Etymology.

First attested in English in 1382, the word "allegory" comes from Latin "allegoria", the latinisation of the Greek ἀλληγορία ("allegoría"), "veiled language, figurative", which in turn comes from both ἄλλος ("allos"), "another, different" and ἀγορεύω ("agoreuo"), "to harangue, to speak in the assembly", which originates from ἀγορά ("agora"), "assembly".

Types.

Northrop Frye discussed what he termed a "continuum of allegory", a spectrum that ranges from what he termed the "naive allegory" of "The Faerie Queene", to the more private allegories of modern paradox literature. In this perspective, the characters in a "naive" allegory are not fully three-dimensional, for each aspect of their individual personalities and the events that befall them embodies some moral quality or other abstraction; the allegory has been selected first, and the details merely flesh it out.

Classical allegory.

The origins of allegory can be traced at least back to Homer in his "quasi-allegorical" use of personifications of, e.g., Terror (Deimos) and Fear (Phobos) at Il. 115 f. The title of "first allegorist," however, is usually awarded to whoever was the earliest to put forth allegorical interpretations of Homer. This approach leads to two possible answers: Theagenes of Rhegium (whom Porphyry calls the "first allegorist," Porph. Quaest. Hom. 1.240.14-241.12 Schrad.) or Pherecydes of Syros, both of whom are presumed to be active in the 6th century B.C.E., though Pherecydes is earlier and as he is often presumed to be the first writer of prose. The debate is complex, since it demands we observe the distinction between two often conflated uses of the Greek verb "allēgoreīn," which can mean both "to speak allegorically" and "to interpret allegorically."  In the case of "interpreting allegorically," Theagenes appears to be our earliest example. Presumably in response to proto-philosophical moral critiques of Homer (e.g. Xenophanes fr. 11 Diels-Kranz ), Theagenes proposed symbolic interpretations whereby the Gods of the Iliad actually stood for physical elements. So, Hephestus represents Fire, for instance (for which see fr. A2 in Diels-Kranz ). Some scholars, however, argue that Pherecydes cosmogonic writings anticipated Theagenes allegorical work, illustrated especially by his early placement of Time (Chronos) in his genealogy of the gods, which is thought to be a reinterpretation of the titan Kronos, from more traditional genealogies.  In classical literature two of the best-known allegories are the Cave in Plato's "Republic" (Book VII) and the story of the stomach and its members in the speech of Menenius Agrippa (Livy ii. 32).  Among the best-known examples of allegory, Plato's Allegory of the Cave, forms a part of his larger work "The Republic." In this allegory, Plato describes a group of people who have lived chained in a cave all of their lives, facing a blank wall (514a–b). The people watch shadows projected on the wall by things passing in front of a fire behind them and begin to ascribe forms to these shadows, using language to identify their world (514c–515a). According to the allegory, the shadows are as close as the prisoners get to viewing reality, until one of them finds his way into the outside world where he sees the actual objects that produced the shadows. He tries to tell the people in the cave of his discovery, but they do not believe him and vehemently resist his efforts to free them so they can see for themselves (516e–518a). This allegory is, on a basic level, about a philosopher who upon finding greater knowledge outside the cave of human understanding, seeks to share it as is his duty, and the foolishness of those who would ignore him because they think themselves educated enough.  In Late Antiquity Martianus Capella organized all the information a fifth-century upper-class male needed to know into an allegory of the wedding of Mercury and "Philologia," with the seven liberal arts the young man needed to know as guests. Also the Neoplatonic philosophy developed a type of allegorical reading of Homer and Plato.

Biblical allegory.

Other early allegories are found in the Hebrew Bible, such as the extended metaphor in Psalm 80 of the Vine and its impressive spread and growth, representing Israel's conquest and peopling of the Promised Land. Also allegorical is Ezekiel 16 and 17, wherein the capture of that same vine by the mighty Eagle represents Israel's exile to Babylon.  Allegorical interpretation of the Bible was a common early Christian practice and continues. For example, the recently re-discovered IVth Commentary on the Gospels by Fortunatianus of Aquileia has a comment by its English translator: "The principal characteristic of Fortunatianus’ exegesis is a figurative approach, relying on a set of concepts associated with key terms in order to create an allegorical decoding of the text." (pXIX)

Medieval allegory.

