magnetic field had only been described in relative terms. The technique used by Gauss was to equate the torque induced on a magnet of known mass by the earth’s magnetic field with the torque induced on an equivalent system under gravity. The resultant calculations enabled him to assign dimensions based on mass, length, a French-inspired initiative for international cooperation in metrology led to the signing in 1875 of the Metre Convention. Initially the convention only covered standards for the metre and the kilogram, one of each was selected at random to become the International prototype metre and International prototype kilogram that replaced the mètre des Archives and kilogramme des Archives respectively. Each member state was entitled to one of each of the prototypes to serve as the national prototype for that country. Initially its prime purpose was a periodic recalibration of national prototype metres. The official language of the Metre Convention is French and the version of all official documents published by or on behalf of the CGPM is the French-language version
9.
SI derived unit
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The International System of Units specifies a set of seven base units from which all other SI units of measurement are derived. Each of these units is either dimensionless or can be expressed as a product of powers of one or more of the base units. For example, the SI derived unit of area is the metre. The degree Celsius has an unclear status, and is arguably an exception to this rule. The names of SI units are written in lowercase, the symbols for units named after persons, however, are always written with an uppercase initial letter. In addition to the two dimensionless derived units radian and steradian,20 other derived units have special names, some other units such as the hour, litre, tonne, bar and electronvolt are not SI units, but are widely used in conjunction with SI units. Until 1995, the SI classified the radian and the steradian as supplementary units, but this designation was abandoned, International System of Quantities International System of Units International Vocabulary of Metrology Metric prefix Metric system Non-SI units mentioned in the SI Planck units SI base unit I. Mills, Tomislav Cvitas, Klaus Homann, Nikola Kallay, IUPAC, Quantities, Units and Symbols in Physical Chemistry. CS1 maint, Multiple names, authors list
10.
Metre
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The metre or meter, is the base unit of length in the International System of Units. The metre is defined as the length of the path travelled by light in a vacuum in 1/299792458 seconds, the metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole. In 1799, it was redefined in terms of a metre bar. In 1960, the metre was redefined in terms of a number of wavelengths of a certain emission line of krypton-86. In 1983, the current definition was adopted, the imperial inch is defined as 0.0254 metres. One metre is about 3 3⁄8 inches longer than a yard, Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations except the United States and the Philippines, which use meter. Measuring devices are spelled -meter in all variants of English, the suffix -meter has the same Greek origin as the unit of length. This range of uses is found in Latin, French, English. Thus calls for measurement and moderation. In 1668 the English cleric and philosopher John Wilkins proposed in an essay a decimal-based unit of length, as a result of the French Revolution, the French Academy of Sciences charged a commission with determining a single scale for all measures. In 1668, Wilkins proposed using Christopher Wrens suggestion of defining the metre using a pendulum with a length which produced a half-period of one second, christiaan Huygens had observed that length to be 38 Rijnland inches or 39.26 English inches. This is the equivalent of what is now known to be 997 mm, no official action was taken regarding this suggestion. In the 18th century, there were two approaches to the definition of the unit of length. One favoured Wilkins approach, to define the metre in terms of the length of a pendulum which produced a half-period of one second. The other approach was to define the metre as one ten-millionth of the length of a quadrant along the Earths meridian, that is, the distance from the Equator to the North Pole. This means that the quadrant would have defined as exactly 10000000 metres at that time. To establish a universally accepted foundation for the definition of the metre, more measurements of this meridian were needed. This portion of the meridian, assumed to be the length as the Paris meridian, was to serve as the basis for the length of the half meridian connecting the North Pole with the Equator
11.
Kilogram
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The kilogram or kilogramme is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype of the Kilogram. The avoirdupois pound, used in both the imperial and US customary systems, is defined as exactly 0.45359237 kg, making one kilogram approximately equal to 2.2046 avoirdupois pounds. Other traditional units of weight and mass around the world are also defined in terms of the kilogram, the gram, 1/1000 of a kilogram, was provisionally defined in 1795 as the mass of one cubic centimeter of water at the melting point of ice. The final kilogram, manufactured as a prototype in 1799 and from which the IPK was derived in 1875, had an equal to the mass of 1 dm3 of water at its maximum density. The kilogram is the only SI base unit with an SI prefix as part of its name and it is also the only SI unit that is still directly defined by an artifact rather than a fundamental physical property that can be reproduced in different laboratories. Three other base units and 17 derived units in the SI system are defined relative to the kilogram, only 8 other units do not require the kilogram in their definition, temperature, time and frequency, length, and angle. At its 2011 meeting, the CGPM agreed in principle that the kilogram should be redefined in terms of the Planck constant, the decision was originally deferred until 2014, in 2014 it was deferred again until the next meeting. There are currently several different proposals for the redefinition, these are described in the Proposed Future Definitions section below, the International Prototype Kilogram is rarely used or handled. In the decree of 1795, the term gramme thus replaced gravet, the French spelling was adopted in the United Kingdom when the word was used for the first time in English in 1797, with the spelling kilogram being adopted in the United States. In the United Kingdom both spellings are used, with kilogram having become by far the more common, UK law regulating the units to be used when trading by weight or measure does not prevent the use of either spelling. In the 19th century the French word kilo, a shortening of kilogramme, was imported into the English language where it has used to mean both kilogram and kilometer. In 1935 this was adopted by the IEC as the Giorgi system, now known as MKS system. In 1948 the CGPM commissioned the CIPM to make recommendations for a practical system of units of measurement. This led to the launch of SI in 1960 and the subsequent publication of the SI Brochure, the kilogram is a unit of mass, a property which corresponds to the common perception of how heavy an object is. Mass is a property, that is, it is related to the tendency of an object at rest to remain at rest, or if in motion to remain in motion at a constant velocity. Accordingly, for astronauts in microgravity, no effort is required to hold objects off the cabin floor, they are weightless. However, since objects in microgravity still retain their mass and inertia, the ratio of the force of gravity on the two objects, measured by the scale, is equal to the ratio of their masses. On April 7,1795, the gram was decreed in France to be the weight of a volume of pure water equal to the cube of the hundredth part of the metre
12.
Second
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The second is the base unit of time in the International System of Units. It is qualitatively defined as the division of the hour by sixty. SI definition of second is the duration of 9192631770 periods of the 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, the microsecond, and the nanosecond. Though SI prefixes may also be used to form multiples of the such as kilosecond. The second is also the unit of time in other systems of measurement, the centimetre–gram–second, metre–kilogram–second, metre–tonne–second. Absolute zero implies no movement, and therefore zero external radiation effects, the second thus defined is consistent with the ephemeris second, which was based on astronomical measurements. The realization of the second is described briefly in a special publication from the National Institute of Standards and Technology. 1 international second is equal to, 1⁄60 minute 1⁄3,600 hour 1⁄86,400 day 1⁄31,557,600 Julian year 1⁄, more generally, = 1⁄, the Hellenistic astronomers Hipparchus and Ptolemy subdivided the day into sixty parts. They also used an hour, simple fractions of an hour. No sexagesimal unit of the day was used as an independent unit of time. The modern second is subdivided using decimals - although the third remains in some languages. The earliest clocks to display seconds appeared during the last half of the 16th century, the second became accurately measurable with the development of mechanical clocks keeping mean time, as opposed to the apparent time displayed by sundials. The earliest spring-driven timepiece with a hand which marked seconds is an unsigned clock depicting Orpheus in the Fremersdorf collection. During the 3rd quarter of the 16th century, Taqi al-Din built a clock with marks every 1/5 minute, in 1579, Jost Bürgi built a clock for William of Hesse that marked seconds. In 1581, Tycho Brahe redesigned clocks that displayed minutes at his observatory so they also displayed seconds, however, they were not yet accurate enough for seconds. In 1587, Tycho complained that his four clocks disagreed by plus or minus four seconds, in 1670, London clockmaker William Clement added this seconds pendulum to the original pendulum clock of Christiaan Huygens. From 1670 to 1680, Clement made many improvements to his clock and this clock used an anchor escapement mechanism with a seconds pendulum to display seconds in a small subdial
13.
Electric current
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An electric current is a flow of electric charge. In electric circuits this charge is carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an ionised gas. The SI unit for measuring a current is the ampere. Electric current is measured using a device called an ammeter, electric currents cause Joule heating, which creates light in incandescent light bulbs. They also create magnetic fields, which are used in motors, inductors and generators, the particles that carry the charge in an electric current are called charge carriers. In metals, one or more electrons from each atom are loosely bound to the atom and these conduction electrons are the charge carriers in metal conductors. The conventional symbol for current is I, which originates from the French phrase intensité de courant, current intensity is often referred to simply as current. The I symbol was used by André-Marie Ampère, after whom the unit of current is named, in formulating the eponymous Ampères force law. The notation travelled from France to Great Britain, where it became standard, in a conductive material, the moving charged particles which constitute the electric current are called charge carriers. In other materials, notably the semiconductors, the carriers can be positive or negative. Positive and negative charge carriers may even be present at the same time, a flow of positive charges gives the same electric current, and has the same effect in a circuit, as an equal flow of negative charges in the opposite direction. Since current can be the flow of positive or negative charges. The direction of current is arbitrarily defined as the same direction as positive charges flow. This is called the direction of current I. If the current flows in the direction, the variable I has a negative value. When analyzing electrical circuits, the direction of current through a specific circuit element is usually unknown. Consequently, the directions of currents are often assigned arbitrarily
14.
