Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measured with a thermometer . It reflects the average kinetic energy of the vibrating and colliding atoms making up a substance.
147-561: Thermometers are calibrated in various temperature scales that historically have relied on various reference points and thermometric substances for definition. The most common scales are the Celsius scale with the unit symbol °C (formerly called centigrade ), the Fahrenheit scale (°F), and the Kelvin scale (K), with the third being used predominantly for scientific purposes. The kelvin
294-431: A quotient set , denoted as M . If the set M has the cardinality of c , then one can construct an injective function f : M → R , by which every thermal system has a parameter associated with it such that when two thermal systems have the same value of that parameter, they are in thermal equilibrium. This parameter is the property of temperature. The specific way of assigning numerical values for temperature
441-410: A cycle of states of its working body. The engine takes in a quantity of heat Q 1 from a hot reservoir and passes out a lesser quantity of waste heat Q 2 < 0 to a cold reservoir. The net heat energy absorbed by the working body is passed, as thermodynamic work, to a work reservoir, and is considered to be the output of the engine. The cycle is imagined to run so slowly that at each point of
588-697: A freely moving particle has an average kinetic energy of k B T /2 where k B denotes the Boltzmann constant . The translational motion of the particle has three degrees of freedom, so that, except at very low temperatures where quantum effects predominate, the average translational kinetic energy of a freely moving particle in a system with temperature T will be 3 k B T /2 . Molecules, such as oxygen (O 2 ), have more degrees of freedom than single spherical atoms: they undergo rotational and vibrational motions as well as translations. Heating results in an increase of temperature due to an increase in
735-421: A fundamental, natural definition of thermodynamic temperature starting with a null point of absolute zero . A scale for thermodynamic temperature is established similarly to the empirical temperature scales, however, needing only one additional fixing point. Empirical scales are based on the measurement of physical parameters that express the property of interest to be measured through some formal, most commonly
882-502: A kilogram is a milligram , not a microkilogram . The BIPM specifies 24 prefixes for the International System of Units (SI): The base units and the derived units formed as the product of powers of the base units with a numerical factor of one form a coherent system of units . Every physical quantity has exactly one coherent SI unit. For example, 1 m/s = 1 m / (1 s) is the coherent derived unit for velocity. With
1029-404: A linear relation between their numerical scale readings, but it does require that the relation between their numerical readings shall be strictly monotonic . A definite sense of greater hotness can be had, independently of calorimetry , of thermodynamics, and of properties of particular materials, from Wien's displacement law of thermal radiation : the temperature of a bath of thermal radiation
1176-408: A list of non-SI units accepted for use with SI , including the hour, minute, degree of angle, litre, and decibel. Although the term metric system is often used as an informal alternative name for the International System of Units, other metric systems exist, some of which were in widespread use in the past or are even still used in particular areas. There are also individual metric units such as
1323-414: A loss of heat from a closed system, without phase change, without change of volume, and without a change in external force fields acting on it, decreases its temperature. While for bodies in their own thermodynamic equilibrium states, the notion of temperature requires that all empirical thermometers must agree as to which of two bodies is the hotter or that they are at the same temperature, this requirement
1470-455: A mercury thermometer have the same two fixed points, namely the freezing and boiling point of water, their readings will not agree with each other except at the fixed points, as the linear 1:1 relationship of expansion between any two thermometric substances may not be guaranteed. Empirical temperature scales are not reflective of the fundamental, microscopic laws of matter. Temperature is a universal attribute of matter, yet empirical scales map
1617-447: A microscopic account of temperature for some bodies of material, especially gases, based on macroscopic systems' being composed of many microscopic particles, such as molecules and ions of various species, the particles of a species being all alike. It explains macroscopic phenomena through the classical mechanics of the microscopic particles. The equipartition theorem of kinetic theory asserts that each classical degree of freedom of
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#17327807707361764-463: A narrow range onto a scale that is known to have a useful functional form for a particular application. Thus, their range is limited. The working material only exists in a form under certain circumstances, beyond which it no longer can serve as a scale. For example, mercury freezes below 234.32 K, so temperatures lower than that cannot be measured in a scale based on mercury. Even ITS-90 , which interpolates among different ranges of temperature, has
1911-599: A particular substance or device. Typically, this is established by fixing two well-defined temperature points and defining temperature increments via a linear function of the response of the thermometric device. For example, both the old Celsius scale and Fahrenheit scale were originally based on the linear expansion of a narrow mercury column within a limited range of temperature, each using different reference points and scale increments. Different empirical scales may not be compatible with each other, except for small regions of temperature overlap. If an alcohol thermometer and
2058-410: A positive or negative power. It can also be combined with other unit symbols to form compound unit symbols. For example, g/cm is an SI unit of density , where cm is to be interpreted as ( cm ) . Prefixes are added to unit names to produce multiples and submultiples of the original unit. All of these are integer powers of ten, and above a hundred or below a hundredth all are integer powers of
2205-438: A range of only 0.65 K to approximately 1358 K (−272.5 °C to 1085 °C). When pressure approaches zero, all real gas will behave like ideal gas, that is, pV of a mole of gas relying only on temperature. Therefore, we can design a scale with pV as its argument. Of course any bijective function will do, but for convenience's sake a linear function is the best. Therefore, we define it as The ideal gas scale
2352-479: A ratio of quantities of energy in processes in an ideal Carnot engine, entirely in terms of macroscopic thermodynamics. That Carnot engine was to work between two temperatures, that of the body whose temperature was to be measured, and a reference, that of a body at the temperature of the triple point of water. Then the reference temperature, that of the triple point, was defined to be exactly 273.16 K . Since May 2019, that value has not been fixed by definition but
2499-485: A reference temperature at the triple point of water, the numerical value of which is defined by measurements using the aforementioned internationally agreed Kelvin scale. Many scientific measurements use the Kelvin temperature scale (unit symbol: K), named in honor of the physicist who first defined it . It is an absolute scale. Its numerical zero point, 0 K , is at the absolute zero of temperature. Since May 2019,
2646-472: A reference temperature. It is known as the Kelvin scale , widely used in science and technology. The kelvin (the unit name is spelled with a lower-case 'k') is the unit of temperature in the International System of Units (SI). The temperature of a body in a state of thermodynamic equilibrium is always positive relative to absolute zero. Besides the internationally agreed Kelvin scale, there