Allegory has an ability to freeze the temporality of a story, while infusing it with a spiritual context. Mediaeval thinking accepted allegory as having a "reality" underlying any rhetorical or fictional uses. The allegory was as true as the facts of surface appearances. Thus, the Papal Bull "Unam Sanctam" (1302) presents themes of the unity of Christendom with the pope as its head in which the allegorical details of the metaphors are adduced as facts on which is based a demonstration with the vocabulary of logic: ""Therefore" of this one and only Church there is one body and one head—not two heads as if it were a monster... If, then, the Greeks or others say that they were not committed to the care of Peter and his successors, they "necessarily" confess that they are not of the sheep of Christ." This text also demonstrates the frequent use of allegory in religious texts during the Mediaeval Period, following the tradition and example of the Bible.  In the late 15th century, the enigmatic "Hypnerotomachia", with its elaborate woodcut illustrations, shows the influence of themed pageants and masques on contemporary allegorical representation, as humanist dialectic conveyed them.  The denial of medieval allegory as found in the 12th-century works of Hugh of St Victor and Edward Topsell's "Historie of Foure-footed Beastes" (London, 1607, 1653) and its replacement in the study of nature with methods of categorisation and mathematics by such figures as naturalist John Ray and the astronomer Galileo is thought to mark the beginnings of early modern science.

Modern allegory.

Since meaningful stories are nearly always applicable to larger issues, allegories may be read into many stories which the author may not have recognised. This is allegoresis, or the act of reading a story as an allegory. Examples of allegory in popular culture that may or may not have been intended include the works of Bertolt Brecht, and even some works of science fiction and fantasy, such as "The Chronicles of Narnia" by C. S. Lewis.  The story of the apple falling onto Isaac Newton's head is another famous allegory. It simplified the idea of gravity by depicting a simple way it was supposedly discovered. It also made the scientific revelation well known by condensing the theory into a short tale.

Poetry and fiction.

While allegoresis may make discovery of allegory in any work, not every resonant work of modern fiction is allegorical, and some are clearly not intended to be viewed this way. According to Henry Littlefield's 1964 article, L. Frank Baum's "The Wonderful Wizard of Oz", may be readily understood as a plot-driven fantasy narrative in an extended fable with talking animals and broadly sketched characters, intended to discuss the politics of the time. Yet, George MacDonald emphasised in 1893 that "A fairy tale is not an allegory."  J. R. R. Tolkien's "The Lord of the Rings" is another example of a well-known work mistakenly perceived as allegorical, as the author himself once stated, "...I cordially dislike allegory in all its manifestations, and always have done so since I grew old and wary enough to detect its presence. I much prefer history – true or feigned– with its varied applicability to the thought and experience of readers. I think that many confuse applicability with allegory, but the one resides in the freedom of the reader, and the other in the purposed domination of the author."  Tolkien specifically resented the suggestion that the book's One Ring, which gives overwhelming power to those possessing it, was intended as an allegory of nuclear weapons. He noted that, had that been his intention, the book would not have ended with the Ring being destroyed but rather with an arms race in which various powers would try to obtain such a Ring for themselves. Then Tolkien went on to outline an alternative plot for "Lord of The Rings", as it would have been written had such an allegory been intended, and which would have made the book into a dystopia. While all this does not mean Tolkien's works may not be treated as having allegorical themes, especially when reinterpreted through postmodern sensibilities, it at least suggests that none were conscious in his writings. This further reinforces the idea of forced allegoresis, as allegory is often a matter of interpretation and only sometimes of original artistic intention.  Like allegorical stories, allegorical poetry has two meanings – a literal meaning and a symbolic meaning.  Some unique specimens of allegory can be found in the following works:

Art.

Some elaborate and successful specimens of allegory are to be found in the following works, arranged in approximate chronological order:

As a literary device, an allegory is a narrative in which a character, place, or event is used to deliver a broader message about real-world issues and occurrences. Authors have used allegory throughout history in all forms of art to illustrate or convey complex ideas and concepts in ways that are comprehensible or striking to its viewers, readers, or listeners.