Kelvin
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The kelvin is a unit of measure for temperature based upon an absolute scale. It is one of the seven units in the International System of Units and is assigned the unit symbol K. The kelvin is defined as the fraction 1⁄273.16 of the temperature of the triple point of water. In other words, it is defined such that the point of water is exactly 273.16 K. The Kelvin scale is named after the Belfast-born, Glasgow University engineer and physicist William Lord Kelvin, unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or typeset as a degree. The kelvin is the unit of temperature measurement in the physical sciences, but is often used in conjunction with the Celsius degree. The definition implies that absolute zero is equivalent to −273.15 °C, Kelvin calculated that absolute zero was equivalent to −273 °C on the air thermometers of the time. This absolute scale is known today as the Kelvin thermodynamic temperature scale, when spelled out or spoken, the unit is pluralised using the same grammatical rules as for other SI units such as the volt or ohm. When reference is made to the Kelvin scale, the word kelvin—which is normally a noun—functions adjectivally to modify the noun scale and is capitalized, as with most other SI unit symbols there is a space between the numeric value and the kelvin symbol. Before the 13th CGPM in 1967–1968, the unit kelvin was called a degree and it was distinguished from the other scales with either the adjective suffix Kelvin or with absolute and its symbol was °K. The latter term, which was the official name from 1948 until 1954, was ambiguous since it could also be interpreted as referring to the Rankine scale. Before the 13th CGPM, the form was degrees absolute. The 13th CGPM changed the name to simply kelvin. Its measured value was 7002273160280000000♠0.01028 °C with an uncertainty of 60 µK, the use of SI prefixed forms of the degree Celsius to express a temperature interval has not been widely adopted. In 2005 the CIPM embarked on a program to redefine the kelvin using a more experimentally rigorous methodology, the current definition as of 2016 is unsatisfactory for temperatures below 20 K and above 7003130000000000000♠1300 K. In particular, the committee proposed redefining the kelvin such that Boltzmanns constant takes the exact value 6977138065049999999♠1. 3806505×10−23 J/K, from a scientific point of view, this will link temperature to the rest of SI and result in a stable definition that is independent of any particular substance. From a practical point of view, the redefinition will pass unnoticed, the kelvin is often used in the measure of the colour temperature of light sources. Colour temperature is based upon the principle that a black body radiator emits light whose colour depends on the temperature of the radiator, black bodies with temperatures below about 7003400000000000000♠4000 K appear reddish, whereas those above about 7003750000000000000♠7500 K appear bluish
15.
Temperature
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A temperature is an objective comparative measurement of hot or cold. It is measured by a thermometer, several scales and units exist for measuring temperature, the most common being Celsius, Fahrenheit, and, especially in science, Kelvin. Absolute zero is denoted as 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, the kinetic theory offers a valuable but limited account of the behavior of the materials of macroscopic bodies, especially of fluids. Temperature is important in all fields of science including physics, geology, chemistry, atmospheric sciences, medicine. The Celsius scale is used for temperature measurements in most of the world. Because of the 100 degree interval, it is called a centigrade scale.15, the United States commonly uses the Fahrenheit scale, on which water freezes at 32°F and boils at 212°F at sea-level atmospheric pressure. Many scientific measurements use the Kelvin temperature scale, named in honor of the Scottish physicist who first defined it and it is a thermodynamic or absolute temperature scale. Its zero point, 0K, is defined to coincide with the coldest physically-possible temperature and its degrees are defined through thermodynamics. The temperature of zero occurs at 0K = −273. 15°C. For historical reasons, the triple point temperature of water is fixed at 273.16 units of the measurement increment, Temperature is one of the principal quantities in the study of thermodynamics. There is a variety of kinds of temperature scale and it may be convenient to classify them as empirically and theoretically based. Empirical temperature scales are historically older, while theoretically based scales arose in the middle of the nineteenth century, empirically based temperature scales rely directly on measurements of simple physical properties of materials. For example, the length of a column of mercury, confined in a capillary tube, is dependent largely on temperature. Such scales are only within convenient ranges of temperature. For example, above the point of mercury, a mercury-in-glass thermometer is impracticable. A material is of no use as a thermometer near one of its phase-change temperatures, in spite of these restrictions, most generally used practical thermometers are of the empirically based kind. Especially, it was used for calorimetry, which contributed greatly to the discovery of thermodynamics, nevertheless, empirical thermometry has serious drawbacks when judged as a basis for theoretical physics. Theoretically based temperature scales are based directly on theoretical arguments, especially those of thermodynamics, kinetic theory and they rely on theoretical properties of idealized devices and materials
16.
Luminous intensity
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The SI unit of luminous intensity is the candela, an SI base unit. Photometry deals with the measurement of light as perceived by human eyes. The human eye can see light in the visible spectrum and has different sensitivities to light of different wavelengths within the spectrum. When adapted for bright conditions, the eye is most sensitive to light at 555 nm. Light with the radiant intensity at other wavelengths has a lower luminous intensity. The curve which measures the response of the eye to light is a defined standard. This curve, denoted V or y ¯, is based on an average of widely differing experimental data from scientists using different measurement techniques, for instance, the measured responses of the eye to violet light varied by a factor of ten. Luminous intensity should not be confused with another unit, luminous flux. Luminous intensity is the power per unit solid angle. If a lamp has a 1 lumen bulb and the optics of the lamp are set up to focus the light evenly into a 1 steradian beam, if the optics were changed to concentrate the beam into 1/2 steradian then the source would have a luminous intensity of 2 candela. The resulting beam is narrower and brighter, though its luminous flux remains unchanged, luminous intensity is also not the same as the radiant intensity, the corresponding objective physical quantity used in the measurement science of radiometry. Like other SI base units, the candela has an operational definition—it is defined by the description of a process that will produce one candela of luminous intensity. The frequency of light used in the definition corresponds to a wavelength of 555 nm, if the source emitted uniformly in all directions, the total radiant flux would be about 18.40 mW, since there are 4π steradians in a sphere. A typical candle produces very roughly one candela of luminous intensity, prior to the definition of the candela, a variety of units for luminous intensity were used in various countries. These were typically based on the brightness of the flame from a candle of defined composition. One of the best-known of these standards was the English standard, one candlepower was the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. Germany, Austria, and Scandinavia used the Hefnerkerze, a based on the output of a Hefner lamp. In 1881, Jules Violle proposed the Violle as a unit of luminous intensity, all of these units were superseded by the definition of the candela
17.
Mole (unit)
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The mole is the unit of measurement in the International System of Units for amount of substance. This number is expressed by the Avogadro constant, which has a value of 6. 022140857×1023 mol−1, the mole is one of the base units of the SI, and has the unit symbol mol. The mole is used in chemistry as a convenient way to express amounts of reactants and products of chemical reactions. For example, the chemical equation 2 H2 + O2 →2 H2O implies that 2 moles of dihydrogen and 1 mole of dioxygen react to form 2 moles of water. The mole may also be used to express the number of atoms, ions, the concentration of a solution is commonly expressed by its molarity, defined as the number of moles of the dissolved substance per litre of solution. For example, the relative molecular mass of natural water is about 18.015, therefore. The term gram-molecule was formerly used for essentially the same concept, the term gram-atom has been used for a related but distinct concept, namely a quantity of a substance that contains Avogadros number of atoms, whether isolated or combined in molecules. Thus, for example,1 mole of MgBr2 is 1 gram-molecule of MgBr2 but 3 gram-atoms of MgBr2, in honor of the unit, some chemists celebrate October 23, which is a reference to the 1023 scale of the Avogadro constant, as Mole Day. Some also do the same for February 6 and June 2, thus, by definition, one mole of pure 12C has a mass of exactly 12 g. It also follows from the definition that X moles of any substance will contain the number of molecules as X moles of any other substance. The mass per mole of a substance is called its molar mass, the number of elementary entities in a sample of a substance is technically called its amount. Therefore, the mole is a convenient unit for that physical quantity, one can determine the chemical amount of a known substance, in moles, by dividing the samples mass by the substances molar mass. Other methods include the use of the volume or the measurement of electric charge. The mass of one mole of a substance depends not only on its molecular formula, since the definition of the gram is not mathematically tied to that of the atomic mass unit, the number NA of molecules in a mole must be determined experimentally. The value adopted by CODATA in 2010 is NA =6. 02214129×1023 ±0. 00000027×1023, in 2011 the measurement was refined to 6. 02214078×1023 ±0. 00000018×1023. The number of moles of a sample is the sample mass divided by the mass of the material. The history of the mole is intertwined with that of mass, atomic mass unit, Avogadros number. The first table of atomic mass was published by John Dalton in 1805
18.