2793-466: A simple linear, functional relationship. For the measurement of temperature, the formal definition of thermal equilibrium in terms of the thermodynamic coordinate spaces of thermodynamic systems, expressed in the zeroth law of thermodynamics , provides the framework to measure temperature. All temperature scales, including the modern thermodynamic temperature scale used in the International System of Units , are calibrated according to thermal properties of
2940-458: A spatially varying local property in that body, and this is because the temperature is an intensive variable. Temperature is a measure of a quality of a state of a material. The quality may be regarded as a more abstract entity than any particular temperature scale that measures it, and is called hotness by some writers. The quality of hotness refers to the state of material only in a particular locality, and in general, apart from bodies held in
3087-415: A specific intensive variable. An example is a diathermic wall that is permeable only to heat; the intensive variable for this case is temperature. When the two bodies have been connected through the specifically permeable wall for a very long time, and have settled to a permanent steady state, the relevant intensive variables are equal in the two bodies; for a diathermal wall, this statement is sometimes called
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#17327807707363234-587: A specification for units of measurement. The International Bureau of Weights and Measures (BIPM) has described SI as "the modern form of metric system". In 1971 the mole became the seventh base unit of the SI. After the metre was redefined in 1960, the International Prototype of the Kilogram (IPK) was the only physical artefact upon which base units (directly the kilogram and indirectly
3381-400: A steady state of thermodynamic equilibrium, hotness varies from place to place. It is not necessarily the case that a material in a particular place is in a state that is steady and nearly homogeneous enough to allow it to have a well-defined hotness or temperature. Hotness may be represented abstractly as a one-dimensional manifold . Every valid temperature scale has its own one-to-one map into
3528-423: A suitable range of processes. This is a matter for study in non-equilibrium thermodynamics . Temperature scale Scale of temperature is a methodology of calibrating the physical quantity temperature in metrology . Empirical scales measure temperature in relation to convenient and stable parameters or reference points , such as the freezing and boiling point of water . Absolute temperature
3675-435: A system undergoing a first-order phase change such as the melting of ice, as a closed system receives heat, without a change in its volume and without a change in external force fields acting on it, its temperature rises. For a system undergoing such a phase change so slowly that departure from thermodynamic equilibrium can be neglected, its temperature remains constant as the system is supplied with latent heat . Conversely,
3822-411: A thousand. For example, kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth, so there are one thousand millimetres to the metre and one thousand metres to the kilometre. The prefixes are never combined, so for example a millionth of a metre is a micrometre , not a millimillimetre . Multiples of the kilogram are named as if the gram were the base unit, so a millionth of
3969-613: A version of the CGPM document (NIST SP 330) which clarifies usage for English-language publications that use American English . The concept of a system of units emerged a hundred years before the SI. In the 1860s, James Clerk Maxwell , William Thomson (later Lord Kelvin), and others working under the auspices of the British Association for the Advancement of Science , building on previous work of Carl Gauss , developed
4116-421: A wide range. For example, driving distances are normally given in kilometres (symbol km ) rather than in metres. Here the metric prefix ' kilo- ' (symbol 'k') stands for a factor of 1000; thus, 1 km = 1000 m . The SI provides twenty-four metric prefixes that signify decimal powers ranging from 10 to 10 , the most recent being adopted in 2022. Most prefixes correspond to integer powers of 1000;
4263-435: Is and the reference temperature T 1 has the value 273.16. (Of course any reference temperature and any positive numerical value could be used—the choice here corresponds to the Kelvin scale.) It follows immediately that Substituting Equation 3 back into Equation 1 gives a relationship for the efficiency in terms of temperature: This is identical to the efficiency formula for Carnot cycle , which effectively employs
4410-506: Is proportional , by a universal constant, to the frequency of the maximum of its frequency spectrum ; this frequency is always positive, but can have values that tend to zero . Thermal radiation is initially defined for a cavity in thermodynamic equilibrium. These physical facts justify a mathematical statement that hotness exists on an ordered one-dimensional manifold . This is a fundamental character of temperature and thermometers for bodies in their own thermodynamic equilibrium. Except for
4557-545: Is a decimal and metric system of units established in 1960 and periodically updated since then. The SI has an official status in most countries, including the United States , Canada , and the United Kingdom , although these three countries are among the handful of nations that, to various degrees, also continue to use their customary systems. Nevertheless, with this nearly universal level of acceptance,
Temperature - Misplaced Pages Continue
4704-511: Is a temperature scale that is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale two years before his death. The degree Celsius (°C) can refer to a specific temperature on the Celsius scale as well as a unit to indicate a temperature interval (a difference between two temperatures). From 1744 until 1954, 0 °C was defined as
4851-437: Is actually 373.1339 K (99.9839 °C) when adhering strictly to the two-point definition of thermodynamic temperature. When calibrated to ITS–90, where one must interpolate between the defining points of gallium and indium, the boiling point of VSMOW water is about 10 mK less, about 99.974 °C. The virtue of ITS–90 is that another lab in another part of the world will measure the very same temperature with ease due to
4998-603: Is also a thermodynamic temperature scale , invented by Lord Kelvin , also with its numerical zero at the absolute zero of temperature, but directly relating to purely macroscopic thermodynamic concepts, including the macroscopic entropy , though microscopically referable to the Gibbs statistical mechanical definition of entropy for the canonical ensemble , that takes interparticle potential energy into account, as well as independent particle motion so that it can account for measurements of temperatures near absolute zero. This scale has
5145-409: Is an intensive variable because it is equal to a differential coefficient of one extensive variable with respect to another, for a given body. It thus has the dimensions of a ratio of two extensive variables. In thermodynamics, two bodies are often considered as connected by contact with a common wall, which has some specific permeability properties. Such specific permeability can be referred to
5292-518: Is arbitrary, and an alternate, less widely used absolute temperature scale exists called the Rankine scale , made to be aligned with the Fahrenheit scale as Kelvin is with Celsius. The thermodynamic definition of temperature is due to Kelvin. It is framed in terms of an idealized device called a Carnot engine , imagined to run in a fictive continuous cycle of successive processes that traverse