托寓(英语:Allegory,来自希腊语的与,解作「公开演说」)是一种有比喻性质的表达方式(),把语意()而非字义()的意思表现出来。一般而言,会被当作一种表现性的修辞手法,但它不一定必须以语言的方式出现。反之,经常以视觉上的方式示人,例如现实主义的雕塑、绘画(寓意画)、,以及其他的模仿或者其他表现艺术(representative art)。


Etymology.

The term "azeotrope" is derived from the Greek words ζέειν (boil) and τρόπος (turning) with the prefix α- (no) to give the overall meaning, "no change on boiling". The term was coined in 1911 by English chemist John Wade and Richard William Merriman.

Types.

Positive and negative azeotropes.

Each azeotrope has a characteristic boiling point. The boiling point of an azeotrope is either less than the boiling point temperatures of any of its constituents (a positive azeotrope), or greater than the boiling point of any of its constituents (a negative azeotrope).  A well-known example of a positive azeotrope is 95.63% ethanol and 4.37% water (by mass) boils at 78.2 °C. Ethanol boils at 78.4 °C, water boils at 100 °C, but the azeotrope boils at 78.2 °C, which is lower than either of its constituents. Indeed, 78.2 °C is the minimum temperature at which any ethanol/water solution can boil at atmospheric pressure. In general, a positive azeotrope boils at a lower temperature than any other ratio of its constituents. Positive azeotropes are also called "minimum boiling mixtures" or "pressure maximum" azeotropes.  In general, a negative azeotrope boils at a higher temperature than any other ratio of its constituents. Negative azeotropes are also called "maximum boiling mixtures" or "pressure minimum" azeotropes. An example of a negative azeotrope is hydrochloric acid at a concentration of 20.2% and 79.8% water (by mass). Hydrogen chloride boils at −84 °C and water at 100 °C, but the azeotrope boils at 110 °C, which is higher than either of its constituents. The maximum temperature at which any hydrochloric acid solution can boil is 110 °C. Other examples:

Homogeneous and heterogeneous azeotropes.

If the constituents of a mixture are completely miscible in all proportions with each other, the type of azeotrope is called a "homogeneous azeotrope". For example, any amount of ethanol can be mixed with any amount of water to form a homogeneous solution. If the constituents are not completely miscible, an azeotrope can be found inside the miscibility gap. This type of azeotrope is called "heterogeneous azeotrope" or heteroazeotrope. A heteroazeotropic distillation will have two liquid phases. For example, acetone / methanol / chloroform form an intermediate boiling (saddle) azeotrope.  For example, if equal volumes of chloroform (water solubility 0.8 g/100 ml at 20 °C) and water are shaken together and then left to stand, the liquid will separate into two layers. Analysis of the layers shows that the top layer is mostly water with a small amount of chloroform dissolved in it, and the bottom layer is mostly chloroform with a small amount of water dissolved in it. If the two layers are heated together, the system of layers will boil at 53.3 °C, which is lower than either the boiling point of chloroform (61.2 °C) or the boiling point of water (100 °C). The vapor will consist of 97.0% chloroform and 3.0% water regardless of how much of each liquid layer is present provided both layers are indeed present. If the vapor is re-condensed, the layers will reform in the condensate, and will do so in a fixed ratio, which in this case is 4.4% of the volume in the top layer and 95.6% in the bottom layer. Such a system of solvents is known as a heteroazeotrope. Heteroazeotropes are always minimum boiling mixtures.  The diagram illustrates how the various phases of a heteroazeotrope are related.

Number of constituents.

Azeotropes consisting of two constituents are called "binary" azeotropes such as diethyl ether (33%) / halothane (66%) a mixture once commonly used in anaesthesia. For example, benzene and hexafluorobenzene form a double binary azeotrope.  Azeotropes consisting of three constituents are called "ternary" azeotropes, e.g. acetone / methanol / chloroform. Azeotropes of more than three constituents are also known.

Zeotropy.