Amount of substance
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Amount of substance is a standards-defined quantity that measures the size of an ensemble of elementary entities, such as atoms, molecules, electrons, and other particles. It is sometimes referred to as chemical amount, the International System of Units defines the amount of substance to be proportional to the number of elementary entities present. The SI unit for amount of substance is the mole and it has the unit symbol mol. The proportionality constant is the inverse of the Avogadro constant, the mole is defined as the amount of substance that contains an equal number of elementary entities as there are atoms in 12g of the isotope carbon-12. This number is called Avogadros number and has the value 6. 022140857×1023 and it is the numerical value of the Avogadro constant which has the unit 1/mol, and relates the molar mass of an amount of substance to its mass. Therefore, the amount of substance of a sample is calculated as the sample mass divided by the mass of the substance. Amount of substance appears in thermodynamic relations such as the gas law. Another unit of amount of substance in use in engineering in the United States is the pound-mole. When quoting an amount of substance, it is necessary to specify the entity involved, unless there is no risk of ambiguity. One mole of chlorine could refer either to chlorine atoms, as in 58.44 g of sodium chloride, or to chlorine molecules, the simplest way to avoid ambiguity is to replace the term substance by the name of the entity or to quote the empirical formula. The main derived quantity in which amount of substance enters into the numerator is amount of substance concentration and this name is often abbreviated to amount concentration, except in clinical chemistry where substance concentration is the preferred term to avoid ambiguity with mass concentration. The term molar concentration is incorrect, but commonly used, the alchemists, and especially the early metallurgists, probably had some notion of amount of substance, but there are no surviving records of any generalization of the idea beyond a set of recipes. In 1758, Mikhail Lomonosov questioned the idea that mass was the measure of the quantity of matter. The development of the concept of amount of substance was coincidental with, and vital to,1777, Wenzel publishes Lessons on Affinity, in which he demonstrates that the proportions of the base component and the acid component remain the same during reactions between two neutral salts. 1789, Lavoisier publishes Treatise of Elementary Chemistry, introducing the concept of a chemical element,1792, Richter publishes the first volume of Stoichiometry or the Art of Measuring the Chemical Elements. The term stoichiometry is used for the first time, the first tables of equivalent weights are published for acid–base reactions. Richter also notes that, for an acid, the equivalent mass of the acid is proportional to the mass of oxygen in the base. 1794, Prousts Law of definite proportions generalizes the concept of equivalent weights to all types of chemical reaction,1805, Dalton publishes his first paper on modern atomic theory, including a Table of the relative weights of the ultimate particles of gaseous and other bodies
19.
Dimensional analysis
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Converting from one dimensional unit to another is often somewhat complex. Dimensional analysis, or more specifically the method, also known as the unit-factor method, is a widely used technique for such conversions using the rules of algebra. The concept of physical dimension was introduced by Joseph Fourier in 1822, Physical quantities that are measurable have the same dimension and can be directly compared to each other, even if they are originally expressed in differing units of measure. If physical quantities have different dimensions, they cannot be compared by similar units, hence, it is meaningless to ask whether a kilogram is greater than, equal to, or less than an hour. Any physically meaningful equation will have the dimensions on their left and right sides. Checking for dimensional homogeneity is an application of dimensional analysis. Dimensional analysis is routinely used as a check of the plausibility of derived equations and computations. It is generally used to categorize types of quantities and units based on their relationship to or dependence on other units. Many parameters and measurements in the sciences and engineering are expressed as a concrete number – a numerical quantity. Often a quantity is expressed in terms of other quantities, for example, speed is a combination of length and time. Compound relations with per are expressed with division, e. g.60 mi/1 h, other relations can involve multiplication, powers, or combinations thereof. A base unit is a unit that cannot be expressed as a combination of other units, for example, units for length and time are normally chosen as base units. Units for volume, however, can be factored into the units of length. Sometimes the names of units obscure that they are derived units, for example, an ampere is a unit of electric current, which is equivalent to electric charge per unit time and is measured in coulombs per second, so 1 A =1 C/s. Similarly, one newton is 1 kg⋅m/s2, percentages are dimensionless quantities, since they are ratios of two quantities with the same dimensions. In other words, the % sign can be read as 1/100, derivatives with respect to a quantity add the dimensions of the variable one is differentiating with respect to on the denominator. Thus, position has the dimension L, derivative of position with respect to time has dimension LT−1 – length from position, time from the derivative, the second derivative has dimension LT−2. In economics, one distinguishes between stocks and flows, a stock has units of units, while a flow is a derivative of a stock, in some contexts, dimensional quantities are expressed as dimensionless quantities or percentages by omitting some dimensions
20.
William Thomson, 1st Baron Kelvin
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William Thomson, 1st Baron Kelvin, OM, GCVO, PC, FRS, FRSE was a Scots-Irish mathematical physicist and engineer who was born in Belfast in 1824. He worked closely with mathematics professor Hugh Blackburn in his work and he also had a career as an electric telegraph engineer and inventor, which propelled him into the public eye and ensured his wealth, fame and honour. For his work on the telegraph project he was knighted in 1866 by Queen Victoria. He had extensive maritime interests and was most noted for his work on the mariners compass, absolute temperatures are stated in units of kelvin in his honour. He was ennobled in 1892 in recognition of his achievements in thermodynamics and he was the first British scientist to be elevated to the House of Lords. The title refers to the River Kelvin, which close by his laboratory at the University of Glasgow. His home was the red sandstone mansion Netherhall, in Largs. William Thomsons father, James Thomson, was a teacher of mathematics and engineering at Royal Belfast Academical Institution, James Thomson married Margaret Gardner in 1817 and, of their children, four boys and two girls survived infancy. Margaret Thomson died in 1830 when William was six years old, William and his elder brother James were tutored at home by their father while the younger boys were tutored by their elder sisters. James was intended to benefit from the share of his fathers encouragement, affection. In 1832, his father was appointed professor of mathematics at Glasgow, the Thomson children were introduced to a broader cosmopolitan experience than their fathers rural upbringing, spending mid-1839 in London and the boys were tutored in French in Paris. Mid-1840 was spent in Germany and the Netherlands, language study was given a high priority. His sister, Anna Thomson, was the mother of James Thomson Bottomley FRSE, Thomson had heart problems and nearly died when he was 9 years old. In school, Thomson showed a keen interest in the classics along with his natural interest in the sciences, at the age of 12 he won a prize for translating Lucian of Samosatas Dialogues of the Gods from Latin to English. In the academic year 1839/1840, Thomson won the prize in astronomy for his Essay on the figure of the Earth which showed an early facility for mathematical analysis. Throughout his life, he would work on the problems raised in the essay as a strategy during times of personal stress. On the title page of this essay Thomson wrote the lines from Alexander Popes Essay on Man. These lines inspired Thomson to understand the world using the power and method of science, Go
21.
Litre
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The litre or liter is an SI accepted metric system unit of volume equal to 1 cubic decimetre,1,000 cubic centimetres or 1/1,000 cubic metre. A cubic decimetre occupies a volume of 10×10×10 centimetres and is equal to one-thousandth of a cubic metre. The original French metric system used the litre as a base unit. The word litre is derived from an older French unit, the litron, whose name came from Greek — where it was a unit of weight, not volume — via Latin, and which equalled approximately 0.831 litres. The litre was also used in subsequent versions of the metric system and is accepted for use with the SI. The spelling used by the International Bureau of Weights and Measures is litre, the less common spelling of liter is more predominantly used in American English. One litre of water has a mass of almost exactly one kilogram. Subsequent redefinitions of the metre and kilogram mean that this relationship is no longer exact, a litre is defined as a special name for a cubic decimetre or 10 centimetres ×10 centimetres ×10 centimetres. Hence 1 L ≡0.001 m3 ≡1000 cm3, from 1901 to 1964, the litre was defined as the volume of one kilogram of pure water at maximum density and standard pressure. The kilogram was in turn specified as the mass of a platinum/iridium cylinder held at Sèvres in France and was intended to be of the mass as the 1 litre of water referred to above. It was subsequently discovered that the cylinder was around 28 parts per million too large and thus, during this time, additionally, the mass-volume relationship of water depends on temperature, pressure, purity and isotopic uniformity. In 1964, the definition relating the litre to mass was abandoned in favour of the current one, although the litre is not an official SI unit, it is accepted by the CGPM for use with the SI. CGPM defines the litre and its acceptable symbols, a litre is equal in volume to the millistere, an obsolete non-SI metric unit customarily used for dry measure. The litre is often used in some calculated measurements, such as density. One litre of water has a mass of almost exactly one kilogram when measured at its maximal density, similarly,1 millilitre of water has a mass of about 1 g,1,000 litres of water has a mass of about 1,000 kg. It is now known that density of water depends on the isotopic ratios of the oxygen and hydrogen atoms in a particular sample. The litre, though not an official SI unit, may be used with SI prefixes, the most commonly used derived unit is the millilitre, defined as one-thousandth of a litre, and also often referred to by the SI derived unit name cubic centimetre. It is a commonly used measure, especially in medicine and cooking, Other units may be found in the table below, where the more often used terms are in bold
22.