5439-542: Is based on thermodynamic principles: using the lowest possible temperature as the zero point, and selecting a convenient incremental unit. Celsius , Kelvin , and Fahrenheit are common temperature scales . Other scales used throughout history include Rankine , Rømer , Newton , Delisle , Réaumur , Gas mark , Leiden and Wedgwood . The zeroth law of thermodynamics describes thermal equilibrium between thermodynamic systems in form of an equivalence relation . Accordingly, all thermal systems may be divided into
5586-410: Is compensated for (an effect that typically amounts to no more than half a millikelvin across the different altitudes and barometric pressures likely to be encountered). The standard even compensates for the pressure effect due to how deeply the temperature probe is immersed into the sample. ITS–90 also draws a distinction between "freezing" and "melting" points. The distinction depends on whether heat
5733-425: Is designed to represent the thermodynamic temperature scale (referencing absolute zero ) as closely as possible throughout its range. Many different thermometer designs are required to cover the entire range. These include helium vapor pressure thermometers, helium gas thermometers, standard platinum resistance thermometers (known as SPRTs, PRTs or Platinum RTDs) and monochromatic radiation thermometers . Although
5880-547: Is directly proportional to the temperature of the black body; this is known as Wien's displacement law and has a theoretical explanation in Planck's law and the Bose–Einstein law . Measurement of the spectrum of noise-power produced by an electrical resistor can also provide accurate temperature measurement. The resistor has two terminals and is in effect a one-dimensional body. The Bose-Einstein law for this case indicates that
6027-491: Is disregarded. In an ideal gas , and in other theoretically understood bodies, the Kelvin temperature is defined to be proportional to the average kinetic energy of non-interactively moving microscopic particles, which can be measured by suitable techniques. The proportionality constant is a simple multiple of the Boltzmann constant. If molecules, atoms, or electrons are emitted from material and their velocities are measured,
Temperature - Misplaced Pages Continue
6174-448: Is establishing a scale of temperature . In practical terms, a temperature scale is always based on usually a single physical property of a simple thermodynamic system, called a thermometer , that defines a scaling function for mapping the temperature to the measurable thermometric parameter. Such temperature scales that are purely based on measurement are called empirical temperature scales . The second law of thermodynamics provides
6321-429: Is going into (melting) or out of (freezing) the sample when the measurement is made. Only gallium is measured while melting, all the other metals are measured while the samples are freezing. There are often small differences between measurements calibrated per ITS–90 and thermodynamic temperature. For instance, precise measurements show that the boiling point of VSMOW water under one standard atmosphere of pressure
6468-434: Is important in all fields of natural science , including physics , chemistry , Earth science , astronomy , medicine , biology , ecology , material science , metallurgy , mechanical engineering and geography as well as most aspects of daily life. Many physical processes are related to temperature; some of them are given below: Temperature scales need two values for definition: the point chosen as zero degrees and
6615-464: Is important not to use the unit alone to specify the quantity. As the SI Brochure states, "this applies not only to technical texts, but also, for example, to measuring instruments (i.e. the instrument read-out needs to indicate both the unit and the quantity measured)". Furthermore, the same coherent SI unit may be a base unit in one context, but a coherent derived unit in another. For example,
6762-452: Is in some sense a "mixed" scale. It relies on the universal properties of gas, a big advance from just a particular substance. But still it is empirical since it puts gas at a special position and thus has limited applicability—at some point no gas can exist. One distinguishing characteristic of ideal gas scale, however, is that it precisely equals thermodynamical scale when it is well defined (see § Equality to ideal gas scale ). ITS-90
6909-556: Is not coherent. The principle of coherence was successfully used to define a number of units of measure based on the CGS, including the erg for energy , the dyne for force , the barye for pressure , the poise for dynamic viscosity and the stokes for kinematic viscosity . A French-inspired initiative for international cooperation in metrology led to the signing in 1875 of the Metre Convention , also called Treaty of
7056-410: Is not fundamental or even unique – it is a matter of convention. The system allows for an unlimited number of additional units, called derived units , which can always be represented as products of powers of the base units, possibly with a nontrivial numeric multiplier. When that multiplier is one, the unit is called a coherent derived unit. For example, the coherent derived SI unit of velocity
7203-455: Is not safe for bodies that are in steady states though not in thermodynamic equilibrium. It can then well be that different empirical thermometers disagree about which is hotter, and if this is so, then at least one of the bodies does not have a well-defined absolute thermodynamic temperature. Nevertheless, any one given body and any one suitable empirical thermometer can still support notions of empirical, non-absolute, hotness, and temperature, for
7350-530: Is not the only way in which a base unit can be determined: the SI Brochure states that "any method consistent with the laws of physics could be used to realise any SI unit". Various consultative committees of the CIPM decided in 2016 that more than one mise en pratique would be developed for determining the value of each unit. These methods include the following: The International System of Units, or SI,
7497-418: Is one of the seven base units in the International System of Units (SI). Absolute zero , i.e., zero kelvin or −273.15 °C, is the lowest point in the thermodynamic temperature scale. Experimentally, it can be approached very closely but not actually reached, as recognized in the third law of thermodynamics . It would be impossible to extract energy as heat from a body at that temperature. Temperature
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#17327807707367644-551: Is only one degree of freedom left to arbitrary choice, rather than two as in relative scales. For the Kelvin scale since May 2019, by international convention, the choice has been made to use knowledge of modes of operation of various thermometric devices, relying on microscopic kinetic theories about molecular motion. The numerical scale is settled by a conventional definition of the value of the Boltzmann constant , which relates macroscopic temperature to average microscopic kinetic energy of particles such as molecules. Its numerical value
7791-468: Is otherwise identical to the SI Brochure. For example, since 1979, the litre may exceptionally be written using either an uppercase "L" or a lowercase "l", a decision prompted by the similarity of the lowercase letter "l" to the numeral "1", especially with certain typefaces or English-style handwriting. The American NIST recommends that within the United States "L" be used rather than "l". Metrologists carefully distinguish between
7938-414: Is said to prevail throughout the body. It makes good sense, for example, to say of the extensive variable U , or of the extensive variable S , that it has a density per unit volume or a quantity per unit mass of the system, but it makes no sense to speak of the density of temperature per unit volume or quantity of temperature per unit mass of the system. On the other hand, it makes no sense to speak of
8085-427: Is the metre per second , with the symbol m/s . The base and coherent derived units of the SI together form a coherent system of units ( the set of coherent SI units ). A useful property of a coherent system is that when the numerical values of physical quantities are expressed in terms of the units of the system, then the equations between the numerical values have exactly the same form, including numerical factors, as
