title: Turtle

text: How sea turtles navigate to their breeding beaches remains unknown. One possibility is imprinting as in salmon, where the young learn the chemical signature, effectively the scent, of their home waters before leaving, and remember that when the time comes for them to return as adults. Another possible cue is the orientation of the earth's magnetic field at the natal beach. There is experimental evidence that turtles have an effective magnetic sense, and that they use this in navigation. Proof that homing occurs is derived from genetic analysis of populations of loggerheads, hawksbills, leatherbacks, and olive ridleys by nesting place. For each of these species, the populations in different places have their own mitochondrial DNA genetic signatures that persist over the years. This shows that the populations are distinct and that homing must be occurring reliably.


title: Turtle

text: Turtles have a wide variety of mating behaviors but do not form pair-bonds or social groups. In green sea turtles, females generally outnumber males. In terrestrial species, males are often larger than females and fighting between males establishes a dominance hierarchy for access to mates. For most semi-aquatic and bottom-walking aquatic species, combat occurs less often. Males of these species instead may use their size advantage to mate forcibly. In fully aquatic species, males are often smaller than females and rely on courtship displays to gain mating access to females.


title: Turtle

text: Courtship varies between species, and with habitat. It is often complex in aquatic species, both marine and freshwater, but simpler in the semi-aquatic mud turtles and snapping turtles. A male tortoise bobs his head, then subdues the female by biting and butting her before mounting. The male scorpion mud turtle approaches the female from the rear, and often resorts to aggressive methods such as biting the female's tail or hind limbs, followed by a mounting.


title: Turtle

text: Female choice is important in some species, and female green sea turtles are not always receptive. As such, they have evolved behaviors to avoid the male's attempts at copulation, such as swimming away, confronting the male followed by biting or taking up a refusal position with her body vertical, her limbs widely outspread, and her plastron facing the male. If the water is too shallow for the refusal position, the females resort to beaching themselves, as the males do not follow them ashore.


title: Turtle

text: All turtles fertilize internally; mounting and copulation can be difficult. In many species, males have a concave plastron that interlocks with the female's carapace. In species like the Russian tortoise, the male has a lighter shell and longer legs. The high, rounded shape of box turtles are particular obstacles for mounting. The male eastern box turtle leans backward and hooks onto the back of the female's plastron. Aquatic turtles mount in water, and female sea turtles support the mounting male while swimming and diving. During copulation, the male turtle aligns his tail with the female's so he can insert his penis into her cloaca. Some female turtles can store sperm from multiple males and their egg clutches can have multiple sires.


title: Turtle

text: Turtles, including sea turtles, lay their eggs on land, although some lay eggs close near water that rises and falls in level, submerging the eggs. While most species build nests and lay eggs where they forage, some travel miles. The common snapping turtle walks on land, while sea turtles travel even further; the leatherback swims some to its nesting beaches. Most turtles create a nest for their eggs. Females usually dig a flask-like chamber in the substrate. Other species lay their eggs in vegetation or crevices. Females choose nesting locations based on environmental factors such as temperature and humidity, which are important for developing embryos. Depending on the species, the number of eggs laid varies from one to over 100. Larger females can lay eggs that are greater in number or bigger in size. Compared to freshwater turtles, tortoises deposit fewer but larger eggs. Females can lay multiple clutches throughout a season, particularly in species that experience unpredictable monsoons.


title: Turtle

text: Most mother turtles do no more in the way of parental care than covering their eggs and immediately leaving, though some species guard their nests for days or weeks. Eggs vary between rounded, oval, elongated, and between hard- and soft-shelled. Most species have their sex determined by temperature. In some species, higher temperatures produce females and lower ones produce males, while in others, milder temperatures produce males and both hot and cold extremes produce females. There is experimental evidence that the embryos of "Mauremys reevesii" can move around inside their eggs to select the best temperature for development, thus influencing their sexual destiny. In other species, sex is determined genetically. The length of incubation for turtle eggs varies from two to three months for temperate species, and four months to over a year for tropical species. Species that live in warm temperate climates can delay their development.


title: Turtle

text: Hatching young turtles break out of the shell using an egg tooth, a sharp projection that exists temporarily on their upper beak. Hatchlings dig themselves out of the nest and find safety in vegetation or water. Some species stay in the nest for longer, be it for overwintering or to wait for the rain to loosen the soil for them to dig out. Young turtles are highly vulnerable to predators, both in the egg and as hatchlings. Mortality is high during this period but significantly decreases when they reach adulthood. Most species grow quickly during their early years and slow down when they are mature.


title: Turtle

text: Turtles can live long lives. The oldest living turtle and land animal is said to be a Seychelles giant tortoise named Jonathan, who turned 187 in 2019. A Galápagos tortoise named Harriet was collected by Charles Darwin in 1835; it died in 2006, having lived for at least 176 years. Most wild turtles do not reach that age. Turtles keep growing new scutes under the previous scutes every year, allowing researchers to estimate how long they have lived. They also age slowly. The survival rate for adult turtles can reach 99% per year.


title: Turtle

text: Zoologists have sought to explain the evolutionary origin of the turtles, and in particular of their unique shells. In 1914, Jan Versluys proposed that bony plates in the dermis, called osteoderms, fused to the ribs beneath them, later called the "Polka Dot Ancestor" by Olivier Rieppel. The theory accounted for the evolution of fossil pareiasaurs from "Bradysaurus" to "Anthodon", but not for how the ribs could have become attached to the bony dermal plates.


title: Turtle

text: More recent discoveries have painted a different scenario for the evolution of the turtle's shell. The stem-turtles "Eunotosaurus" of the Middle Permian, "Pappochelys" of the Middle Triassic, and "Eorhynchochelys" of the Late Triassic lacked carapaces and plastrons but had shortened torsos, expanded ribs, and lengthened dorsal vertebrae. Also in the Late Triassic, "Odontochelys" had a partial shell consisting of a complete bony plastron and an incomplete carapace. The development of a shell reached completion with the Late Triassic "Proganochelys", with its fully developed carapace and plastron. Adaptations that lead to the evolution of the shell may have originally been for digging and a fossorial lifestyle.


title: Turtle

text: The oldest known members of the Pleurodira lineage are the Platychelyidae, from the Late Jurassic. The oldest known unambiguous cryptodire is "Sinaspideretes," a close relative of softshell turtles, from the Late Jurassic of China. During the Late Cretaceous and Cenozoic, members of the pleurodire families Bothremydidae and Podocnemididae became widely distributed in the Northern Hemisphere due to their coastal habits. The oldest known soft-shelled turtles and sea turtles appeared during the Early Cretaceous. Tortoises originated in Asia during the Eocene. A late surviving group of stem-turtles, the Meiolaniidae, survived in Australasia into the Pleistocene and Holocene.


title: Turtle

text: The turtles' exact ancestry has been disputed. It was believed they were the only surviving branch of the ancient evolutionary grade Anapsida, which includes groups such as procolophonids and pareiasaurs. All anapsid skulls lack a temporal opening while all other living amniotes have temporal openings. It was later suggested that the anapsid-like turtle skulls may be due to backward evolution rather than to anapsid descent. Fossil evidence has shown that early stem-turtles possessed small temporal openings.


title: Turtle

text: Some early morphological phylogenetic studies have placed turtles closer to Lepidosauria (tuataras, lizards, and snakes) than to Archosauria (crocodilians and birds). By contrast, several molecular studies place turtles either within Archosauria, or, more commonly, as a sister group to extant archosaurs, though an analysis conducted by Tyler Lyson and colleagues (2012) recovered turtles as the sister group of lepidosaurs instead. Ylenia Chiari and colleagues (2012) analyzed 248 nuclear genes from 16 vertebrates and suggested that turtles share a more recent common ancestor with birds and crocodilians. The date of separation of turtles and birds and crocodilians was estimated to be during the Permian. Through genomic-scale phylogenetic study of ultra-conserved elements (UCEs) to clarify the placement of turtles within reptiles, Nicholas Crawford and colleagues (2012) similarly found that turtles are closer to birds and crocodilians.