Combinations of solvents that do not form an azeotrope when mixed in any proportion are said to be zeotropic. Azeotropes are useful in separating zeotropic mixtures. An example is acetic acid and water, which do not form an azeotrope. Despite this, it is very difficult to separate pure acetic acid (boiling point: 118.1 °C) from a solution of acetic acid and water by distillation alone. As progressive distillations produce solutions with less and less water, each further distillation becomes less effective at removing the remaining water. Distilling the solution to dry acetic acid is therefore economically impractical. But ethyl acetate forms an azeotrope with water that boils at 70.4 °C. By adding ethyl acetate as an entrainer, it is possible to distill away the azeotrope and leave nearly pure acetic acid as the residue.

Mechanism.

Condition of existence.

The condition relates activity coefficients in liquid phase to total pressure and the vapour pressures of pure components. Azeotropes can form only when a mixture deviates from Raoult's law, the equality of compositions in liquid phase and vapor phases, in vapour-liquid equilibrium and Dalton's law the equality of pressures for total pressure being equal to the sum of the partial pressures in real mixtures.  In other words: Raoult's law predicts the vapor pressures of ideal mixtures as a function of composition ratio. More simply: per Raoult's law molecules of the constituents stick to each other to the same degree as they do to themselves. For example, if the constituents are X and Y, then X sticks to Y with roughly equal energy as X does with X and Y does with Y. A so-called "positive deviation" from Raoult's law results when the constituents have a disaffinity for each other – that is X sticks to X and Y to Y better than X sticks to Y. Because this results in the mixture having less total affinity of the molecules than the pure constituents, they more readily escape from the stuck-together phase, which is to say the liquid phase, and into the vapor phase. When X sticks to Y more aggressively than X does to X and Y does to Y, the result is a "negative deviation" from Raoult's law. In this case because the molecules in the mixture are sticking together more than in the pure constituents, they are more reluctant to escape the stuck-together liquid phase.  When the deviation is great enough to cause a maximum or minimum in the vapor pressure versus composition function, it is a mathematical consequence that at that point, the vapor will have the same composition as the liquid, resulting in an azeotrope.  The adjacent diagram illustrates total vapor pressure of three hypothetical mixtures of constituents, X, and Y. The temperature throughout the plot is assumed to be constant.  The center trace is a straight line, which is what Raoult's law predicts for an ideal mixture. In general solely mixtures of chemically similar solvents, such as "n"-hexane with "n"-heptane, form nearly ideal mixtures that come close to obeying Raoult's law. The top trace illustrates a nonideal mixture that has a positive deviation from Raoult's law, where the total combined vapor pressure of constituents, X and Y, is greater than what is predicted by Raoult's law. The top trace deviates sufficiently that there is a point on the curve where its tangent is horizontal. Whenever a mixture has a positive deviation and has a point at which the tangent is horizontal, the composition at that point is a positive azeotrope. At that point the total vapor pressure is at a maximum. Likewise the bottom trace illustrates a nonideal mixture that has a negative deviation from Raoult's law, and at the composition where tangent to the trace is horizontal there is a negative azeotrope. This is also the point where total vapor pressure is minimum.

"Minimum-boiling" or "Positive" azeotrope.

The boiling and recondensation of a mixture of two solvents are changes of chemical state; as such, they are best illustrated with a phase diagram. If the pressure is held constant, the two variable parameters are the temperature and the composition.  The phase diagram on the right shows a "positive" azeotrope of hypothetical constituents, X and Y. The bottom trace illustrates the boiling temperature of various compositions. Below the bottom trace, only the liquid phase is in equilibrium. The top trace illustrates the vapor composition above the liquid at a given temperature. Above the top trace, only the vapor is in equilibrium. Between the two traces, liquid and vapor phases exist simultaneously in equilibrium: for example, heating a 25% X : 75% Y mixture to temperature AB would generate vapor of composition B over liquid of composition A. The azeotrope is the point on the diagram where the two curves touch. The horizontal and vertical steps show the path of repeated distillations. Point A is the boiling point of a nonazeotropic mixture. The vapor that separates at that temperature has composition B. The shape of the curves requires that the vapor at B be richer in constituent X than the liquid at point A. The vapor is physically separated from the VLE (vapor-liquid equilibrium) system and is cooled to point C, where it condenses. The resulting liquid (point C) is now richer in X than it was at point A. If the collected liquid is boiled again, it progresses to point D, and so on. The stepwise progression shows how repeated distillation can never produce a distillate that is richer in constituent X than the azeotrope. Note that starting to the right of the azeotrope point results in the same stepwise process closing in on the azeotrope point from the other direction.