8232-424: Is the inverse of electrical resistance , with the consequence that the siemens is the inverse of the ohm, and similarly, the ohm and siemens can be replaced with a ratio of an ampere and a volt, because those quantities bear a defined relationship to each other. Other useful derived quantities can be specified in terms of the SI base and derived units that have no named units in the SI, such as acceleration, which has
8379-526: Is to be measured through microscopic phenomena, involving the Boltzmann constant, as described above. The microscopic statistical mechanical definition does not have a reference temperature. A material on which a macroscopically defined temperature scale may be based is the ideal gas . The pressure exerted by a fixed volume and mass of an ideal gas is directly proportional to its temperature. Some natural gases show so nearly ideal properties over suitable temperature range that they can be used for thermometry; this
8526-571: The Boltzmann constant , to the Maxwell–Boltzmann distribution , and to the Boltzmann statistical mechanical definition of entropy , as distinct from the Gibbs definition, for independently moving microscopic particles, disregarding interparticle potential energy, by international agreement, a temperature scale is defined and said to be absolute because it is independent of the characteristics of particular thermometric substances and thermometer mechanisms. Apart from absolute zero, it does not have
8673-524: The Boltzmann constant . That constant refers to chosen kinds of motion of microscopic particles in the constitution of the body. In those kinds of motion, the particles move individually, without mutual interaction. Such motions are typically interrupted by inter-particle collisions, but for temperature measurement, the motions are chosen so that, between collisions, the non-interactive segments of their trajectories are known to be accessible to accurate measurement. For this purpose, interparticle potential energy
8820-534: The ISO/IEC 80000 series of standards, which define the International System of Quantities (ISQ), specifies base and derived quantities that necessarily have the corresponding SI units. Many non-SI units continue to be used in the scientific, technical, and commercial literature. Some units are deeply embedded in history and culture, and their use has not been entirely replaced by their SI alternatives. The CIPM recognised and acknowledged such traditions by compiling
8967-472: The International Bureau of Weights and Measures (abbreviated BIPM from French : Bureau international des poids et mesures ) it is the only system of measurement with official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce. The SI comprises a coherent system of units of measurement starting with seven base units , which are
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#17327807707369114-399: The centimetre–gram–second system of units or cgs system in 1874. The systems formalised the concept of a collection of related units called a coherent system of units. In a coherent system, base units combine to define derived units without extra factors. For example, using meters per second is coherent in a system that uses meter for length and seconds for time, but kilometre per hour
9261-440: The second (symbol s, the unit of time ), metre (m, length ), kilogram (kg, mass ), ampere (A, electric current ), kelvin (K, thermodynamic temperature ), mole (mol, amount of substance ), and candela (cd, luminous intensity ). The system can accommodate coherent units for an unlimited number of additional quantities. These are called coherent derived units , which can always be represented as products of powers of
9408-564: The speed of light in vacuum c , the hyperfine transition frequency of caesium Δ ν Cs , the Planck constant h , the elementary charge e , the Boltzmann constant k , the Avogadro constant N A , and the luminous efficacy K cd . The nature of the defining constants ranges from fundamental constants of nature such as c to the purely technical constant K cd . The values assigned to these constants were fixed to ensure continuity with previous definitions of
9555-419: The sverdrup and the darcy that exist outside of any system of units. Most of the units of the other metric systems are not recognised by the SI. Sometimes, SI unit name variations are introduced, mixing information about the corresponding physical quantity or the conditions of its measurement; however, this practice is unacceptable with the SI. "Unacceptability of mixing information with units: When one gives
9702-464: The zeroth law of thermodynamics says that they all measure the same quality. This means that for a body in its own state of internal thermodynamic equilibrium, every correctly calibrated thermometer, of whatever kind, that measures the temperature of the body, records one and the same temperature. For a body that is not in its own state of internal thermodynamic equilibrium, different thermometers can record different temperatures, depending respectively on
9849-410: The 100-degree interval. Since the standardization of the kelvin in the International System of Units, it has subsequently been redefined in terms of the equivalent fixing points on the Kelvin scale, so that a temperature increment of one degree Celsius is the same as an increment of one kelvin, though numerically the scales differ by an exact offset of 273.15. The Fahrenheit scale is in common use in
9996-500: The BIPM publishes a mises en pratique , ( French for 'putting into practice; implementation', ) describing the current best practical realisations of the unit. The separation of the defining constants from the definitions of units means that improved measurements can be developed leading to changes in the mises en pratique as science and technology develop, without having to revise the definitions. The published mise en pratique
10143-414: The Boltzmann constant as a primarily defined reference of exactly defined value, a measurement of the speed of sound can provide a more precise measurement of the temperature of the gas. Measurement of the spectrum of electromagnetic radiation from an ideal three-dimensional black body can provide an accurate temperature measurement because the frequency of maximum spectral radiance of black-body radiation
10290-477: The Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature with symbol K. Absolute zero, the lowest temperature possible, is defined as being exactly 0 K and −273.15 °C. Until 19 May 2019, the temperature of the triple point of water was defined as exactly 273.16 K (0.01 °C). This means that a temperature difference of one degree Celsius and that of one kelvin are exactly
10437-478: The IPK. During extraordinary verifications carried out in 2014 preparatory to redefinition of metric standards, continuing divergence was not confirmed. Nonetheless, the residual and irreducible instability of a physical IPK undermined the reliability of the entire metric system to precision measurement from small (atomic) to large (astrophysical) scales. By avoiding the use of an artefact to define units, all issues with
10584-535: The International Committee for Weights and Measures (CIPM ), and the International Bureau of Weights and Measures (BIPM ). All the decisions and recommendations concerning units are collected in a brochure called The International System of Units (SI) , which is published in French and English by the BIPM and periodically updated. The writing and maintenance of the brochure is carried out by one of
10731-759: The International System of Units defined a scale and unit for the kelvin as a thermodynamic temperature , by using the reliably reproducible temperature of the triple point of water as a second reference point, the first reference point being 0 K at absolute zero. Historically, the temperature of the triple point of water was defined as exactly 273.16 K. Today it is an empirically measured quantity. The freezing point of water at sea-level atmospheric pressure occurs at very close to 273.15 K ( 0 °C ). There are various kinds of temperature scale. It may be convenient to classify them as empirically and theoretically based. Empirical temperature scales are historically older, while theoretically based scales arose in