title: Turtle

text: Using the draft (unfinished) genome sequences of the green sea turtle and the Chinese softshell turtle, Zhuo Wang and colleagues (2013) concluded that turtles are likely a sister group of crocodilians and birds. The external phylogeny of the turtles is shown in the cladogram below.


title: Turtle

text: Modern turtles and their extinct relatives with a complete shell are classified within the clade Testudinata. The most recent common ancestor of living turtles, corresponding to the split between Pleurodira (side-necked species) and Cryptodira (hidden necked species), is estimated to have occurred around during the Late Triassic. Robert Thompson and colleagues (2021) comment that living turtles have low diversity, relative to how long they existed. Diversity has been stable, according to their analysis, except for a single rapid increase around the Eocene-Oligocene boundary some 30 million years ago, and a large regional extinction at roughly the same time. They suggest that global climate change caused both events, as the cooling and drying caused the land to become arid and turtles to become extinct there, while new continental margins opened up by the climate change provided habitats for other species to evolve.


title: Turtle

text: The cladogram, from Nicholas Crawford and colleagues 2015, shows the internal phylogeny of the Testudines down to the level of families. The analysis by Thompson and colleagues in 2021 supports the same structure down to the family level.


title: Turtle

text: Turtles are divided into two living suborders: Cryptodira and Pleurodira. The two groups differ in the way the neck is retracted for protection. Pleurodirans retract their neck to the side and in front of the shoulder girdles, whereas cryptodirans retract their neck backward into their shell. These motions are enabled by the morphology and arrangement of neck vertebrae. Sea turtles (which belong to Cryptodira) have mostly lost the ability to retract their heads.


title: Turtle

text: The adductor muscles in the lower jaw create a pulley-like system in both subgroups. However, the bones that the muscles articulate with differ. In Pleurodira, the pulley is formed with the pterygoid bones of the palate, but in Cryptodira the pulley is formed with the otic capsule. Both systems help to vertically redirect the adductor muscles and maintain a powerful bite.


title: Turtle

text: A further difference between the suborders is the attachment of the pelvis. In Cryptodira, the pelvis is free, linked to the shell only by ligaments. In Pleurodira, the pelvis is sutured, joined with bony connections, to the carapace and to the plastron, creating a pair of large columns of bone at the back end of the turtle, linking the two parts of the shell.


title: Turtle

text: Turtles are widely distributed across the world's continents, oceans, and islands with terrestrial, fully aquatic, and semi-aquatic species. Sea turtles are mainly tropical and subtropical, but leatherbacks can be found in colder areas of the Atlantic and Pacific. Living Pleurodira all live in freshwater and are found only in the Southern Hemisphere. The Cryptodira include terrestrial, freshwater, and marine species, and these range more widely. The world regions richest in non-marine turtle species are the Amazon basin, the Gulf of Mexico drainages of the United States, and parts of South and Southeast Asia.


title: Turtle

text: For turtles in colder climates, their distribution is limited by constraints on reproduction, which is reduced by long hibernations. North American species barely range above the southern Canadian border. Some turtles are found at high altitudes, for example, the species "Terrapene ornata" occurs up to in New Mexico. Conversely, the leatherback sea turtle can dive over . Species of the genus "Gopherus" can tolerate both below freezing and over in body temperature, though they are most active at .


title: Turtle

text: Among vertebrate orders, turtles are second only to primates in the percentage of threatened species. 360 modern species have existed since 1500 AD. Of these, 51–56% are considered threatened and 60% considered threatened or extinct. Turtles face many threats, including habitat destruction, harvesting for consumption, the pet trade, light pollution, and climate change. Asian species have a particularly high extinction risk, primarily due to their long-term unsustainable exploitation for food and medicine, and about 83% of Asia's non-marine turtle species are considered threatened. As of 2021, turtle extinction is progressing much faster than during the Cretaceous-Tertiary extinction. At this rate, all turtles could be extinct in a few centuries.


title: Turtle

text: Turtle hatcheries can be set up when protection against flooding, erosion, predation, or heavy poaching is required. Chinese markets have sought to satisfy an increasing demand for turtle meat with farmed turtles. In 2007 it was estimated that over a thousand turtle farms operated in China. All the same, wild turtles continue to be caught and sent to market in large numbers, resulting in what conservationists have called "the Asian turtle crisis". In the words of the biologist George Amato, the hunting of turtles "vacuumed up entire species from areas in Southeast Asia", even as biologists still did not know how many species lived in the region. In 2000, all the Asian box turtles were placed on the CITES list of endangered species.


title: Turtle

text: Harvesting wild turtles is legal in some American states, and there has been a growing demand for American turtles in China. The Florida Fish and Wildlife Conservation Commission estimated in 2008 that around 3,000 pounds of softshell turtles were exported weekly via Tampa International Airport. However, the great majority of turtles exported from the US between 2002 and 2005 were farmed.


title: Turtle

text: Large numbers of sea turtles are accidentally killed in longlines, gillnets, and trawling nets as bycatch. A 2010 study suggested that over 8 million had been killed between 1990 and 2008; the Eastern Pacific and the Mediterranean were identified as among the areas worst affected. Since the 1980s, the United States has required all shrimp trawlers to fit their nets with turtle excluder devices that prevent turtles from being entangled in the net and drowning.


title: Turtle

text: More locally, other human activities are affecting marine turtles. In Australia, Queensland's shark culling program, which uses shark nets and drum lines, has killed over 5,000 turtles as bycatch between 1962 and 2015; including 719 loggerhead turtles and 33 hawksbill sea turtles, which are listed as critically endangered.


title: Turtle

text: Native turtle populations can also be threatened by invasive ones. The central North American red-eared slider turtle has been listed among the "world's worst invasive species", pet turtle having been released globally. They appear to compete with native turtle species in eastern and western North America, Europe, and Japan.


title: Turtle

text: Turtles have featured in human cultures across the world since ancient times. They are generally viewed positively despite not being "cuddly" or flashy; their association with the ancient times and old age have contributed to their endearing image.


title: Turtle

text: In Hindu mythology, the World Turtle, named Kurma or Kacchapa, supports four elephants on his back; they, in turn, carry the weight of the whole world on their backs. The turtle is one of the ten avatars or incarnations of the god Vishnu. The yoga pose Kurmasana is named for the avatar. World Turtles are found in Native American cultures including the Algonquian, Iroquois, and Lenape. They tell many versions of the creation story of Turtle Island. One version has Muskrat pile up earth on Turtle's back, creating the continent of North America. An Iroquois version has the pregnant Sky Woman fall through a hole in the sky between a tree's roots, where she is caught by birds who land her safely on Turtle's back; the Earth grows around her. The turtle here is altruistic, but the world is a heavy burden, and the turtle sometimes shakes itself to relieve the load, causing earthquakes.


title: Turtle

text: A turtle was the symbol of the Ancient Mesopotamian god Enki from the 3rd millennium BCE onward. An ancient Greek origin myth told that only the tortoise refused the invitation of the gods Zeus and Hera to their wedding, as it preferred to stay at home. Zeus then ordered it to carry its house with it, ever after. Another of their gods, Hermes, invented a seven-stringed lyre made with the shell of a tortoise. In the Shang dynasty Chinese practice of plastromancy, dating back to 1200 BCE, oracles were obtained by inscribing questions on turtle plastrons using the oldest known form of Chinese characters, burning the plastron, and interpreting the resulting cracks. Later, the turtle was one of the four sacred animals in Confucianism, while in the Han period, steles were mounted on top of stone turtles, later linked with Bixi, the turtle-shelled son of the Dragon King. Marine turtles feature significantly in Australian Aboriginal art. The army of Ancient Rome used the ("tortoise") formation where soldiers would form a shield wall for protection.