Earth
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Earth, otherwise known as the World, or the Globe, is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets, according to radiometric dating and other sources of evidence, Earth formed about 4.54 billion years ago. Earths gravity interacts with objects in space, especially the Sun. During one orbit around the Sun, Earth rotates about its axis over 365 times, thus, Earths axis of rotation is tilted, producing seasonal variations on the planets surface. The gravitational interaction between the Earth and Moon causes ocean tides, stabilizes the Earths orientation on its axis, Earths lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earths surface is covered with water, mostly by its oceans, the remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earths polar regions are covered in ice, including the Antarctic ice sheet, Earths interior remains active with a solid iron inner core, a liquid outer core that generates the Earths magnetic field, and a convecting mantle that drives plate tectonics. Within the first billion years of Earths history, life appeared in the oceans and began to affect the Earths atmosphere and surface, some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earths distance from the Sun, physical properties, in the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely, over 7.4 billion humans live on Earth and depend on its biosphere and minerals for their survival. Humans have developed diverse societies and cultures, politically, the world has about 200 sovereign states, the modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most often spelled eorðe. It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō, originally, earth was written in lowercase, and from early Middle English, its definite sense as the globe was expressed as the earth. By early Modern English, many nouns were capitalized, and the became the Earth. More recently, the name is simply given as Earth. House styles now vary, Oxford spelling recognizes the lowercase form as the most common, another convention capitalizes Earth when appearing as a name but writes it in lowercase when preceded by the. It almost always appears in lowercase in colloquial expressions such as what on earth are you doing, the oldest material found in the Solar System is dated to 4. 5672±0.0006 billion years ago. By 4. 54±0.04 Gya the primordial Earth had formed, the formation and evolution of Solar System bodies occurred along with the Sun
23.
Paris
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Paris is the capital and most populous city of France. It has an area of 105 square kilometres and a population of 2,229,621 in 2013 within its administrative limits, the agglomeration has grown well beyond the citys administrative limits. By the 17th century, Paris was one of Europes major centres of finance, commerce, fashion, science, and the arts, and it retains that position still today. The aire urbaine de Paris, a measure of area, spans most of the Île-de-France region and has a population of 12,405,426. It is therefore the second largest metropolitan area in the European Union after London, the Metropole of Grand Paris was created in 2016, combining the commune and its nearest suburbs into a single area for economic and environmental co-operation. Grand Paris covers 814 square kilometres and has a population of 7 million persons, the Paris Region had a GDP of €624 billion in 2012, accounting for 30.0 percent of the GDP of France and ranking it as one of the wealthiest regions in Europe. The city is also a rail, highway, and air-transport hub served by two international airports, Paris-Charles de Gaulle and Paris-Orly. Opened in 1900, the subway system, the Paris Métro. It is the second busiest metro system in Europe after Moscow Metro, notably, Paris Gare du Nord is the busiest railway station in the world outside of Japan, with 262 millions passengers in 2015. In 2015, Paris received 22.2 million visitors, making it one of the top tourist destinations. The association football club Paris Saint-Germain and the rugby union club Stade Français are based in Paris, the 80, 000-seat Stade de France, built for the 1998 FIFA World Cup, is located just north of Paris in the neighbouring commune of Saint-Denis. Paris hosts the annual French Open Grand Slam tennis tournament on the red clay of Roland Garros, Paris hosted the 1900 and 1924 Summer Olympics and is bidding to host the 2024 Summer Olympics. The name Paris is derived from its inhabitants, the Celtic Parisii tribe. Thus, though written the same, the name is not related to the Paris of Greek mythology. In the 1860s, the boulevards and streets of Paris were illuminated by 56,000 gas lamps, since the late 19th century, Paris has also been known as Panam in French slang. Inhabitants are known in English as Parisians and in French as Parisiens and they are also pejoratively called Parigots. The Parisii, a sub-tribe of the Celtic Senones, inhabited the Paris area from around the middle of the 3rd century BC. One of the areas major north-south trade routes crossed the Seine on the île de la Cité, this place of land and water trade routes gradually became a town
24.
Water
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Water is a transparent and nearly colorless chemical substance that is the main constituent of Earths streams, lakes, and oceans, and the fluids of most living organisms. Its chemical formula is H2O, meaning that its molecule contains one oxygen, Water strictly refers to the liquid state of that substance, that prevails at standard ambient temperature and pressure, but it often refers also to its solid state or its gaseous state. It also occurs in nature as snow, glaciers, ice packs and icebergs, clouds, fog, dew, aquifers, Water covers 71% of the Earths surface. It is vital for all forms of life. Only 2. 5% of this water is freshwater, and 98. 8% of that water is in ice and groundwater. Less than 0. 3% of all freshwater is in rivers, lakes, and the atmosphere, a greater quantity of water is found in the earths interior. Water on Earth moves continually through the cycle of evaporation and transpiration, condensation, precipitation. Evaporation and transpiration contribute to the precipitation over land, large amounts of water are also chemically combined or adsorbed in hydrated minerals. Safe drinking water is essential to humans and other even though it provides no calories or organic nutrients. There is a correlation between access to safe water and gross domestic product per capita. However, some observers have estimated that by 2025 more than half of the population will be facing water-based vulnerability. A report, issued in November 2009, suggests that by 2030, in developing regions of the world. Water plays an important role in the world economy, approximately 70% of the freshwater used by humans goes to agriculture. Fishing in salt and fresh water bodies is a source of food for many parts of the world. Much of long-distance trade of commodities and manufactured products is transported by boats through seas, rivers, lakes, large quantities of water, ice, and steam are used for cooling and heating, in industry and homes. Water is an excellent solvent for a variety of chemical substances, as such it is widely used in industrial processes. Water is also central to many sports and other forms of entertainment, such as swimming, pleasure boating, boat racing, surfing, sport fishing, Water is a liquid at the temperatures and pressures that are most adequate for life. Specifically, at atmospheric pressure of 1 bar, water is a liquid between the temperatures of 273.15 K and 373.15 K
25.
Day
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In common usage, it is either an interval equal to 24 hours or daytime, the consecutive period of time during which the Sun is above the horizon. The period of time during which the Earth completes one rotation with respect to the Sun is called a solar day, several definitions of this universal human concept are used according to context, need and convenience. In 1960, the second was redefined in terms of the motion of the Earth. The unit of measurement day, redefined in 1960 as 86400 SI seconds and symbolized d, is not an SI unit, but is accepted for use with SI. The word day may also refer to a day of the week or to a date, as in answer to the question. The life patterns of humans and many species are related to Earths solar day. In recent decades the average length of a day on Earth has been about 86400.002 seconds. A day, understood as the span of time it takes for the Earth to make one rotation with respect to the celestial background or a distant star, is called a stellar day. This period of rotation is about 4 minutes less than 24 hours, mainly due to tidal effects, the Earths rotational period is not constant, resulting in further minor variations for both solar days and stellar days. Other planets and moons have stellar and solar days of different lengths to Earths, besides the day of 24 hours, the word day is used for several different spans of time based on the rotation of the Earth around its axis. An important one is the day, defined as the time it takes for the Sun to return to its culmination point. Because the Earth orbits the Sun elliptically as the Earth spins on an inclined axis, on average over the year this day is equivalent to 24 hours. A day, in the sense of daytime that is distinguished from night-time, is defined as the period during which sunlight directly reaches the ground. The length of daytime averages slightly more than half of the 24-hour day, two effects make daytime on average longer than nights. The Sun is not a point, but has an apparent size of about 32 minutes of arc, additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground even when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc, the difference in time depends on the angle at which the Sun rises and sets, but can amount to around seven minutes. Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon, the exact moment of, and the interval between, two sunrises or sunsets depends on the geographical position, and the time of year. A more constant day can be defined by the Sun passing through the local meridian, the exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year
26.
Newton (unit)
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The newton is the International System of Units derived unit of force. It is named after Isaac Newton in recognition of his work on classical mechanics, see below for the conversion factors. One newton is the force needed to one kilogram of mass at the rate of one metre per second squared in direction of the applied force. In 1948, the 9th CGPM resolution 7 adopted the name newton for this force, the MKS system then became the blueprint for todays SI system of units. The newton thus became the unit of force in le Système International dUnités. This SI unit is named after Isaac Newton, as with every International System of Units unit named for a person, the first letter of its symbol is upper case. Note that degree Celsius conforms to this rule because the d is lowercase. — Based on The International System of Units, section 5.2. Newtons second law of motion states that F = ma, where F is the applied, m is the mass of the object receiving the force. The newton is therefore, where the symbols are used for the units, N for newton, kg for kilogram, m for metre. In dimensional analysis, F = M L T2 where F is force, M is mass, L is length, at average gravity on earth, a kilogram mass exerts a force of about 9.8 newtons. An average-sized apple exerts about one newton of force, which we measure as the apples weight, for example, the tractive effort of a Class Y steam train and the thrust of an F100 fighter jet engine are both around 130 kN. One kilonewton,1 kN, is 102.0 kgf,1 kN =102 kg ×9.81 m/s2 So for example, a platform rated at 321 kilonewtons will safely support a 32,100 kilograms load. Specifications in kilonewtons are common in safety specifications for, the values of fasteners, Earth anchors. Working loads in tension and in shear, thrust of rocket engines and launch vehicles clamping forces of the various moulds in injection moulding machines used to manufacture plastic parts
27.