10878-442: The Kelvin and Celsius scales are defined using absolute zero (0 K) and the triple point of water (273.16 K and 0.01 °C), it is impractical to use this definition at temperatures that are very different from the triple point of water. Accordingly, ITS–90 uses numerous defined points, all of which are based on various thermodynamic equilibrium states of fourteen pure chemical elements and one compound (water). Most of
11025-483: The Metre Convention". This working document was Practical system of units of measurement . Based on this study, the 10th CGPM in 1954 defined an international system derived six base units: the metre, kilogram, second, ampere, degree Kelvin, and candela. The 9th CGPM also approved the first formal recommendation for the writing of symbols in the metric system when the basis of the rules as they are now known
11172-475: The Metre, by 17 nations. The General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), which was established by the Metre Convention, brought together many international organisations to establish the definitions and standards of a new system and to standardise the rules for writing and presenting measurements. Initially the convention only covered standards for
11319-402: The SI "has been used around the world as the preferred system of units, the basic language for science, technology, industry, and trade." The only other types of measurement system that still have widespread use across the world are the imperial and US customary measurement systems . The international yard and pound are defined in terms of the SI. The quantities and equations that provide
11466-509: The SI Brochure notes that the name of the unit with the symbol °C is correctly spelled as 'degree Celsius ': the first letter of the name of the unit, 'd', is in lowercase, while the modifier 'Celsius' is capitalised because it is a proper name. The English spelling and even names for certain SI units and metric prefixes depend on the variety of English used. US English uses the spelling deka- , meter , and liter , and International English uses deca- , metre , and litre . The name of
11613-455: The SI unit m/s . A combination of base and derived units may be used to express a derived unit. For example, the SI unit of force is the newton (N), the SI unit of pressure is the pascal (Pa) – and the pascal can be defined as one newton per square metre (N/m ). Like all metric systems, the SI uses metric prefixes to systematically construct, for the same physical quantity, a set of units that are decimal multiples of each other over
11760-498: The SI units. The ISQ is formalised, in part, in the international standard ISO/IEC 80000 , which was completed in 2009 with the publication of ISO 80000-1 , and has largely been revised in 2019–2020. The SI is regulated and continually developed by three international organisations that were established in 1875 under the terms of the Metre Convention . They are the General Conference on Weights and Measures (CGPM ),
11907-452: The United States. Water freezes at 32 °F and boils at 212 °F at sea-level atmospheric pressure. At the absolute zero of temperature, no energy can be removed from matter as heat, a fact expressed in the third law of thermodynamics . At this temperature, matter contains no macroscopic thermal energy, but still has quantum-mechanical zero-point energy as predicted by the uncertainty principle , although this does not enter into
12054-480: The advantages of a comprehensive international calibration standard featuring many conveniently spaced, reproducible, defining points spanning a wide range of temperatures. OV is a specialized scale used in Japan to measure female basal body temperature for fertility awareness . The range of 35.5 °C (OV 0) to 38.0 °C (OV 50) is divided into 50 equal parts. Celsius (known until 1948 as centigrade)
12201-521: The ampere is a base unit when it is a unit of electric current, but a coherent derived unit when it is a unit of magnetomotive force. According to the SI Brochure, unit names should be treated as common nouns of the context language. This means that they should be typeset in the same character set as other common nouns (e.g. Latin alphabet in English, Cyrillic script in Russian, etc.), following
12348-428: The ampere, mole and candela) depended for their definition, making these units subject to periodic comparisons of national standard kilograms with the IPK. During the 2nd and 3rd Periodic Verification of National Prototypes of the Kilogram, a significant divergence had occurred between the mass of the IPK and all of its official copies stored around the world: the copies had all noticeably increased in mass with respect to
12495-409: The average kinetic energy of particles (see equipartition theorem ). In experiments ITS-90 is used to approximate thermodynamic scale due to simpler realization. Lord Kelvin devised the thermodynamic scale based on the efficiency of heat engines as shown below: The efficiency of an engine is the work divided by the heat introduced to the system or where w cy is the work done per cycle. Thus,
12642-421: The average translational kinetic energy of the molecules. Heating will also cause, through equipartitioning , the energy associated with vibrational and rotational modes to increase. Thus a diatomic gas will require more energy input to increase its temperature by a certain amount, i.e. it will have a greater heat capacity than a monatomic gas. As noted above, the speed of sound in a gas can be calculated from
12789-515: The base units. The SI selects seven units to serve as base units , corresponding to seven base physical quantities. They are the second , with the symbol s , which is the SI unit of the physical quantity of time ; the metre , symbol m , the SI unit of length ; kilogram ( kg , the unit of mass ); ampere ( A , electric current ); kelvin ( K , thermodynamic temperature ); mole ( mol , amount of substance ); and candela ( cd , luminous intensity ). The base units are defined in terms of
12936-445: The base units. After the redefinition, the SI is defined by fixing the numerical values of seven defining constants. This has the effect that the distinction between the base units and derived units is, in principle, not needed, since all units, base as well as derived, may be constructed directly from the defining constants. Nevertheless, the distinction is retained because "it is useful and historically well established", and also because
13083-416: The base units. Twenty-two coherent derived units have been provided with special names and symbols. The seven base units and the 22 coherent derived units with special names and symbols may be used in combination to express other coherent derived units. Since the sizes of coherent units will be convenient for only some applications and not for others, the SI provides twenty-four prefixes which, when added to
13230-408: The body is described by stating its entropy S as a function of its internal energy U , and other state variables V , N , with S = S ( U , V , N ) , then the reciprocal of the temperature is equal to the partial derivative of the entropy with respect to the internal energy: The above definition, equation (1), of the absolute temperature, is due to Kelvin. It refers to systems closed to
13377-483: The boiling point of mercury , a mercury-in-glass thermometer is impracticable. Most materials expand with temperature increase, but some materials, such as water, contract with temperature increase over some specific range, and then they are hardly useful as thermometric materials. A material is of no use as a thermometer near one of its phase-change temperatures, for example, its boiling-point. In spite of these limitations, most generally used practical thermometers are of