title: Turtle

text: In Aesop's Fables, "The Tortoise and the Hare" tells how an unequal race may be won by the slower partner. Lewis Carroll's 1865 "Alice's Adventures in Wonderland" features a Mock Turtle, named for a soup meant to imitate the expensive soup made from real turtle meat. In 1896, the French playwright Léon Gandillot wrote a comedy in three acts named that was "a Parisian sensation" in its run in France, and came to the Manhattan Theatre, Broadway, New York, in 1898 as "The Turtle". A "cosmic turtle" and the island motif reappear in Gary Snyder's 1974 novel "Turtle Island", and again in Terry Pratchett's "Discworld" series as Great A'Tuin, starting with the 1983 novel "The Colour of Magic". It is supposedly of the species "Chelys galactica", the galactic turtle, complete with four elephants on its back to support Discworld. Turtles have been featured in comic books and animations such as the 1984 Teenage Mutant Ninja Turtles.


title: Turtle

text: Some turtles, particularly small terrestrial and freshwater species, are kept as pets. The demand for pet turtles increased in the 1950s, with the US being the main supplier, particularly of farm-bred red-eared sliders. The popularity for exotic pets has led to an increase in illegal wildlife trafficking. Around 21% of the value of live animal trade is in reptiles, and turtles are among the more popularly traded species. Poor husbandry of tortoises can cause chronic rhinitis (nasal swelling), overgrown beaks, hyperparathyroidism (which softens their skeleton), constipation, various reproductive problems, and injuries from dogs. In the early 20th century, people in the United States have organized and gambled on turtle races.


title: Turtle

text: The flesh of captured wild turtles continues to be eaten in Asian cultures, while turtle soup was once a popular dish in English cuisine. Gopher tortoise stew has been popular with some groups in Florida. The supposed aphrodisiac or medicinal properties of turtle eggs created a large trade for them in Southeast Asia. Hard-shell turtle plastrons and soft-shell carapaces are widely used in traditional Chinese medicine; Taiwan imported nearly 200 metric tons of hard-shells from its neighbors yearly from 1999 to 2008. A popular medicinal preparation based on herbs and turtle shells is "guilinggao" jelly. The substance tortoiseshell, usually from the hawksbill turtle, has been used for centuries to make jewelry, tools, and ornaments around the Western Pacific. Hawksbills have accordingly been hunted for their shells. The trading of tortoiseshell was internationally banned in 1977 by CITES. Some cultures have used turtle shells to make music: Native American shamans made them into ceremonial rattles, while Aztecs, Mayas, and Mixtecs made "" drums.


title: Temperature

text: Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.


title: Temperature

text: 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), the latter being used predominantly for scientific purposes. The kelvin is one of the seven base units in the International System of Units (SI).


title: Temperature

text: Absolute zero, i.e. zero kelvin or −273.15 °C, is the lowest point in the thermodynamic temperature scale. Experimentally, it can be approached only 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.


title: Temperature

text: Temperature 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.


title: Temperature

text: Temperature scales need two values for definition: the point chosen as zero degrees and the magnitudes of the incremental unit of temperature.


title: Temperature

text: 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 being defined as the freezing point of water, and as the boiling point of water, both at atmospheric pressure at sea level. Because of the 100-degree interval, it was called a centigrade scale. 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, and 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 offset of exactly 273.15.


title: Temperature

text: The Fahrenheit scale is in common use in the United States. Water freezes at and boils at at sea-level atmospheric pressure.


title: Temperature

text: 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 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 100 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. The absolute zero, defined as , is exactly equal to , or .


title: Temperature

text: Referring to 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 the absolute zero, it does not have 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 the absolute zero.


title: Temperature

text: Besides the internationally agreed Kelvin scale, there 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 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.


title: Temperature

text: 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, , is at the absolute zero of temperature. Since May, 2019, 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.


title: Temperature

text: 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, 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 at absolute zero.


title: Temperature

text: 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 ().


title: Temperature

text: 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 the middle of the nineteenth century.


title: Temperature

text: 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 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.


title: Temperature

text: In spite of these limitations, 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. 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.


title: Temperature

text: 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.


title: Temperature

text: In physics, 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, 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.


title: Temperature

text: 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 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 is disregarded.


title: Temperature

text: 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, 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.


title: Temperature

text: The speed of sound in a gas can be calculated theoretically from the molecular character of the gas, from its temperature and pressure, and from the value of 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 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.


title: Temperature

text: 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 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.


title: Temperature

text: 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 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.


title: Temperature

text: Historically, till May 2019, the definition of the Kelvin scale was that invented by Kelvin, based on 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 . Since May 2019, that value has not been fixed by definition but 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.


title: Temperature

text: 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 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, 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.


title: Temperature

text: The magnitude of the kelvin is now defined in terms of kinetic theory, derived from the value of the Boltzmann constant.


title: Temperature

text: Kinetic theory provides 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 a freely moving particle has an average kinetic energy of where 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 will be .


title: Temperature

text: Molecules, such as oxygen (O), 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 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.


title: Temperature

text: As noted above, the speed of sound in a gas can be calculated from the molecular character of the gas, from its temperature and pressure, and from the value of 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.


title: Temperature

text: It is possible to measure the average kinetic energy of constituent microscopic particles if they are allowed to escape from 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.


title: Temperature

text: Temperature is one of the principal quantities in 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.


title: Temperature

text: 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 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 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 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.


title: Temperature

text: 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 a cycle of states of its working body. The engine takes in a quantity of heat from a hot reservoir and passes out a lesser quantity of waste heat 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 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, and , of the reservoirs are defined such that


title: Temperature

text: 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.


title: Temperature

text: Kelvin's original work postulating absolute temperature was published in 1848. It was based on the work of Carnot, before 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 the definition just stated, was printed in 1853, a paper read in 1851.


title: Temperature

text: 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.


title: Temperature

text: In thermodynamic terms, temperature 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 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 the zeroth law of thermodynamics.


title: Temperature

text: In particular, when the body is described by stating its internal energy , an extensive variable, as a function of its entropy , also an extensive variable, and other state variables , with ), then the temperature is equal to the partial derivative of the internal energy with respect to the entropy:


title: Temperature

text: Likewise, when the body is described by stating its entropy as a function of its internal energy , and other state variables , with , then the reciprocal of the temperature is equal to the partial derivative of the entropy with respect to the internal energy:


title: Temperature

text: The above definition, equation (1), of the absolute temperature, is due to Kelvin. It refers to systems closed to 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.


title: Temperature

text: Real-world bodies are often not in thermodynamic equilibrium and not homogeneous. For 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 is said to prevail throughout the body.


title: Temperature

text: It makes good sense, for example, to say of the extensive variable , or of the extensive variable , 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 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.


title: Temperature

text: Thus, when local thermodynamic equilibrium prevails in a body, the temperature can be regarded as a spatially varying local property in that body, and this is because the temperature is an intensive variable.


title: Temperature

text: 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 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 the hotness manifold.


title: Temperature

text: 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.


title: Temperature

text: Experimental physicists, for example Galileo and Newton, found that there are indefinitely many empirical temperature scales. Nevertheless, 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 the mechanisms of operation of the thermometers.


title: Temperature

text: 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 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 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.


title: Temperature

text: Except for 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, 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.


title: Temperature

text: 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 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, anyone has given body and any one suitable empirical thermometer can still support notions of empirical, non-absolute, hotness, and temperature, for a suitable range of processes. This is a matter for study in non-equilibrium thermodynamics.


title: Temperature

text: When a body is not in a steady-state, then the notion of temperature becomes even less safe than for a body in a steady state not in thermodynamic equilibrium. This is also a matter for study in non-equilibrium thermodynamics.