Silver nitrate
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Silver nitrate is an inorganic compound with chemical formula AgNO3. This compound is a precursor to many other silver compounds. It is far less sensitive to light than the halides and it was once called lunar caustic because silver was called luna by the ancient alchemists, who believed that silver was associated with the moon. In solid silver nitrate, the ions are three-coordinated in a trigonal planar arrangement. Albertus Magnus, in the 13th century, documented the ability of nitric acid to separate gold, Magnus noted that the resulting solution of silver nitrate could blacken skin. Silver nitrate can be prepared by reacting silver, such as a silver bullion or silver foil, with acid, resulting in silver nitrate, water. Reaction byproducts depend upon the concentration of acid used. 3 Ag +4 HNO3 →3 AgNO3 +2 H2O + NO Ag +2 HNO3 → AgNO3 + H2O + NO2 This is performed under a fume hood because of nitrogen oxide evolved during the reaction. A typical reaction with silver nitrate is to suspend a rod of copper in a solution of silver nitrate, silver nitrate is the least expensive salt of silver, it offers several other advantages as well. It is non-hygroscopic, in contrast to silver fluoroborate and silver perchlorate and it is relatively stable to light. Finally, it dissolves in solvents, including water. The nitrate can be replaced by other ligands, rendering AgNO3 versatile. Treatment with solutions of halide ions gives a precipitate of AgX, similarly, silver nitrate is used to prepare some silver-based explosives, such as the fulminate, azide, or acetylide, through a precipitation reaction. This reaction is used in inorganic chemistry to abstract halides, Ag+ + X− → AgX where X− = Cl−, Br−. Other silver salts with non-coordinating anions, namely silver tetrafluoroborate and silver hexafluorophosphate are used for demanding applications. Similarly, this reaction is used in chemistry to confirm the presence of chloride, bromide. Samples are typically acidified with nitric acid to remove interfering ions, e. g. carbonate ions. This step avoids confusion of silver sulfide or silver carbonate precipitates with that of silver halides, the color of precipitate varies with the halide, white, pale yellow/cream, yellow
28.
Thermodynamic temperature
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Thermodynamic temperature is the absolute measure of temperature and is one of the principal parameters of thermodynamics. Thermodynamic temperature is defined by the law of thermodynamics in which the theoretically lowest temperature is the null or zero point. At this point, absolute zero, the constituents of matter have minimal motion. In the quantum-mechanical description, matter at absolute zero is in its ground state, the International System of Units specifies a particular scale for thermodynamic temperature. It uses the Kelvin scale for measurement and selects the point of water at 273.16 K as the fundamental fixing point. Other scales have been in use historically, the Rankine scale, using the degree Fahrenheit as its unit interval, is still in use as part of the English Engineering Units in the United States in some engineering fields. ITS-90 gives a means of estimating the thermodynamic temperature to a very high degree of accuracy. Internal energy is called the heat energy or thermal energy in conditions when no work is done upon the substance by its surroundings. Internal energy may be stored in a number of ways within a substance, each way constituting a degree of freedom. At equilibrium, each degree of freedom will have on average the energy, k B T /2 where k B is the Boltzmann constant. Temperature is a measure of the random submicroscopic motions and vibrations of the constituents of matter. These motions comprise the internal energy of a substance, more specifically, the thermodynamic temperature of any bulk quantity of matter is the measure of the average kinetic energy per classical degree of freedom of its constituent particles. Translational motions are almost always in the classical regime, translational motions are ordinary, whole-body movements in three-dimensional space in which particles move about and exchange energy in collisions. Figure 1 below shows translational motion in gases, Figure 4 below shows translational motion in solids, Zero kinetic energy remains in a substance at absolute zero. Throughout the scientific world where measurements are made in SI units, many engineering fields in the U. S. however, measure thermodynamic temperature using the Rankine scale. By international agreement, the kelvin and its scale are defined by two points, absolute zero, and the triple point of Vienna Standard Mean Ocean Water. Absolute zero, the lowest possible temperature, is defined as being precisely 0 K, the triple point of water is defined as being precisely 273.16 K and 0.01 °C. This definition does three things, It fixes the magnitude of the unit as being precisely 1 part in 273.15 kelvins
29.
Triple point
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In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases of that substance coexist in thermodynamic equilibrium. For example, the point of mercury occurs at a temperature of −38.83440 °C. In addition to the point for solid, liquid, and gas phases. Helium-4 is a case that presents a triple point involving two different fluid phases. The triple point of water is used to define the kelvin, the value of the triple point of water is fixed by definition, rather than measured. The triple points of substances are used to define points in the ITS-90 international temperature scale. The term triple point was coined in 1873 by James Thomson, at that point, it is possible to change all of the substance to ice, water, or vapor by making arbitrarily small changes in pressure and temperature. Strictly speaking, the surfaces separating the different phases should also be perfectly flat, the gas–liquid–solid triple point of water corresponds to the minimum pressure at which liquid water can exist. At pressures below the point, solid ice when heated at constant pressure is converted directly into water vapor in a process known as sublimation. Above the triple point, solid ice when heated at constant pressure first melts to form liquid water, for most substances the gas–liquid–solid triple point is also the minimum temperature at which the liquid can exist. For water, however, this is not true because the point of ordinary ice decreases as a function of pressure. At temperatures just below the point, compression at constant temperature transforms water vapor first to solid. The triple point pressure of water was used during the Mariner 9 mission to Mars as a point to define sea level. More recent missions use laser altimetry and gravity measurements instead of pressure to define elevation on Mars, at high pressures, water has a complex phase diagram with 15 known phases of ice and several triple points including ten whose coordinates are shown in the diagram. For example, the point at 251 K and 210 MPa corresponds to the conditions for the coexistence of ice Ih, ice III and liquid water. There are also triple points for the coexistence of three phases, for example ice II, ice V and ice VI at 218 K and 620 MPa. For those high-pressure forms of ice which can exist in equilibrium with liquid, at temperatures above 273 K, increasing the pressure on water vapor results first in liquid water and then a high-pressure form of ice. In the range 251–273 K, ice I is formed first, followed by water and then ice III or ice V
30.
Celsius
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Celsius, also known as centigrade, is a metric scale and unit of measurement for temperature. As an SI derived unit, it is used by most countries in the world and it is named after the Swedish astronomer Anders Celsius, who developed a similar temperature scale. The degree Celsius can refer to a temperature on the Celsius scale as well as a unit to indicate a temperature interval. Before being renamed to honour Anders Celsius in 1948, the unit was called centigrade, from the Latin centum, which means 100, and gradus, which means steps. The scale is based on 0° for the point of water. This scale is widely taught in schools today, by international agreement the unit degree Celsius and the Celsius scale are currently defined by two different temperatures, absolute zero, and the triple point of VSMOW. This definition also precisely relates the Celsius scale to the Kelvin scale, absolute zero, the lowest temperature possible, is defined as being precisely 0 K and −273.15 °C. The temperature of the point of water is defined as precisely 273.16 K at 611.657 pascals pressure. This definition fixes the magnitude of both the degree Celsius and the kelvin as precisely 1 part in 273.16 of the difference between absolute zero and the point of water. Thus, it sets the magnitude of one degree Celsius and that of one kelvin as exactly the same, additionally, it establishes the difference between the two scales null points as being precisely 273.15 degrees. In his paper Observations of two persistent degrees on a thermometer, he recounted his experiments showing that the point of ice is essentially unaffected by pressure. He also determined with precision how the boiling point of water varied as a function of atmospheric pressure. He proposed that the point of his temperature scale, being the boiling point. This pressure is known as one standard atmosphere, the BIPMs 10th General Conference on Weights and Measures later defined one standard atmosphere to equal precisely 1013250dynes per square centimetre. On 19 May 1743 he published the design of a mercury thermometer, in 1744, coincident with the death of Anders Celsius, the Swedish botanist Carolus Linnaeus reversed Celsiuss scale. In it, Linnaeus recounted the temperatures inside the orangery at the University of Uppsala Botanical Garden, since the 19th century, the scientific and thermometry communities worldwide referred to this scale as the centigrade scale. Temperatures on the scale were often reported simply as degrees or. More properly, what was defined as centigrade then would now be hectograde.2 gradians, for scientific use, Celsius is the term usually used, with centigrade otherwise continuing to be in common but decreasing use, especially in informal contexts in English-speaking countries
31.