13524-417: The bulk of the system, through a small hole in the containing wall. The spectrum of velocities has to be measured, and the average calculated from that. It is not necessarily the case that the particles that escape and are measured have the same velocity distribution as the particles that remain in the bulk of the system, but sometimes a good sample is possible. Temperature is one of the principal quantities in
13671-401: The coherent set and the multiples and sub-multiples of coherent units formed by using the SI prefixes. The kilogram is the only coherent SI unit whose name and symbol include a prefix. For historical reasons, the names and symbols for multiples and sub-multiples of the unit of mass are formed as if the gram were the base unit. Prefix names and symbols are attached to the unit name gram and
13818-467: The committees of the CIPM. The definitions of the terms "quantity", "unit", "dimension", etc. that are used in the SI Brochure are those given in the international vocabulary of metrology . The brochure leaves some scope for local variations, particularly regarding unit names and terms in different languages. For example, the United States' National Institute of Standards and Technology (NIST) has produced
13965-501: The constituent particles of matter, so that they have a limiting specific heat of zero for zero temperature, according to the third law of thermodynamics. Nevertheless, a thermodynamic temperature does in fact have a definite numerical value that has been arbitrarily chosen by tradition and is dependent on the property of particular materials; it is simply less arbitrary than relative "degrees" scales such as Celsius and Fahrenheit . Being an absolute scale with one fixed point (zero), there
14112-485: The context in which the SI units are defined are now referred to as the International System of Quantities (ISQ). The ISQ is based on the quantities underlying each of the seven base units of the SI . Other quantities, such as area , pressure , and electrical resistance , are derived from these base quantities by clear, non-contradictory equations. The ISQ defines the quantities that are measured with
14259-595: The corresponding equations between the physical quantities. Twenty-two coherent derived units have been provided with special names and symbols as shown in the table below. The radian and steradian have no base units but are treated as derived units for historical reasons. The derived units in the SI are formed by powers, products, or quotients of the base units and are unlimited in number. Derived units apply to some derived quantities , which may by definition be expressed in terms of base quantities , and thus are not independent; for example, electrical conductance
14406-426: The cycle the working body is in a state of thermodynamic equilibrium. The successive processes of the cycle are thus imagined to run reversibly with no entropy production . Then the quantity of entropy taken in from the hot reservoir when the working body is heated is equal to that passed to the cold reservoir when the working body is cooled. Then the absolute or thermodynamic temperatures, T 1 and T 2 , of
14553-409: The defined points are based on a phase transition ; specifically the melting / freezing point of a pure chemical element. However, the deepest cryogenic points are based exclusively on the vapor pressure /temperature relationship of helium and its isotopes whereas the remainder of its cold points (those less than room temperature) are based on triple points . Examples of other defining points are
14700-501: The defining constants. For example, the kilogram is defined by taking the Planck constant h to be 6.626 070 15 × 10 J⋅s , giving the expression in terms of the defining constants All units in the SI can be expressed in terms of the base units, and the base units serve as a preferred set for expressing or analysing the relationships between units. The choice of which and even how many quantities to use as base quantities
14847-437: The definition just stated, was printed in 1853, a paper read in 1851. Numerical details were formerly settled by making one of the heat reservoirs a cell at the triple point of water, which was defined to have an absolute temperature of 273.16 K. Nowadays, the numerical value is instead obtained from measurement through the microscopic statistical mechanical international definition, as above. In thermodynamic terms, temperature
14994-410: The definition of a unit and its realisation. The SI units are defined by declaring that seven defining constants have certain exact numerical values when expressed in terms of their SI units. The realisation of the definition of a unit is the procedure by which the definition may be used to establish the value and associated uncertainty of a quantity of the same kind as the unit. For each base unit
15141-471: The definition of absolute temperature. Experimentally, absolute zero can be approached only very closely; it can never be reached (the lowest temperature attained by experiment is 38 pK). Theoretically, in a body at a temperature of absolute zero, all classical motion of its particles has ceased and they are at complete rest in this classical sense. Absolute zero, defined as 0 K , is exactly equal to −273.15 °C , or −459.67 °F . Referring to
15288-403: The definitions. A consequence is that as science and technologies develop, new and superior realisations may be introduced without the need to redefine the unit. One problem with artefacts is that they can be lost, damaged, or changed; another is that they introduce uncertainties that cannot be reduced by advancements in science and technology. The original motivation for the development of the SI
15435-517: The development of the CGS system. The International System of Units consists of a set of defining constants with corresponding base units, derived units, and a set of decimal-based multipliers that are used as prefixes. The seven defining constants are the most fundamental feature of the definition of the system of units. The magnitudes of all SI units are defined by declaring that seven constants have certain exact numerical values when expressed in terms of their SI units. These defining constants are
15582-487: The efficiency depends only on q C / q H . Because of Carnot theorem , any reversible heat engine operating between temperatures T 1 and T 2 must have the same efficiency, meaning, the efficiency is the function of the temperatures only: In addition, a reversible heat engine operating between temperatures T 1 and T 3 must have the same efficiency as one consisting of two cycles, one between T 1 and another (intermediate) temperature T 2 , and
15729-537: The electrical units in terms of length, mass, and time using dimensional analysis was beset with difficulties – the dimensions depended on whether one used the ESU or EMU systems. This anomaly was resolved in 1901 when Giovanni Giorgi published a paper in which he advocated using a fourth base unit alongside the existing three base units. The fourth unit could be chosen to be electric current , voltage , or electrical resistance . Electric current with named unit 'ampere'
15876-863: 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. Empirically based thermometers, beyond their base as simple direct measurements of ordinary physical properties of thermometric materials, can be re-calibrated, by use of theoretical physical reasoning, and this can extend their range of adequacy. Theoretically based temperature scales are based directly on theoretical arguments, especially those of kinetic theory and thermodynamics. They are more or less ideally realized in practically feasible physical devices and materials. Theoretically based temperature scales are used to provide calibrating standards for practical empirically based thermometers. In physics,
16023-439: The exception of the kilogram (for which the prefix kilo- is required for a coherent unit), when prefixes are used with the coherent SI units, the resulting units are no longer coherent, because the prefix introduces a numerical factor other than one. For example, the metre, kilometre, centimetre, nanometre, etc. are all SI units of length, though only the metre is a coherent SI unit. The complete set of SI units consists of both