title: Temperature

text: For the axiomatic treatment of thermodynamic equilibrium, since the 1930s, it has become customary to refer to a zeroth law of thermodynamics. The customarily stated minimalist version of such a law postulates only that all bodies, which when thermally connected would be in thermal equilibrium, should be said to have the same temperature by definition, but by itself does not establish temperature as a quantity expressed as a real number on a scale. A more physically informative version of such a law views empirical temperature as a chart on a hotness manifold. While the zeroth law permits the definitions of many different empirical scales of temperature, the second law of thermodynamics selects the definition of a single preferred, absolute temperature, unique up to an arbitrary scale factor, whence called the thermodynamic temperature. If internal energy is considered as a function of the volume and entropy of a homogeneous system in thermodynamic equilibrium, thermodynamic absolute temperature appears as the partial derivative of internal energy with respect the entropy at constant volume. Its natural, intrinsic origin or null point is absolute zero at which the entropy of any system is at a minimum. Although this is the lowest absolute temperature described by the model, the third law of thermodynamics postulates that absolute zero cannot be attained by any physical system.


title: Temperature

text: When an energy transfer to or from a body is only as heat, the state of the body changes. Depending on the surroundings and the walls separating them from the body, various changes are possible in the body. They include chemical reactions, increase of pressure, increase of temperature and phase change. For each kind of change under specified conditions, the heat capacity is the ratio of the quantity of heat transferred to the magnitude of the change.


title: Temperature

text: For example, if the change is an increase in temperature at constant volume, with no phase change and no chemical change, then the temperature of the body rises and its pressure increases. The quantity of heat transferred, , divided by the observed temperature change, , is the body's heat capacity at constant volume:


title: Temperature

text: If heat capacity is measured for a well-defined amount of substance, the specific heat is the measure of the heat required to increase the temperature of such a unit quantity by one unit of temperature. For example, raising the temperature of water by one kelvin (equal to one degree Celsius) requires 4186 joules per kilogram (J/kg).


title: Temperature

text: Temperature measurement using modern scientific thermometers and temperature scales goes back at least as far as the early 18th century, when Daniel Gabriel Fahrenheit adapted a thermometer (switching to mercury) and a scale both developed by Ole Christensen Rømer. Fahrenheit's scale is still in use in the United States for non-scientific applications.


title: Temperature

text: Temperature is measured with thermometers that may be calibrated to a variety of temperature scales. In most of the world (except for Belize, Myanmar, Liberia and the United States), the Celsius scale is used for most temperature measuring purposes. Most scientists measure temperature using the Celsius scale and thermodynamic temperature using the Kelvin scale, which is the Celsius scale offset so that its null point is = , or absolute zero. Many engineering fields in the US, notably high-tech and US federal specifications (civil and military), also use the Kelvin and Celsius scales. Other engineering fields in the US also rely upon the Rankine scale (a shifted Fahrenheit scale) when working in thermodynamic-related disciplines such as combustion.


title: Temperature

text: The basic unit of temperature in the International System of Units (SI) is the kelvin. It has the symbol K.


title: Temperature

text: For everyday applications, it is often convenient to use the Celsius scale, in which corresponds very closely to the freezing point of water and is its boiling point at sea level. Because liquid droplets commonly exist in clouds at sub-zero temperatures, is better defined as the melting point of ice. In this scale, a temperature difference of 1 degree Celsius is the same as a increment, but the scale is offset by the temperature at which ice melts ().


title: Temperature

text: By international agreement, until May 2019, the Kelvin and Celsius scales were defined by two fixing points: absolute zero and the triple point of Vienna Standard Mean Ocean Water, which is water specially prepared with a specified blend of hydrogen and oxygen isotopes. Absolute zero was defined as precisely and . It is the temperature at which all classical translational motion of the particles comprising matter ceases and they are at complete rest in the classical model. Quantum-mechanically, however, zero-point motion remains and has an associated energy, the zero-point energy. Matter is in its ground state, and contains no thermal energy. The temperatures and were defined as those of the triple point of water. This definition served the following purposes: it fixed the magnitude of the kelvin as being precisely 1 part in 273.16 parts of the difference between absolute zero and the triple point of water; it established that one kelvin has precisely the same magnitude as one degree on the Celsius scale; and it established the difference between the null points of these scales as being ( = and = ). Since 2019, there has been a new definition based on the Boltzmann constant, but the scales are scarcely changed.


title: Temperature

text: In the United States, the Fahrenheit scale is the most widely used. On this scale the freezing point of water corresponds to and the boiling point to . The Rankine scale, still used in fields of chemical engineering in the US, is an absolute scale based on the Fahrenheit increment.


title: Temperature

text: The following temperature scales are in use or have historically been used for measuring temperature:


title: Temperature

text: The field of plasma physics deals with phenomena of electromagnetic nature that involve very high temperatures. It is customary to express temperature as energy in a unit related to the electronvolt or kiloelectronvolt (eV/"k" or keV/"k"). The corresponding energy, which is dimensionally distinct from temperature, is then calculated as the product of the Boltzmann constant and temperature, formula_2. Then, 1eV/"k" is . In the study of QCD matter one routinely encounters temperatures of the order of a few hundred MeV/"k", equivalent to about .


title: Temperature

text: Historically, there are several scientific approaches to the explanation of temperature: the classical thermodynamic description based on macroscopic empirical variables that can be measured in a laboratory; the kinetic theory of gases which relates the macroscopic description to the probability distribution of the energy of motion of gas particles; and a microscopic explanation based on statistical physics and quantum mechanics. In addition, rigorous and purely mathematical treatments have provided an axiomatic approach to classical thermodynamics and temperature. Statistical physics provides a deeper understanding by describing the atomic behavior of matter and derives macroscopic properties from statistical averages of microscopic states, including both classical and quantum states. In the fundamental physical description, the temperature may be measured directly in units of energy. However, in the practical systems of measurement for science, technology, and commerce, such as the modern metric system of units, the macroscopic and the microscopic descriptions are interrelated by the Boltzmann constant, a proportionality factor that scales temperature to the microscopic mean kinetic energy.


title: Temperature

text: The microscopic description in statistical mechanics is based on a model that analyzes a system into its fundamental particles of matter or into a set of classical or quantum-mechanical oscillators and considers the system as a statistical ensemble of microstates. As a collection of classical material particles, the temperature is a measure of the mean energy of motion, called translational kinetic energy, of the particles, whether in solids, liquids, gases, or plasmas. The kinetic energy, a concept of classical mechanics, is half the mass of a particle times its speed squared. In this mechanical interpretation of thermal motion, the kinetic energies of material particles may reside in the velocity of the particles of their translational or vibrational motion or in the inertia of their rotational modes. In monatomic perfect gases and, approximately, in most gas and in simple metals, the temperature is a measure of the mean particle translational kinetic energy, 3/2 "k""T". It also determines the probability distribution function of energy. In condensed matter, and particularly in solids, this purely mechanical description is often less useful and the oscillator model provides a better description to account for quantum mechanical phenomena. Temperature determines the statistical occupation of the microstates of the ensemble. The microscopic definition of temperature is only meaningful in the thermodynamic limit, meaning for large ensembles of states or particles, to fulfill the requirements of the statistical model.


title: Temperature

text: Kinetic energy is also considered as a component of thermal energy. The thermal energy may be partitioned into independent components attributed to the degrees of freedom of the particles or to the modes of oscillators in a thermodynamic system. In general, the number of these degrees of freedom that are available for the equipartitioning of energy depends on the temperature, i.e. the energy region of the interactions under consideration. For solids, the thermal energy is associated primarily with the vibrations of its atoms or molecules about their equilibrium position. In an ideal monatomic gas, the kinetic energy is found exclusively in the purely translational motions of the particles. In other systems, vibrational and rotational motions also contribute degrees of freedom.