Absolute zero
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Absolute zero is the lower limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reaches its minimum value, taken as 0. The corresponding Kelvin and Rankine temperature scales set their zero points at absolute zero by definition, in the quantum-mechanical description, matter at absolute zero is in its ground state, the point of lowest internal energy. And a system at absolute zero still possesses quantum mechanical zero-point energy, the kinetic energy of the ground state cannot be removed. Scientists and technologists routinely achieve temperatures close to zero, where matter exhibits quantum effects such as superconductivity and superfluidity. At temperatures near 0 K, nearly all molecular motion ceases and ΔS =0 for any adiabatic process, in such a circumstance, pure substances can form perfect crystals as T →0. Max Plancks strong form of the law of thermodynamics states the entropy of a perfect crystal vanishes at absolute zero. The Nernst postulate identifies the isotherm T =0 as coincident with the adiabat S =0, although other isotherms, as no two adiabats intersect, no other adiabat can intersect the T =0 isotherm. Consequently no adiabatic process initiated at nonzero temperature can lead to zero temperature, a perfect crystal is one in which the internal lattice structure extends uninterrupted in all directions. The perfect order can be represented by translational symmetry along three axes, every lattice element of the structure is in its proper place, whether it is a single atom or a molecular grouping. For substances that exist in two crystalline forms, such as diamond and graphite for carbon, there is a kind of chemical degeneracy. The question remains whether both can have zero entropy at T =0 even though each is perfectly ordered, using the Debye model, the specific heat and entropy of a pure crystal are proportional to T3, while the enthalpy and chemical potential are proportional to T4. These quantities drop toward their T =0 limiting values and approach with zero slopes, for the specific heats at least, the limiting value itself is definitely zero, as borne out by experiments to below 10 K. Even the less detailed Einstein model shows this curious drop in specific heats, in fact, all specific heats vanish at absolute zero, not just those of crystals. Likewise for the coefficient of thermal expansion, maxwells relations show that various other quantities also vanish. Since the relation between changes in Gibbs free energy, the enthalpy and the entropy is Δ G = Δ H − T Δ S thus, as T decreases, ΔG and ΔH approach each other. Experimentally, it is found that all spontaneous processes result in a decrease in G as they proceed toward equilibrium, if ΔS and/or T are small, the condition ΔG <0 may imply that ΔH <0, which would indicate an exothermic reaction. However, this is not required, endothermic reactions can proceed spontaneously if the TΔS term is large enough, moreover, the slopes of the derivatives of ΔG and ΔH converge and are equal to zero at T =0. e. An actual process is the most exothermic one, one model that estimates the properties of an electron gas at absolute zero in metals is the Fermi gas
32.
Relative atomic mass
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Relative atomic mass is a dimensionless physical quantity. In its modern definition, it is the ratio of the mass of atoms of an element in a given sample to one unified atomic mass unit. The unified atomic mass unit, symbol u, is defined being 1⁄12 of the mass of a carbon-12 atom, the mass of atoms can vary, due to the presence of various isotopes of that element. Since both values in the ratio are expressed in the unit, the resulting value is dimensionless. Within one source, it is a average over the individual atom weights present. Between sources, the weight can vary when the sources origin resulted in different isotopic concentrations. These differences are real and measurable, and can be used to identify a sample to its origin, for example, a sample of elemental carbon from volcanic methane will have a different relative atomic mass than one collected from plant or animal tissues. The well-known standard atomic weight, or atomic weight, is a usage of relative atomic mass, it is the relative atomic mass. For these sources, research reports are used by the CIAAW of the International Union of Pure, the standard atomic weights are reprinted in a wide variety of textbooks, commercial catalogues, and periodic table wall charts. They are what chemists loosely call atomic weights and it is the most published form of the relative atomic mass. Both terms are officially sanctioned by IUPAC, the term relative atomic mass now seems to be gaining as the preferred term over atomic weight, although in the case of standard atomic weight, this shorter term continues to be used. This comparison is the quotient of the two weights, which makes the value dimensionless and this quotient also explains the word relative, the sample mass value is made relative to carbon-12. It is uses as a synonym for atomic weight, in some circumstances may even be synonymous with standard atomic weight. Here the unified atomic mass unit refers to 1⁄12 of the mass of an atom of 12C in its ground state. The IUPAC definition of relative atomic mass is, An atomic weight of an element from a source is the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of 12C. The definition deliberately specifies An atomic weight…, as an element will have different relative atomic masses depending on the source, for example, boron from Turkey has a lower relative atomic mass than boron from California, because of its different isotopic composition. Nevertheless, given the cost and difficulty of isotope analysis, it is usual to use the values of standard atomic weights which are ubiquitous in chemical laboratories. Older historical relative scales used either the relative isotopic mass for reference
33.
Steradian
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The steradian or square radian is the SI unit of solid angle. It is used in geometry, and is analogous to the radian which quantifies planar angles. The name is derived from the Greek stereos for solid and the Latin radius for ray and it is useful, however, to distinguish between dimensionless quantities of a different nature, so the symbol sr is used to indicate a solid angle. For example, radiant intensity can be measured in watts per steradian, the steradian was formerly an SI supplementary unit, but this category was abolished in 1995 and the steradian is now considered an SI derived unit. A steradian can be defined as the angle subtended at the center of a unit sphere by a unit area on its surface. For a general sphere of radius r, any portion of its surface with area A = r2 subtends one steradian, because the surface area A of a sphere is 4πr2, the definition implies that a sphere measures 4π steradians. By the same argument, the solid angle that can be subtended at any point is 4π sr. Since A = r2, it corresponds to the area of a cap. Therefore one steradian corresponds to the angle of the cross-section of a simple cone subtending the plane angle 2θ, with θ given by, θ = arccos = arccos = arccos ≈0.572 rad. This angle corresponds to the plane angle of 2θ ≈1.144 rad or 65. 54°. A steradian is also equal to the area of a polygon having an angle excess of 1 radian, to 1/4π of a complete sphere. The solid angle of a cone whose cross-section subtends the angle 2θ is, Ω =2 π s r. In two dimensions, an angle is related to the length of the arc that it spans, θ = l r r a d where l is arc length, r is the radius of the circle. For example, a measurement of the width of an object would be given in radians. At the same time its visible area over ones visible field would be given in steradians. Just as the area of a circle is related to its diameter or radius. One-dimensional circular measure has units of radians or degrees, while two-dimensional spherical measure is expressed in steradians, in higher dimensional mathematical spaces, units for analogous solid angles have not been explicitly named. When they are used, they are dealt with by analogy with the circular or spherical cases and that is, as a proportion of the relevant unit hypersphere taken up by the generalized angle, or point set expressed in spherical coordinates
34.
Candlepower
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Candlepower is an obsolete unit expressing luminous intensity, equal to 0.981 candelas. It expresses levels of intensity in terms of the light emitted by a candle of specific size. In modern usage candlepower equates directly to the known as the candela. Spermaceti is found in the heads of sperm whales, and was used to make high-quality candles. At this time the French standard of light was based upon the illumination from a Carcel burner, the unit was defined as that illumination emanating from a lamp burning pure colza oil at a defined rate. It was accepted that ten standard candles were about equal to one Carcel burner, in 1909 a meeting took place to come up with an international standard. It was attended by representatives of the Laboratoire Central de l’Electricité, the National Physical Laboratory, the Bureau of Standards, the majority redefined the candle in term of an electric lamp with a carbon filament. The Germans, however, dissented and decided to use a definition equal to 9/10 of the output of a Hefner lamp, in 1921, the Commission Internationale de lEclairage redefined the international candle again in terms of a carbon filament incandescent lamp. In 1948 the term candlepower was replaced by the unit known as the candela. One old candlepower unit is about 0.981 candela, in general modern use, a candlepower now equates directly to the number of candelas—an implicit increase from its old value. The candlepower of a lamp was measured by judging by eye the relative brightness of adjacent surfaces, one illuminated only by a standard lamp, the distance of one of the lamps was adjusted until the two were judged to give equal brightness. The candlepower of the lamp under test could then be calculated from the two distances and the square law. Candlepower is largely an obsolete term, however, it is still sometimes used to describe the luminous intensity of high powered flashlights and spotlights. A given lamp will have a higher rating if its light is more tightly focused. List of obsolete units of measurement Rowlett, Russ, a Dictionary of Units of Measurement. University of North Carolina at Chapel Hill, International candle at Sizes. com Last revised,27 June 2007. Accessed July 2007 Candle History - Candlepower 2003 Bob Sherman at Craftcave, brief History Of Lighting 2004 by The Wolfstone Group. A History of Light and Lighting by Bill Williams Edition,2.3 - Accessed July 2007, the genesis of incandescent lamp - manufacture
35.