16170-455: The formulation of the first law of thermodynamics. Carnot had no sound understanding of heat and no specific concept of entropy. He wrote of 'caloric' and said that all the caloric that passed from the hot reservoir was passed into the cold reservoir. Kelvin wrote in his 1848 paper that his scale was absolute in the sense that it was defined "independently of the properties of any particular kind of matter". His definitive publication, which sets out
16317-444: The freezing point of water and 100 °C was defined as the boiling point of water, both at a pressure of one standard atmosphere . Although these defining correlations are commonly taught in schools today, by international agreement, between 1954 and 2019 the unit degree Celsius and the Celsius scale were defined by absolute zero and the triple point of VSMOW (specially prepared water). This definition also precisely related
16464-401: The gas's molecular character, temperature, pressure, and the Boltzmann constant. For a gas of known molecular character and pressure, this provides a relation between temperature and the Boltzmann constant. Those quantities can be known or measured more precisely than can the thermodynamic variables that define the state of a sample of water at its triple point. Consequently, taking the value of
16611-418: The gas's molecular character, temperature, pressure, and the Boltzmann constant. Taking the value of the Boltzmann constant as a primarily defined reference of exactly defined value, a measurement of the speed of sound can provide a more precise measurement of the temperature of the gas. It is possible to measure the average kinetic energy of constituent microscopic particles if they are allowed to escape from
16758-483: The hotness manifold. When two systems in thermal contact are at the same temperature no heat transfers between them. When a temperature difference does exist heat flows spontaneously from the warmer system to the colder system until they are in thermal equilibrium . Such heat transfer occurs by conduction or by thermal radiation. Experimental physicists, for example Galileo and Newton , found that there are indefinitely many empirical temperature scales . Nevertheless,
16905-416: The ideal gas does not liquefy or solidify, no matter how cold it is. Alternatively thinking, the ideal gas law, refers to the limit of infinitely high temperature and zero pressure; these conditions guarantee non-interactive motions of the constituent molecules. The magnitude of the kelvin is now defined in terms of kinetic theory, derived from the value of the Boltzmann constant . Kinetic theory provides
17052-412: The ideal gas scale. This means that the two scales equal numerically at every point. International System of Units The International System of Units , internationally known by the abbreviation SI (from French Système international d'unités ), is the modern form of the metric system and the world's most widely used system of measurement . Coordinated by
17199-419: The internal energy at a point, while when local thermodynamic equilibrium prevails, it makes good sense to speak of the temperature at a point. Consequently, the temperature can vary from point to point in a medium that is not in global thermodynamic equilibrium, but in which there is local thermodynamic equilibrium. Thus, when local thermodynamic equilibrium prevails in a body, the temperature can be regarded as
17346-407: The internationally agreed conventional temperature scale is called the Kelvin scale. It is calibrated through the internationally agreed and prescribed value of the Boltzmann constant, referring to motions of microscopic particles, such as atoms, molecules, and electrons, constituent in the body whose temperature is to be measured. In contrast with the thermodynamic temperature scale invented by Kelvin,
17493-451: The kelvin has been defined through particle kinetic theory , and statistical mechanics. In the International System of Units (SI), the magnitude of the kelvin is defined in terms of the Boltzmann constant , the value of which is defined as fixed by international convention. Since May 2019, the magnitude of the kelvin is defined in relation to microscopic phenomena, characterized in terms of statistical mechanics. Previously, but since 1954,
17640-499: The loss, damage, and change of the artefact are avoided. A proposal was made that: The new definitions were adopted at the 26th CGPM on 16 November 2018, and came into effect on 20 May 2019. The change was adopted by the European Union through Directive (EU) 2019/1258. Prior to its redefinition in 2019, the SI was defined through the seven base units from which the derived units were constructed as products of powers of
17787-427: The magnitudes of the incremental unit of temperature. The Celsius scale (°C) is used for common temperature measurements in most of the world. It is an empirical scale that developed historically, which led to its zero point 0 °C being defined as the freezing point of water , and 100 °C as the boiling point of water, both at atmospheric pressure at sea level. It was called a centigrade scale because of
17934-418: The mechanisms of operation of the thermometers. For experimental physics, hotness means that, when comparing any two given bodies in their respective separate thermodynamic equilibria , any two suitably given empirical thermometers with numerical scale readings will agree as to which is the hotter of the two given bodies, or that they have the same temperature. This does not require the two thermometers to have
18081-582: The metre and the kilogram. This became the foundation of the MKS system of units. At the close of the 19th century three different systems of units of measure existed for electrical measurements: a CGS-based system for electrostatic units , also known as the Gaussian or ESU system, a CGS-based system for electromechanical units (EMU), and an International system based on units defined by the Metre Convention for electrical distribution systems. Attempts to resolve
18228-442: The middle of the nineteenth century. Empirically based temperature scales rely directly on measurements of simple macroscopic physical properties of materials. For example, the length of a column of mercury, confined in a glass-walled capillary tube, is dependent largely on temperature and is the basis of the very useful mercury-in-glass thermometer. Such scales are valid only within convenient ranges of temperature. For example, above
18375-431: The name and symbol of a coherent unit produce twenty-four additional (non-coherent) SI units for the same quantity; these non-coherent units are always decimal (i.e. power-of-ten) multiples and sub-multiples of the coherent unit. The current way of defining the SI is a result of a decades-long move towards increasingly abstract and idealised formulation in which the realisations of the units are separated conceptually from
18522-436: The noise-power is directly proportional to the temperature of the resistor and to the value of its resistance and to the noise bandwidth. In a given frequency band, the noise-power has equal contributions from every frequency and is called Johnson noise . If the value of the resistance is known then the temperature can be found. Historically, till May 2019, the definition of the Kelvin scale was that invented by Kelvin, based on
18669-453: The only ones that do not are those for 10, 1/10, 100, and 1/100. The conversion between different SI units for one and the same physical quantity is always through a power of ten. This is why the SI (and metric systems more generally) are called decimal systems of measurement units . The grouping formed by a prefix symbol attached to a unit symbol (e.g. ' km ', ' cm ') constitutes a new inseparable unit symbol. This new symbol can be raised to
18816-454: The presently conventional Kelvin temperature is not defined through comparison with the temperature of a reference state of a standard body, nor in terms of macroscopic thermodynamics. Apart from the absolute zero of temperature, the Kelvin temperature of a body in a state of internal thermodynamic equilibrium is defined by measurements of suitably chosen of its physical properties, such as have precisely known theoretical explanations in terms of