title: Temperature

text: Maxwell and Boltzmann developed a kinetic theory that yields a fundamental understanding of temperature in gases.


title: Temperature

text: This theory also explains the ideal gas law and the observed heat capacity of monatomic (or 'noble') gases.


title: Temperature

text: The ideal gas law is based on observed empirical relationships between pressure ("p"), volume ("V"), and temperature ("T"), and was recognized long before the kinetic theory of gases was developed (see Boyle's and Charles's laws). The ideal gas law states:


title: Temperature

text: This relationship gives us our first hint that there is an absolute zero on the temperature scale, because it only holds if the temperature is measured on an absolute scale such as Kelvin's. The ideal gas law allows one to measure temperature on this absolute scale using the gas thermometer. The temperature in kelvins can be defined as the pressure in pascals of one mole of gas in a container of one cubic meter, divided by the gas constant.


title: Temperature

text: Although it is not a particularly convenient device, the gas thermometer provides an essential theoretical basis by which all thermometers can be calibrated. As a practical matter, it is not possible to use a gas thermometer to measure absolute zero temperature since the gases condense into a liquid long before the temperature reaches zero. It is possible, however, to extrapolate to absolute zero by using the ideal gas law, as shown in the figure.


title: Temperature

text: The kinetic theory assumes that pressure is caused by the force associated with individual atoms striking the walls, and that all energy is translational kinetic energy. Using a sophisticated symmetry argument, Boltzmann deduced what is now called the Maxwell–Boltzmann probability distribution function for the velocity of particles in an ideal gas. From that probability distribution function, the average kinetic energy (per particle) of a monatomic ideal gas is


title: Temperature

text: where the Boltzmann constant is the ideal gas constant divided by the Avogadro number, and formula_5 is the root-mean-square speed. This direct proportionality between temperature and mean molecular kinetic energy is a special case of the equipartition theorem, and holds only in the classical limit of a perfect gas. It does not hold exactly for most substances.


title: Temperature

text: When two otherwise isolated bodies are connected together by a rigid physical path impermeable to matter, there is the spontaneous transfer of energy as heat from the hotter to the colder of them. Eventually, they reach a state of mutual thermal equilibrium, in which heat transfer has ceased, and the bodies' respective state variables have settled to become unchanging.


title: Temperature

text: One statement of the zeroth law of thermodynamics is that if two systems are each in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other.


title: Temperature

text: This statement helps to define temperature but it does not, by itself, complete the definition. An empirical temperature is a numerical scale for the hotness of a thermodynamic system. Such hotness may be defined as existing on a one-dimensional manifold, stretching between hot and cold. Sometimes the zeroth law is stated to include the existence of a unique universal hotness manifold, and of numerical scales on it, so as to provide a complete definition of empirical temperature. To be suitable for empirical thermometry, a material must have a monotonic relation between hotness and some easily measured state variable, such as pressure or volume, when all other relevant coordinates are fixed. An exceptionally suitable system is the ideal gas, which can provide a temperature scale that matches the absolute Kelvin scale. The Kelvin scale is defined on the basis of the second law of thermodynamics.


title: Temperature

text: As an alternative to considering or defining the zeroth law of thermodynamics, it was the historical development in thermodynamics to define temperature in terms of the second law of thermodynamics which deals with entropy. The second law states that any process will result in either no change or a net increase in the entropy of the universe. This can be understood in terms of probability.


title: Temperature

text: For example, in a series of coin tosses, a perfectly ordered system would be one in which either every toss comes up heads or every toss comes up tails. This means the outcome is always 100% the same result. In contrast, many mixed ("disordered") outcomes are possible, and their number increases with each toss. Eventually, the combinations of ~50% heads and ~50% tails dominate, and obtaining an outcome significantly different from 50/50 becomes increasingly unlikely. Thus the system naturally progresses to a state of maximum disorder or entropy.


title: Temperature

text: As temperature governs the transfer of heat between two systems and the universe tends to progress toward a maximum of entropy, it is expected that there is some relationship between temperature and entropy. A heat engine is a device for converting thermal energy into mechanical energy, resulting in the performance of work. An analysis of the Carnot heat engine provides the necessary relationships. According to energy conservation and energy being a state function that does not change over a full cycle, the work from a heat engine over a full cycle is equal to the net heat, i.e. the sum of the heat put into the system at high temperature, "q" > 0, and the waste heat given off at the low temperature, "q" < 0.


title: Temperature

text: where "w" is the work done per cycle. The efficiency depends only on |"q"|/"q". Because "q" and "q" correspond to heat transfer at the temperatures "T" and "T", respectively, |"q"|/"q" should be some function of these temperatures:


title: Temperature

text: Carnot's theorem states that all reversible engines operating between the same heat reservoirs are equally efficient. Thus, a heat engine operating between "T" and "T" must have the same efficiency as one consisting of two cycles, one between "T" and "T", and the second between "T" and "T". This can only be the case if


title: Temperature

text: Since the first function is independent of "T", this temperature must cancel on the right side, meaning "f"("T", "T") is of the form "g"("T")/"g"("T") (i.e. = = = , where "g" is a function of a single temperature. A temperature scale can now be chosen with the property that


title: Temperature

text: For "T" = 0K the efficiency is 100% and that efficiency becomes greater than 100% below 0K. Since an efficiency greater than 100% violates the first law of thermodynamics, this implies that 0K is the minimum possible temperature. In fact, the lowest temperature ever obtained in a macroscopic system was 20nK, which was achieved in 1995 at NIST. Subtracting the right hand side of (5) from the middle portion and rearranging gives


title: Temperature

text: where the negative sign indicates heat ejected from the system. This relationship suggests the existence of a state function, "S", whose change characteristically vanishes for a complete cycle if it is defined by


title: Temperature

text: where the subscript indicates a reversible process. This function corresponds to the entropy of the system, which was described previously. Rearranging (8) gives a formula for temperature in terms of fictive infinitesimal quasi-reversible elements of entropy and heat:


title: Temperature

text: For a constant-volume system where entropy "S"("E") is a function of its energy "E", d"E" = d"q" and (9) gives


title: Temperature

text: i.e. the reciprocal of the temperature is the rate of increase of entropy with respect to energy at constant volume.


title: Temperature

text: Statistical mechanics defines temperature based on a system's fundamental degrees of freedom. Eq.(10) is the defining relation of temperature, where the entropy formula_9 is defined (up to a constant) by the logarithm of the number of microstates of the system in the given macrostate (as specified in the microcanonical ensemble):


title: Temperature

text: where formula_11 is the Boltzmann constant and "W" is the number of microstates with the energy "E" of the system (degeneracy).


title: Temperature

text: When two systems with different temperatures are put into purely thermal connection, heat will flow from the higher temperature system to the lower temperature one; thermodynamically this is understood by the second law of thermodynamics: The total change in entropy following a transfer of energy formula_12 from system 1 to system 2 is:


title: Temperature

text: From the point of view of statistical mechanics, the total number of microstates in the combined system 1 + system 2 is formula_15, the logarithm of which (times the Boltzmann constant) is the sum of their entropies; thus a flow of heat from high to low temperature, which brings an increase in total entropy, is more likely than any other scenario (normally it is much more likely), as there are more microstates in the resulting macrostate.


title: Temperature

text: It is possible to extend the definition of temperature even to systems of few particles, like in a quantum dot. The generalized temperature is obtained by considering time ensembles instead of configuration-space ensembles given in statistical mechanics in the case of thermal and particle exchange between a small system of fermions ("N" even less than 10) with a single/double-occupancy system. The finite quantum grand canonical ensemble, obtained under the hypothesis of ergodicity and orthodicity, allows expressing the generalized temperature from the ratio of the average time of occupation formula_16 and formula_17 of the single/double-occupancy system:


title: Temperature

text: where "E" is the Fermi energy. This generalized temperature tends to the ordinary temperature when "N" goes to infinity.