Proposed redefinition of SI base units
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The metric system was originally conceived as a system of measurement that was derivable from nature. When the metric system was first introduced in France in 1799 technical limitations necessitated the use of such as the prototype metre. In 1960 the metre was redefined in terms of the wavelength of light from a source, making it derivable from nature. If the proposed redefinition is accepted, the system will, for the first time. The proposal can be summarised as follows, There will still be the seven base units. The second, metre and candela are already defined by physical constants, the new definitions will improve the SI without changing the size of any units, thus ensuring continuity with present measurements. Further details are found in the chapter of the Ninth SI Units Brochure. The last major overhaul of the system was in 1960 when the International System of Units was formally published as a coherent set of units of measure. SI is structured around seven base units that have apparently arbitrary definitions, although the set of units form a coherent system, the definitions do not. The proposal before the CIPM seeks to remedy this by using the quantities of nature as the basis for deriving the base units. This will mean, amongst other things, that the prototype kilogram will cease to be used as the replica of the kilogram. The second and the metre are already defined in such a manner, the basic structure of SI was developed over a period of about 170 years. Since 1960 technological advances have made it possible to address weaknesses in SI. Specifically, the metre was defined as one ten-millionth of the distance from the North Pole to the Equator, although these definitions were chosen so that nobody would own the units, they could not be measured with sufficient convenience or precision for practical use. Instead copies were created in the form of the mètre des Archives, in 1875, by which time the use of the metric system had become widespread in Europe and in Latin America, twenty industrially developed nations met for the Convention of the Metre. They were, CGPM —The Conference meets every four to six years, CIPM —The Committee consists of eighteen eminent scientists, each from a different country, nominated by the CGPM. The CIPM meets annually and is tasked to advise the CGPM, the CIPM has set up a number of sub-committees, each charged with a particular area of interest. One of these, the Consultative Committee for Units, amongst other things, the first CGPM formally approved the use of 40 prototype metres and 40 prototype kilograms from the British firm Johnson Matthey as the standards mandated by the Convention of the Metre
36.
Metre Convention
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Metre Convention, also known as the Treaty of the Metre, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations. The treaty set up an institute for the purpose of coordinating international metrology, the treaty also set up associated organizations to oversee the running of the institute. In 1960, at the 11th meetings of the CGPM, the system of units it had established was overhauled and relaunched as the International System of Units. The Convention created three main organizations, The CGPM – a meeting every four to six years of delegates from all member states. The International Committee for Weights and Measures – an advisory body to the CGPM consisting of 18 prominent metrologists from 18 different countries, in England in 1215, clause 25 of the Magna Carta set out the standards of measure that were to be applied throughout the realm prefixed. The wording of the clause emphasised that There is to be a single measure, five centuries later, when in 1707 England and Scotland were united into a single kingdom, the Scots agreed to use the same units of measure that were already established in England. During the eighteenth century, in order to trade, Peter the Great. Abuse of units of measures were one of the causes of the French Revolution, between 1850 and 1870, a number of countries adopted the metric system as their system of measure including Spain, many South American republics and many of the Italian and German states. In 1863, the International Postal Union used grams to express permitted weights of letters, in the 1860s, inspections of the prototype metre revealed wear and tear at the measuring faces of the bar and also that the bar was wont to flex slightly when in use. In July 1870, two weeks before the conference was due to start, the Franco-Prussian War broke out, although the delegates did meet, it was agreed that the conference should be recalled once all the delegates were present. In 1872 the new republican government reissued the invitations and in 1875 scientists from thirty European and American countries met in Paris, the conference did not concern itself with other units of measure. The conference had undertones of Franco-German political manoeuvring, particularly since the French had been humiliated by the Prussians during the war a few years previously. Although France lost control of the system, they ensured that it passed to international rather than German control. They were eventually reassured when the German-born Swiss delegate Adolphe Hirsch said no serious scientist would in our day and age contemplate a metre deduced from the size of the earth. When the conference was reconvened in 1875, it was proposed that new prototype metre, although the new standard metre had the same value as the old metre, it had an X cross-section rather than a rectangular cross-section as this reduced the flexing when taking measurements. Moreover, the new bar, rather than being exactly one metre in length was a longer than one metre and had lines engraved on them that were exactly one metre apart. The London firm Johnson Matthey delivered 30 prototype metres and 40 prototype kilograms, at the first meeting of the CGPM in 1889 bar No.6 and cylinder No. X were chosen by lot as the international prototypes, the remainder were either kept as BIPM working copies or distributed by lot to member states as national prototypes
37.
Platinum
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Platinum is a chemical element with symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious and its name is derived from the Spanish term platina, translated into little silver. Platinum is a member of the group of elements and group 10 of the periodic table of elements. It has six naturally occurring isotopes and it is one of the rarer elements in Earths crust with an average abundance of approximately 5 μg/kg. It occurs in some nickel and copper ores along with some deposits, mostly in South Africa. Because of its scarcity in Earths crust, only a few hundred tonnes are produced annually, Platinum is one of the least reactive metals. It has remarkable resistance to corrosion, even at high temperatures, consequently, platinum is often found chemically uncombined as native platinum. Because it occurs naturally in the sands of various rivers. Platinum is used in catalytic converters, laboratory equipment, electrical contacts and electrodes, platinum resistance thermometers, dentistry equipment, and jewelry. Being a heavy metal, it leads to health issues upon exposure to its salts, compounds containing platinum, such as cisplatin, oxaliplatin and carboplatin, are applied in chemotherapy against certain types of cancer. Pure platinum is a lustrous, ductile, and malleable, silver-white metal, Platinum is more ductile than gold, silver or copper, thus being the most ductile of pure metals, but it is less malleable than gold. The metal has excellent resistance to corrosion, is stable at temperatures and has stable electrical properties. Platinum reacts with oxygen slowly at high temperatures. It reacts vigorously with fluorine at 500 °C to form platinum tetrafluoride and it is also attacked by chlorine, bromine, iodine, and sulfur. Platinum is insoluble in hydrochloric and nitric acid, but dissolves in hot aqua regia to form chloroplatinic acid and its physical characteristics and chemical stability make it useful for industrial applications. Its resistance to wear and tarnish is well suited to use in fine jewelry, the most common oxidation states of platinum are +2 and +4. The +1 and +3 oxidation states are common, and are often stabilized by metal bonding in bimetallic species. As is expected, tetracoordinate platinum compounds tend to adopt 16-electron square planar geometries, Platinum has six naturally occurring isotopes, 190Pt, 192Pt, 194Pt, 195Pt, 196Pt, and 198Pt
38.
Iridium
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Iridium is a chemical element with symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group and it is also the most corrosion-resistant metal, even at temperatures as high as 2000 °C. Although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is more reactive. Iridium was discovered in 1803 among insoluble impurities in natural platinum, Smithson Tennant, the primary discoverer, named iridium for the Greek goddess Iris, personification of the rainbow, because of the striking and diverse colors of its salts. Iridium is one of the rarest elements in Earths crust, with production and consumption of only three tonnes. 191Ir and 193Ir are the two naturally occurring isotopes of iridium, as well as the only stable isotopes, the latter is the more abundant of the two. Iridium radioisotopes are used in some radioisotope thermoelectric generators, Iridium is found in meteorites in much higher abundance than in the Earths crust. Similarly, an anomaly in core samples from the Pacific Ocean suggested the Eltanin impact of about 2.5 million years ago. A member of the platinum metals, iridium is white, resembling platinum. Because of its hardness, brittleness, and very high melting point, solid iridium is difficult to machine, form, or work, thus powder metallurgy is commonly employed and it is the only metal to maintain good mechanical properties in air at temperatures above 1,600 °C. The measured density of iridium is only lower than that of osmium. Iridium is the most corrosion-resistant metal known, it is not attacked by almost any acid, aqua regia, molten metals, or silicates at high temperatures. It can, however, be attacked by some molten salts, such as cyanide and potassium cyanide, as well as oxygen. Iridium forms compounds in oxidation states between −3 and +9, the most common oxidation states are +3 and +4, well-characterized examples of the high +6 oxidation state are rare, but include IrF6 and two mixed oxides Sr 2MgIrO6 and Sr 2CaIrO6. In addition, it was reported in 2009 that iridium oxide was prepared under matrix isolation conditions by UV irradiation of an iridium-peroxo complex and this species, however, is not expected to be stable as a bulk solid at higher temperatures. The highest oxidation state, which is also the highest recorded for any element, is known in one cation, IrO+4. Iridium dioxide, IrO2, a powder, is the only well-characterized oxide of iridium. A sesquioxide, Ir 2O3, has described as a blue-black powder which is oxidized to IrO2 by HNO3
39.