18963-486: The quantity symbols, formatting of numbers and the decimal marker, expressing measurement uncertainty, multiplication and division of quantity symbols, and the use of pure numbers and various angles. In the United States, the guideline produced by the National Institute of Standards and Technology (NIST) clarifies language-specific details for American English that were left unclear by the SI Brochure, but
19110-402: The reservoirs are defined such that The zeroth law of thermodynamics allows this definition to be used to measure the absolute or thermodynamic temperature of an arbitrary body of interest, by making the other heat reservoir have the same temperature as the body of interest. Kelvin's original work postulating absolute temperature was published in 1848. It was based on the work of Carnot, before
19257-506: The same as the older defined value to within the limits of accuracy of contemporary metrology . The degree Celsius remains exactly equal to the kelvin, and 0 K remains exactly −273.15 °C. Thermodynamic scale differs from empirical scales in that it is absolute. It is based on the fundamental laws of thermodynamics or statistical mechanics instead of some arbitrary chosen working material. Besides it covers full range of temperature and has simple relation with microscopic quantities like
19404-456: The same. On 20 May 2019, the kelvin was redefined so that its value is now determined by the definition of the Boltzmann constant rather than being defined by the triple point of VSMOW. This means that the triple point is now a measured value, not a defined value. The newly-defined exact value of the Boltzmann constant was selected so that the measured value of the VSMOW triple point is exactly
19551-413: The second between T 2 and T 3 . This can only be the case if Specializing to the case that T 1 {\displaystyle T_{1}} is a fixed reference temperature: the temperature of the triple point of water. Then for any T 2 and T 3 , Therefore, if thermodynamic temperature is defined by then the function f , viewed as a function of thermodynamic temperature,
19698-494: The spectrum of their velocities often nearly obeys a theoretical law called the Maxwell–Boltzmann distribution , which gives a well-founded measurement of temperatures for which the law holds. There have not yet been successful experiments of this same kind that directly use the Fermi–Dirac distribution for thermometry, but perhaps that will be achieved in the future. The speed of sound in a gas can be calculated theoretically from
19845-407: The study by methods of classical irreversible thermodynamics, a body is usually spatially and temporally divided conceptually into 'cells' of small size. If classical thermodynamic equilibrium conditions for matter are fulfilled to good approximation in such a 'cell', then it is homogeneous and a temperature exists for it. If this is so for every 'cell' of the body, then local thermodynamic equilibrium
19992-469: The study of thermodynamics . Formerly, the magnitude of the kelvin was defined in thermodynamic terms, but nowadays, as mentioned above, it is defined in terms of kinetic theory. The thermodynamic temperature is said to be absolute for two reasons. One is that its formal character is independent of the properties of particular materials. The other reason is that its zero is, in a sense, absolute, in that it indicates absence of microscopic classical motion of
20139-427: The transfer of matter and has a special emphasis on directly experimental procedures. A presentation of thermodynamics by Gibbs starts at a more abstract level and deals with systems open to the transfer of matter; in this development of thermodynamics, the equations (2) and (3) above are actually alternative definitions of temperature. Real-world bodies are often not in thermodynamic equilibrium and not homogeneous. For
20286-403: The triple point of hydrogen (−259.3467 °C) and the freezing point of aluminum (660.323 °C). Thermometers calibrated per ITS–90 use complex mathematical formulas to interpolate between its defined points. ITS–90 specifies rigorous control over variables to ensure reproducibility from lab to lab. For instance, the small effect that atmospheric pressure has upon the various melting points
20433-465: The unit symbol g respectively. For example, 10 kg is written milligram and mg , not microkilogram and μkg . Several different quantities may share the same coherent SI unit. For example, the joule per kelvin (symbol J/K ) is the coherent SI unit for two distinct quantities: heat capacity and entropy ; another example is the ampere, which is the coherent SI unit for both electric current and magnetomotive force . This illustrates why it
20580-560: The unit whose symbol is t and which is defined according to 1 t = 10 kg is 'metric ton' in US English and 'tonne' in International English. Symbols of SI units are intended to be unique and universal, independent of the context language. The SI Brochure has specific rules for writing them. In addition, the SI Brochure provides style conventions for among other aspects of displaying quantities units:
20727-435: The usual grammatical and orthographical rules of the context language. For example, in English and French, even when the unit is named after a person and its symbol begins with a capital letter, the unit name in running text should start with a lowercase letter (e.g., newton, hertz, pascal) and is capitalised only at the beginning of a sentence and in headings and publication titles . As a nontrivial application of this rule,
20874-405: The zeroth law of thermodynamics. In particular, when the body is described by stating its internal energy U , an extensive variable, as a function of its entropy S , also an extensive variable, and other state variables V , N , with U = U ( S , V , N ), then the temperature is equal to the partial derivative of the internal energy with respect to the entropy: Likewise, when
21021-465: Was chosen as the base unit, and the other electrical quantities derived from it according to the laws of physics. When combined with the MKS the new system, known as MKSA, was approved in 1946. In 1948, the 9th CGPM commissioned a study to assess the measurement needs of the scientific, technical, and educational communities and "to make recommendations for a single practical system of units of measurement, suitable for adoption by all countries adhering to
21168-402: Was established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and to standardise the rules for writing and presenting measurements. The system was published in 1960 as a result of an initiative that began in 1948, and is based on the metre–kilogram–second system of units (MKS) combined with ideas from
21315-406: Was important during the development of thermodynamics and is still of practical importance today. The ideal gas thermometer is, however, not theoretically perfect for thermodynamics. This is because the entropy of an ideal gas at its absolute zero of temperature is not a positive semi-definite quantity, which puts the gas in violation of the third law of thermodynamics. In contrast to real materials,
21462-464: Was laid down. These rules were subsequently extended and now cover unit symbols and names, prefix symbols and names, how quantity symbols should be written and used, and how the values of quantities should be expressed. The 10th CGPM in 1954 resolved to create an international system of units and in 1960, the 11th CGPM adopted the International System of Units , abbreviated SI from the French name Le Système international d'unités , which included
21609-399: Was the diversity of units that had sprung up within the centimetre–gram–second (CGS) systems (specifically the inconsistency between the systems of electrostatic units and electromagnetic units ) and the lack of coordination between the various disciplines that used them. The General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), which
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