title: Temperature

text: On the empirical temperature scales that are not referenced to absolute zero, a negative temperature is one below the zero-point of the scale used. For example, dry ice has a sublimation temperature of which is equivalent to . On the absolute Kelvin scale this temperature is . No body can be brought to exactly (the temperature of the ideally coldest possible body) by any finite practicable process; this is a consequence of the third law of thermodynamics.


title: Temperature

text: The international kinetic theory temperature of a body cannot take negative values. The thermodynamic temperature scale, however, is not so constrained.


title: Temperature

text: For a body of matter, there can sometimes be conceptually defined, in terms of microscopic degrees of freedom, namely particle spins, a subsystem, with a temperature other than that of the whole body. When the body is in its own state of internal thermodynamic equilibrium, the temperatures of the whole body and of the subsystem must be the same. The two temperatures can differ when, by work through externally imposed force fields, energy can be transferred to and from the subsystem, separately from the rest of the body; then the whole body is not in its own state of internal thermodynamic equilibrium. There is an upper limit of energy such a spin subsystem can attain.


title: Temperature

text: Considering the subsystem to be in a temporary state of virtual thermodynamic equilibrium, it is possible to obtain a negative temperature on the thermodynamic scale. Thermodynamic temperature is the inverse of the derivative of the subsystem's entropy with respect to its internal energy. As the subsystem's internal energy increases, the entropy increases for some range, but eventually attains a maximum value and then begins to decrease as the highest energy states begin to fill. At the point of maximum entropy, the temperature function shows the behavior of a singularity, because the slope of the entropy as a function of energy decreases to zero and then turns negative. As the subsystem's entropy reaches its maximum, its thermodynamic temperature goes to positive infinity, switching to negative infinity as the slope turns negative. Such negative temperatures are hotter than any positive temperature. Over time, when the subsystem is exposed to the rest of the body, which has a positive temperature, energy is transferred as heat from the negative temperature subsystem to the positive temperature system. The kinetic theory temperature is not defined for such subsystems.


title: George Bernard Shaw

text: George Bernard Shaw (26 July 1856 – 2 November 1950), known at his insistence simply as Bernard Shaw, was an Irish playwright, critic, polemicist and political activist. His influence on Western theatre, culture and politics extended from the 1880s to his death and beyond. He wrote more than sixty plays, including major works such as "Man and Superman" (1902), "Pygmalion" (1913) and "Saint Joan" (1923). With a range incorporating both contemporary satire and historical allegory, Shaw became the leading dramatist of his generation, and in 1925 was awarded the Nobel Prize in Literature.


title: George Bernard Shaw

text: Born in Dublin, Shaw moved to London in 1876, where he struggled to establish himself as a writer and novelist, and embarked on a rigorous process of self-education. By the mid-1880s he had become a respected theatre and music critic. Following a political awakening, he joined the gradualist Fabian Society and became its most prominent pamphleteer. Shaw had been writing plays for years before his first public success, "Arms and the Man" in 1894. Influenced by Henrik Ibsen, he sought to introduce a new realism into English-language drama, using his plays as vehicles to disseminate his political, social and religious ideas. By the early twentieth century his reputation as a dramatist was secured with a series of critical and popular successes that included "Major Barbara", "The Doctor's Dilemma", and "Caesar and Cleopatra".


title: George Bernard Shaw

text: Shaw's expressed views were often contentious; he promoted eugenics and alphabet reform, and opposed vaccination and organised religion. He courted unpopularity by denouncing both sides in the First World War as equally culpable, and although not a republican, castigated British policy on Ireland in the postwar period. These stances had no lasting effect on his standing or productivity as a dramatist; the inter-war years saw a series of often ambitious plays, which achieved varying degrees of popular success. In 1938 he provided the screenplay for a filmed version of "Pygmalion" for which he received an Academy Award. His appetite for politics and controversy remained undiminished; by the late 1920s, he had largely renounced Fabian Society gradualism, and often wrote and spoke favourably of dictatorships of the right and left—he expressed admiration for both Mussolini and Stalin. In the final decade of his life, he made fewer public statements but continued to write prolifically until shortly before his death, aged ninety-four, having refused all state honours, including the Order of Merit in 1946.


title: George Bernard Shaw

text: Since Shaw's death scholarly and critical opinion about his works has varied, but he has regularly been rated among British dramatists as second only to Shakespeare; analysts recognise his extensive influence on generations of English-language playwrights. The word "Shavian" has entered the language as encapsulating Shaw's ideas and his means of expressing them.


title: George Bernard Shaw

text: Shaw was born at 3 Upper Synge Street in Portobello, a lower-middle-class part of Dublin. He was the youngest child and only son of George Carr Shaw (1814–1885) and Lucinda Elizabeth (Bessie) Shaw ("née" Gurly; 1830–1913). His elder siblings were Lucinda (Lucy) Frances (1853–1920) and Elinor Agnes (1855–1876). The Shaw family was of English descent and belonged to the dominant Protestant Ascendancy in Ireland; George Carr Shaw, an ineffectual alcoholic, was among the family's less successful members. His relatives secured him a sinecure in the civil service, from which he was pensioned off in the early 1850s; thereafter he worked irregularly as a corn merchant. In 1852 he married Bessie Gurly; in the view of Shaw's biographer Michael Holroyd she married to escape a tyrannical great-aunt. If, as Holroyd and others surmise, George's motives were mercenary, then he was disappointed, as Bessie brought him little of her family's money. She came to despise her ineffectual and often drunken husband, with whom she shared what their son later described as a life of "shabby-genteel poverty".


title: George Bernard Shaw

text: By the time of Shaw's birth, his mother had become close to George John Lee, a flamboyant figure well known in Dublin's musical circles. Shaw retained a lifelong obsession that Lee might have been his biological father; there is no consensus among Shavian scholars on the likelihood of this. The young Shaw suffered no harshness from his mother, but he later recalled that her indifference and lack of affection hurt him deeply. He found solace in the music that abounded in the house. Lee was a conductor and teacher of singing; Bessie had a fine mezzo-soprano voice and was much influenced by Lee's unorthodox method of vocal production. The Shaws' house was often filled with music, with frequent gatherings of singers and players.


title: George Bernard Shaw

text: In 1862, Lee and the Shaws agreed to share a house, No. 1 Hatch Street, in an affluent part of Dublin, and a country cottage on Dalkey Hill, overlooking Killiney Bay. Shaw, a sensitive boy, found the less salubrious parts of Dublin shocking and distressing, and was happier at the cottage. Lee's students often gave him books, which the young Shaw read avidly; thus, as well as gaining a thorough musical knowledge of choral and operatic works, he became familiar with a wide spectrum of literature.


title: George Bernard Shaw

text: Between 1865 and 1871, Shaw attended four schools, all of which he hated. His experiences as a schoolboy left him disillusioned with formal education: "Schools and schoolmasters", he later wrote, were "prisons and turnkeys in which children are kept to prevent them disturbing and chaperoning their parents." In October 1871 he left school to become a junior clerk in a Dublin firm of land agents, where he worked hard, and quickly rose to become head cashier. During this period, Shaw was known as "George Shaw"; after 1876, he dropped the "George" and styled himself "Bernard Shaw".


title: George Bernard Shaw

text: In June 1873, Lee left Dublin for London and never returned. A fortnight later, Bessie followed him; the two girls joined her. Shaw's explanation of why his mother followed Lee was that without the latter's financial contribution the joint household had to be broken up. Left in Dublin with his father, Shaw compensated for the absence of music in the house by teaching himself to play the piano.


title: George Bernard Shaw

text: Early in 1876 Shaw learned from his mother that Agnes was dying of tuberculosis. He resigned from the land agents, and in March travelled to England to join his mother and Lucy at Agnes's funeral. He never again lived in Ireland, and did not visit it for twenty-nine years.