Metrology
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The field of metrology is important for establishing a common understanding of units, which is crucial in linking human activities. This led to the creation of the decimal based metric system in 1795 to establish a set of standards for types of measurements. This has since evolved into the International System of Units as a result of a made in the 11th Conference Generale des Poids et Mesures in 1960. The NMS greatly effects how measurements are undertaken in the country and this has wide reaching impacts in a variety of different regions of society. It has implications in the area of economies, energy, environment, health, manufacturing, industry, consumer confidence, the effects of metrology on trade and the economy are some of the easiest observed societal impacts. The field of metrology is important for establishing a common understanding of units and these base concepts of metrology are propagated by the three main fields of metrology, which are, scientific or fundamental metrology, applied, technical or industrial metrology, and legal metrology. While there is no definition of fundamental metrology it is considered to be the top level of scientific metrology that strives for the highest level of accuracy. The BIPM maintains a database of the calibration and measurement capabilities of various institutes around the world. These institutes, whose activities are peer-reviewed, provide the reference points for metrological traceability. In the area of measurement, the BIPM has identified nine metrology areas, including length, mass, although the emphasis in this area of metrology is on the measurements themselves, traceability of the calibration of the measurement devices is necessary to ensure confidence in the measurements. Industrial metrology is important to the economical and industrial development of a country, such statutory requirements might arise from, amongst others, the needs for protection of health, public safety, the environment, enabling taxation, protection of consumers and fair trade. The International Organization for Legal Metrology was established to assist in harmonising such regulations across national boundaries to ensure that legal requirements do not inhibit trade. In Europe WELMEC was established in 1990 to promote cooperation on the field of metrology in the European Union. Throughout history the ability to make measurements has been instrumental in the progress of mankind, the ability to measure alone is not sufficient, rather the ability to compare separate measurements and have agreeability is crucial for the measurements to be meaningful. The first record of a permanent standard is from 2900 BC, the cubit was decreed to be the length of the Pharaohs forearm plus the width of his hand and replica standards were given out to builders. The success of standardized length during the building of the pyramids is evidenced by the lengths of the pyramid bases differing by no more than 0. 05%, other civilizations produced their own measurement standards that were accepted throughout their nations. The architecture of the Roman and Greek empires were based on their own systems of measurement. Nonetheless, the fall of great empires and the rise of the Dark Ages caused much of measurement knowledge
40.
Planck constant
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The Planck constant is a physical constant that is the quantum of action, central in quantum mechanics. The light quantum behaved in some respects as a neutral particle. It was eventually called the photon, the Planck–Einstein relation connects the particulate photon energy E with its associated wave frequency f, E = h f This energy is extremely small in terms of ordinarily perceived everyday objects. Since the frequency f, wavelength λ, and speed of c are related by f = c λ. This leads to another relationship involving the Planck constant, with p denoting the linear momentum of a particle, the de Broglie wavelength λ of the particle is given by λ = h p. In applications where it is natural to use the frequency it is often useful to absorb a factor of 2π into the Planck constant. The resulting constant is called the reduced Planck constant or Dirac constant and it is equal to the Planck constant divided by 2π, and is denoted ħ, ℏ = h 2 π. The energy of a photon with angular frequency ω, where ω = 2πf, is given by E = ℏ ω, while its linear momentum relates to p = ℏ k and this was confirmed by experiments soon afterwards. This holds throughout quantum theory, including electrodynamics and these two relations are the temporal and spatial component parts of the special relativistic expression using 4-Vectors. P μ = = ℏ K μ = ℏ Classical statistical mechanics requires the existence of h, eventually, following upon Plancks discovery, it was recognized that physical action cannot take on an arbitrary value. Instead, it must be multiple of a very small quantity. This is the old quantum theory developed by Bohr and Sommerfeld, in which particle trajectories exist but are hidden. Thus there is no value of the action as classically defined, related to this is the concept of energy quantization which existed in old quantum theory and also exists in altered form in modern quantum physics. Classical physics cannot explain either quantization of energy or the lack of a particle motion. In many cases, such as for light or for atoms, quantization of energy also implies that only certain energy levels are allowed. The Planck constant has dimensions of physical action, i. e. energy multiplied by time, or momentum multiplied by distance, in SI units, the Planck constant is expressed in joule-seconds or or. The value of the Planck constant is, h =6.626070040 ×10 −34 J⋅s =4.135667662 ×10 −15 eV⋅s. The value of the reduced Planck constant is, ℏ = h 2 π =1.054571800 ×10 −34 J⋅s =6.582119514 ×10 −16 eV⋅s
41.
Avogadro constant
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In chemistry and physics, the Avogadro constant is the number of constituent particles, usually atoms or molecules, that are contained in the amount of substance given by one mole. Thus, it is the proportionality factor that relates the mass of a compound to the mass of a sample. Avogadros constant, often designated with the symbol NA or L, has the value 7023602214085700000♠6. 022140857×1023 mol−1 in the International System of Units and this number is also known as Loschmidt constant in German literature. The constant was later redefined as the number of atoms in 12 grams of the isotope carbon-12, for instance, to a first approximation,1 gram of hydrogen element, having the atomic number 1, has 7023602200000000000♠6. 022×1023 hydrogen atoms. Similarly,12 grams of 12C, with the mass number 12, has the number of carbon atoms. Avogadros number is a quantity, and has the same numerical value of the Avogadro constant given in base units. In contrast, the Avogadro constant has the dimension of reciprocal amount of substance, the Avogadro constant can also be expressed as 0.602214. ML mol−1 Å−3, which can be used to convert from volume per molecule in cubic ångströms to molar volume in millilitres per mole, revisions in the base set of SI units necessitated redefinitions of the concepts of chemical quantity. Avogadros number, and its definition, was deprecated in favor of the Avogadro constant, the French physicist Jean Perrin in 1909 proposed naming the constant in honor of Avogadro. Perrin won the 1926 Nobel Prize in Physics, largely for his work in determining the Avogadro constant by several different methods, accurate determinations of Avogadros number require the measurement of a single quantity on both the atomic and macroscopic scales using the same unit of measurement. This became possible for the first time when American physicist Robert Millikan measured the charge on an electron in 1910, the electric charge per mole of electrons is a constant called the Faraday constant and had been known since 1834 when Michael Faraday published his works on electrolysis. By dividing the charge on a mole of electrons by the charge on a single electron the value of Avogadros number is obtained, since 1910, newer calculations have more accurately determined the values for the Faraday constant and the elementary charge. Perrin originally proposed the name Avogadros number to refer to the number of molecules in one gram-molecule of oxygen, with this recognition, the Avogadro constant was no longer a pure number, but had a unit of measurement, the reciprocal mole. While it is rare to use units of amount of other than the mole, the Avogadro constant can also be expressed in units such as the pound mole. NA = 7026273159734000000♠2. 73159734×1026 −1 = 7025170724843400000♠1. 707248434×1025 −1 Avogadros constant is a factor between macroscopic and microscopic observations of nature. As such, it provides the relationship between other physical constants and properties. The Avogadro constant also enters into the definition of the atomic mass unit. The earliest accurate method to measure the value of the Avogadro constant was based on coulometry
42.
Boltzmann constant
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The Boltzmann constant, which is named after Ludwig Boltzmann, is a physical constant relating the average kinetic energy of particles in a gas with the temperature of the gas. It is the gas constant R divided by the Avogadro constant NA, the Boltzmann constant has the dimension energy divided by temperature, the same as entropy. The accepted value in SI units is 6977138064851999999♠1. 38064852×10−23 J/K, the Boltzmann constant, k, is a bridge between macroscopic and microscopic physics. Introducing the Boltzmann constant transforms the gas law into an alternative form, p V = N k T. For n =1 mol, N is equal to the number of particles in one mole, given a thermodynamic system at an absolute temperature T, the average thermal energy carried by each microscopic degree of freedom in the system is on the order of magnitude of 1/2kT. In classical statistical mechanics, this average is predicted to hold exactly for homogeneous ideal gases, monatomic ideal gases possess three degrees of freedom per atom, corresponding to the three spatial directions, which means a thermal energy of 3/2kT per atom. This corresponds very well with experimental data, the thermal energy can be used to calculate the root-mean-square speed of the atoms, which turns out to be inversely proportional to the square root of the atomic mass. The root mean square speeds found at room temperature accurately reflect this, ranging from 7003137000000000000♠1370 m/s for helium, kinetic theory gives the average pressure p for an ideal gas as p =13 N V m v 2 ¯. Combination with the gas law p V = N k T shows that the average translational kinetic energy is 12 m v 2 ¯ =32 k T. Considering that the translational motion velocity vector v has three degrees of freedom gives the energy per degree of freedom equal to one third of that. Diatomic gases, for example, possess a total of six degrees of freedom per molecule that are related to atomic motion. Again, it is the energy-like quantity kT that takes central importance, consequences of this include the Arrhenius equation in chemical kinetics. This equation, which relates the details, or microstates. Such is its importance that it is inscribed on Boltzmanns tombstone, the constant of proportionality k serves to make the statistical mechanical entropy equal to the classical thermodynamic entropy of Clausius, Δ S = ∫ d Q T. One could choose instead a rescaled dimensionless entropy in terms such that S ′ = ln W, Δ S ′ = ∫ d Q k T. This is a natural form and this rescaled entropy exactly corresponds to Shannons subsequent information entropy. The characteristic energy kT is thus the required to increase the rescaled entropy by one nat. The iconic terse form of the equation S = k ln W on Boltzmanns tombstone is in due to Planck