title: George Bernard Shaw

text: Initially, Shaw refused to seek clerical employment in London. His mother allowed him to live free of charge in her house in South Kensington, but he nevertheless needed an income. He had abandoned a teenage ambition to become a painter, and had not yet thought of writing for a living, but Lee found a little work for him, ghost-writing a musical column printed under Lee's name in a satirical weekly, "The Hornet". Lee's relations with Bessie deteriorated after their move to London. Shaw maintained contact with Lee, who found him work as a rehearsal pianist and occasional singer.


title: George Bernard Shaw

text: Eventually Shaw was driven to applying for office jobs. In the interim he secured a reader's pass for the British Museum Reading Room (the forerunner of the British Library) and spent most weekdays there, reading and writing. His first attempt at drama, begun in 1878, was a blank-verse satirical piece on a religious theme. It was abandoned unfinished, as was his first try at a novel. His first completed novel, "Immaturity" (1879), was too grim to appeal to publishers and did not appear until the 1930s. He was employed briefly by the newly formed Edison Telephone Company in 1879–80, and as in Dublin achieved rapid promotion. Nonetheless, when the Edison firm merged with the rival Bell Telephone Company, Shaw chose not to seek a place in the new organisation. Thereafter he pursued a full-time career as an author.


title: George Bernard Shaw

text: For the next four years Shaw made a negligible income from writing, and was subsidised by his mother. In 1881, for the sake of economy, and increasingly as a matter of principle, he became a vegetarian. He grew a beard to hide a facial scar left by smallpox. In rapid succession he wrote two more novels: "The Irrational Knot" (1880) and "Love Among the Artists" (1881), but neither found a publisher; each was serialised a few years later in the socialist magazine "Our Corner".


title: George Bernard Shaw

text: In 1880 Shaw began attending meetings of the Zetetical Society, whose objective was to "search for truth in all matters affecting the interests of the human race". Here he met Sidney Webb, a junior civil servant who, like Shaw, was busy educating himself. Despite difference of style and temperament, the two quickly recognised qualities in each other and developed a lifelong friendship. Shaw later reflected: "You knew everything that I didn't know and I knew everything you didn't know ... We had everything to learn from one another and brains enough to do it".


title: George Bernard Shaw

text: Shaw's next attempt at drama was a one-act playlet in French, "Un Petit Drame", written in 1884 but not published in his lifetime. In the same year the critic William Archer suggested a collaboration, with a plot by Archer and dialogue by Shaw. The project foundered, but Shaw returned to the draft as the basis of "Widowers' Houses" in 1892, and the connection with Archer proved of immense value to Shaw's career.


title: George Bernard Shaw

text: On 5 September 1882 Shaw attended a meeting at the Memorial Hall, Farringdon, addressed by the political economist Henry George. Shaw then read George's book "Progress and Poverty", which awakened his interest in economics. He began attending meetings of the Social Democratic Federation (SDF), where he discovered the writings of Karl Marx, and thereafter spent much of 1883 reading "Das Kapital". He was not impressed by the SDF's founder, H. M. Hyndman, whom he found autocratic, ill-tempered and lacking leadership qualities. Shaw doubted the ability of the SDF to harness the working classes into an effective radical movement and did not join it—he preferred, he said, to work with his intellectual equals.


title: George Bernard Shaw

text: After reading a tract, "Why Are The Many Poor?", issued by the recently formed Fabian Society, Shaw went to the society's next advertised meeting, on 16 May 1884. He became a member in September, and before the year's end had provided the society with its first manifesto, published as Fabian Tract No. 2. He joined the society's executive committee in January 1885, and later that year recruited Webb and also Annie Besant, a fine orator.


title: George Bernard Shaw

text: From 1885 to 1889 Shaw attended the fortnightly meetings of the British Economic Association; it was, Holroyd observes, "the closest Shaw had ever come to university education." This experience changed his political ideas; he moved away from Marxism and became an apostle of gradualism. When in 1886–87 the Fabians debated whether to embrace anarchism, as advocated by Charlotte Wilson, Besant and others, Shaw joined the majority in rejecting this approach. After a rally in Trafalgar Square addressed by Besant was violently broken up by the authorities on 13 November 1887 ("Bloody Sunday"), Shaw became convinced of the folly of attempting to challenge police power. Thereafter he largely accepted the principle of "permeation" as advocated by Webb: the notion whereby socialism could best be achieved by infiltration of people and ideas into existing political parties.


title: George Bernard Shaw

text: Throughout the 1880s the Fabian Society remained small, its message of moderation frequently unheard among more strident voices. Its profile was raised in 1889 with the publication of "Fabian Essays in Socialism", edited by Shaw who also provided two of the essays. The second of these, "Transition", details the case for gradualism and permeation, asserting that "the necessity for cautious and gradual change must be obvious to everyone". In 1890 Shaw produced Tract No. 13, "What Socialism Is", a revision of an earlier tract in which Charlotte Wilson had defined socialism in anarchistic terms. In Shaw's new version, readers were assured that "socialism can be brought about in a perfectly constitutional manner by democratic institutions".


title: George Bernard Shaw

text: The mid-1880s marked a turning point in Shaw's life, both personally and professionally: he lost his virginity, had two novels published, and began a career as a critic. He had been celibate until his twenty-ninth birthday, when his shyness was overcome by Jane (Jenny) Patterson, a widow some years his senior. Their affair continued, not always smoothly, for eight years. Shaw's sex life has caused much speculation and debate among his biographers, but there is a consensus that the relationship with Patterson was one of his few non-platonic romantic liaisons.


title: George Bernard Shaw

text: The published novels, neither commercially successful, were his two final efforts in this genre: "Cashel Byron's Profession" written in 1882–83, and "An Unsocial Socialist", begun and finished in 1883. The latter was published as a serial in "ToDay" magazine in 1884, although it did not appear in book form until 1887. "Cashel Byron" appeared in magazine and book form in 1886.


title: George Bernard Shaw

text: In 1884 and 1885, through the influence of Archer, Shaw was engaged to write book and music criticism for London papers. When Archer resigned as art critic of "The World" in 1886 he secured the succession for Shaw. The two figures in the contemporary art world whose views Shaw most admired were William Morris and John Ruskin, and he sought to follow their precepts in his criticisms. Their emphasis on morality appealed to Shaw, who rejected the idea of art for art's sake, and insisted that all great art must be didactic.


title: George Bernard Shaw

text: Of Shaw's various reviewing activities in the 1880s and 1890s it was as a music critic that he was best known. After serving as deputy in 1888, he became musical critic of "The Star" in February 1889, writing under the pen-name Corno di Bassetto. In May 1890 he moved back to "The World", where he wrote a weekly column as "G.B.S." for more than four years. In the 2016 version of the "Grove Dictionary of Music and Musicians", Robert Anderson writes, "Shaw's collected writings on music stand alone in their mastery of English and compulsive readability." Shaw ceased to be a salaried music critic in August 1894, but published occasional articles on the subject throughout his career, his last in 1950.


title: George Bernard Shaw

text: From 1895 to 1898, Shaw was the theatre critic for "The Saturday Review", edited by his friend Frank Harris. As at "The World", he used the by-line "G.B.S." He campaigned against the artificial conventions and hypocrisies of the Victorian theatre and called for plays of real ideas and true characters. By this time he had embarked in earnest on a career as a playwright: "I had rashly taken up the case; and rather than let it collapse I manufactured the evidence".


title: George Bernard Shaw

text: After using the plot of the aborted 1884 collaboration with Archer to complete "Widowers' Houses" (it was staged twice in London, in December 1892), Shaw continued writing plays. At first he made slow progress; "The Philanderer", written in 1893 but not published until 1898, had to wait until 1905 for a stage production. Similarly, "Mrs Warren's Profession" (1893) was written five years before publication and nine years before reaching the stage.


title: George Bernard Shaw

text: Shaw's first play to bring him financial success was "Arms and the Man" (1894), a mock-Ruritanian comedy satirising conventions of love, military honour and class. The press found the play overlong, and accused Shaw of mediocrity, sneering at heroism and patriotism, heartless cleverness, and copying W.S.Gilbert's style. The public took a different view, and the management of the theatre staged extra matinée performances to meet the demand. The play ran from April to July, toured the provinces and was staged in New York. It earned him £341 in royalties in its first year, a sufficient sum to enable him to give up his salaried post as a music critic. Among the cast of the London production was Florence Farr, with whom Shaw had a romantic relationship between 1890 and 1894, much resented by Jenny Patterson.


title: George Bernard Shaw

text: The success of "Arms and the Man" was not immediately replicated. "Candida", which presented a young woman making a conventional romantic choice for unconventional reasons, received a single performance in South Shields in 1895; in 1897 a playlet about Napoleon called "The Man of Destiny" had a single staging at Croydon. In the 1890s Shaw's plays were better known in print than on the West End stage; his biggest success of the decade was in New York in 1897, when Richard Mansfield's production of the historical melodrama "The Devil's Disciple" earned the author more than £2,000 in royalties.


title: George Bernard Shaw

text: In January 1893, as a Fabian delegate, Shaw attended the Bradford conference which led to the foundation of the Independent Labour Party. He was sceptical about the new party, and scorned the likelihood that it could switch the allegiance of the working class from sport to politics. He persuaded the conference to adopt resolutions abolishing indirect taxation, and taxing unearned income "to extinction". Back in London, Shaw produced what Margaret Cole, in her Fabian history, terms a "grand philippic" against the minority Liberal administration that had taken power in 1892. "To Your Tents, O Israel" excoriated the government for ignoring social issues and concentrating solely on Irish Home Rule, a matter Shaw declared of no relevance to socialism. In 1894 the Fabian Society received a substantial bequest from a sympathiser, Henry Hunt Hutchinson—Holroyd mentions £10,000. Webb, who chaired the board of trustees appointed to supervise the legacy, proposed to use most of it to found a school of economics and politics. Shaw demurred; he thought such a venture was contrary to the specified purpose of the legacy. He was eventually persuaded to support the proposal, and the London School of Economics and Political Science (LSE) opened in the summer of 1895.


title: George Bernard Shaw

text: By the later 1890s Shaw's political activities lessened as he concentrated on making his name as a dramatist. In 1897 he was persuaded to fill an uncontested vacancy for a "vestryman" (parish councillor) in London's St Pancras district. At least initially, Shaw took to his municipal responsibilities seriously; when London government was reformed in 1899 and the St Pancras vestry became the Metropolitan Borough of St Pancras, he was elected to the newly formed borough council.


title: George Bernard Shaw

text: In 1898, as a result of overwork, Shaw's health broke down. He was nursed by Charlotte Payne-Townshend, a rich Anglo-Irish woman whom he had met through the Webbs. The previous year she had proposed that she and Shaw should marry. He had declined, but when she insisted on nursing him in a house in the country, Shaw, concerned that this might cause scandal, agreed to their marriage. The ceremony took place on 1 June 1898, in the register office in Covent Garden. The bride and bridegroom were both aged forty-one. In the view of the biographer and critic St John Ervine, "their life together was entirely felicitous". There were no children of the marriage, which it is generally believed was never consummated; whether this was wholly at Charlotte's wish, as Shaw liked to suggest, is less widely credited. In the early weeks of the marriage Shaw was much occupied writing his Marxist analysis of Wagner's "Ring" cycle, published as "The Perfect Wagnerite" late in 1898. In 1906 the Shaws found a country home in Ayot St Lawrence, Hertfordshire; they renamed the house "Shaw's Corner", and lived there for the rest of their lives. They retained a London flat in the Adelphi and later at Whitehall Court.


title: George Bernard Shaw

text: During the first decade of the twentieth century, Shaw secured a firm reputation as a playwright. In 1904 J. E. Vedrenne and Harley Granville-Barker established a company at the Royal Court Theatre in Sloane Square, Chelsea to present modern drama. Over the next five years they staged fourteen of Shaw's plays. The first, "John Bull's Other Island", a comedy about an Englishman in Ireland, attracted leading politicians and was seen by Edward VII, who laughed so much that he broke his chair. The play was withheld from Dublin's Abbey Theatre, for fear of the affront it might provoke, although it was shown at the city's Royal Theatre in November 1907. Shaw later wrote that William Butler Yeats, who had requested the play, "got rather more than he bargained for... It was uncongenial to the whole spirit of the neo-Gaelic movement, which is bent on creating a new Ireland after its own ideal, whereas my play is a very uncompromising presentment of the real old Ireland." Nonetheless, Shaw and Yeats were close friends; Yeats and Lady Gregory tried unsuccessfully to persuade Shaw to take up the vacant co-directorship of the Abbey Theatre after J. M. Synge's death in 1909. Shaw admired other figures in the Irish Literary Revival, including George Russell and James Joyce, and was a close friend of Seán O'Casey, who was inspired to become a playwright after reading "John Bull's Other Island".


title: George Bernard Shaw

text: "Man and Superman", completed in 1902, was a success both at the Royal Court in 1905 and in Robert Loraine's New York production in the same year. Among the other Shaw works presented by Vedrenne and Granville-Barker were "Major Barbara" (1905), depicting the contrasting morality of arms manufacturers and the Salvation Army; "The Doctor's Dilemma" (1906), a mostly serious piece about professional ethics; and "Caesar and Cleopatra", Shaw's counterblast to Shakespeare's "Antony and Cleopatra", seen in New York in 1906 and in London the following year.


title: George Bernard Shaw

text: Now prosperous and established, Shaw experimented with unorthodox theatrical forms described by his biographer Stanley Weintraub as "discussion drama" and "serious farce". These plays included "Getting Married" (premiered 1908), "The Shewing-Up of Blanco Posnet" (1909), "Misalliance" (1910), and "Fanny's First Play" (1911). "Blanco Posnet" was banned on religious grounds by the Lord Chamberlain (the official theatre censor in England), and was produced instead in Dublin; it filled the Abbey Theatre to capacity. "Fanny's First Play", a comedy about suffragettes, had the longest initial run of any Shaw play—622 performances.


title: George Bernard Shaw

text: "Androcles and the Lion" (1912), a less heretical study of true and false religious attitudes than "Blanco Posnet", ran for eight weeks in September and October 1913. It was followed by one of Shaw's most successful plays, "Pygmalion", written in 1912 and staged in Vienna the following year, and in Berlin shortly afterwards. Shaw commented, "It is the custom of the English press when a play of mine is produced, to inform the world that it is not a play—that it is dull, blasphemous, unpopular, and financially unsuccessful. ... Hence arose an urgent demand on the part of the managers of Vienna and Berlin that I should have my plays performed by them first." The British production opened in April 1914, starring Sir Herbert Tree and Mrs Patrick Campbell as, respectively, a professor of phonetics and a cockney flower-girl. There had earlier been a romantic liaison between Shaw and Campbell that caused Charlotte Shaw considerable concern, but by the time of the London premiere it had ended. The play attracted capacity audiences until July, when Tree insisted on going on holiday, and the production closed. His co-star then toured with the piece in the US.


title: George Bernard Shaw

text: In 1899, when the Boer War began, Shaw wished the Fabians to take a neutral stance on what he deemed, like Home Rule, to be a "non-Socialist" issue. Others, including the future Labour prime minister Ramsay MacDonald, wanted unequivocal opposition, and resigned from the society when it followed Shaw. In the Fabians' war manifesto, "Fabianism and the Empire" (1900), Shaw declared that "until the Federation of the World becomes an accomplished fact we must accept the most responsible Imperial federations available as a substitute for it".