"Maximum-boiling" or "Negative" azeotrope.

The phase diagram on the right shows a "negative" azeotrope of ideal constituents, X and Y. Again the bottom trace illustrates the boiling temperature at various compositions, and again, below the bottom trace the mixture must be entirely liquid phase. The top trace again illustrates the condensation temperature of various compositions, and again, above the top trace the mixture must be entirely vapor phase. The point, A, shown here is a boiling point with a composition chosen very near to the azeotrope. The vapor is collected at the same temperature at point B. That vapor is cooled, condensed, and collected at point C. Because this example is a negative azeotrope rather than a positive one, the distillate is "farther" from the azeotrope than the original liquid mixture at point A was. So the distillate is poorer in constituent X and richer in constituent Y than the original mixture. Because this process has removed a greater fraction of Y from the liquid than it had originally, the residue must be poorer in Y and richer in X after distillation than before.  If the point, A had been chosen to the right of the azeotrope rather than to the left, the distillate at point C would be farther to the right than A, which is to say that the distillate would be richer in X and poorer in Y than the original mixture. So in this case too, the distillate moves away from the azeotrope and the residue moves toward it. This is characteristic of negative azeotropes. No amount of distillation, however, can make either the distillate or the residue arrive on the opposite side of the azeotrope from the original mixture. This is characteristic of "all" azeotropes.

Traces.

The traces in the phase diagrams separate whenever the composition of the vapor differs from the composition of the liquid at the same temperature. Suppose the total composition were 50/50%. You could make this composition using 50% of 50/50% vapor and 50% of 50/50% liquid, but you could also make it from 83.33% of 45/55% vapor and 16.67% of 75%/25% liquid, as well as from many other combinations. The separation of the two traces represents the range of combinations of liquid and vapor that can make each total composition.

Temperature-pressure dependence.

For both the top and bottom traces, the temperature point of the azeotrope is the constant temperature chosen for the graph. If the ambient pressure is controlled to be equal to the total vapor pressure at the azeotropic mixture, then the mixture will boil at this fixed temperature.  Vapor pressure of both pure liquids as well as mixtures is a sensitive function of temperature. As a rule, vapor pressure of a liquid increases nearly exponentially as a function of temperature. If the graph were replotted for a different fixed temperature, then the total vapor pressure at the azeotropic composition will certainly change, but it is also possible that the composition at which the azeotrope occurs will change. This implies that the composition of an azeotrope is affected by the pressure chosen at which to boil the mixture. Ordinarily distillation is done at atmospheric pressure, but with proper equipment it is possible to carry out distillation at a wide variety of pressures, both above and below atmospheric pressure.

Separation of constituents.

If two solvents can form a negative azeotrope, then distillation of any mixture of those constituents will result in the residue being closer to the composition at the azeotrope than the original mixture. For example, if a hydrochloric acid solution contains less than 20.2% hydrogen chloride, boiling the mixture will leave behind a solution that is richer in hydrogen chloride than the original. If the solution initially contains more than 20.2% hydrogen chloride, then boiling will leave behind a solution that is poorer in hydrogen chloride than the original. Boiling of any hydrochloric acid solution long enough will cause the solution left behind to approach the azeotropic ratio.. On the other hand, if two solvents can form a positive azeotrope, then distillation of any mixture of those constituents will result in the residue away from the composition at the azeotrope than the original mixture. For example, if a 50/50 mixture of ethanol and water is distilled once, the distillate will be 80% ethanol and 20% water, which is closer to the azeotropic mixture than the original, which means the solution left behind will be poorer in ethanol. Distilling the 80/20% mixture produces a distillate that is 87% ethanol and 13% water. Further repeated distillations will produce mixtures that are progressively closer to the azeotropic ratio of 95.5/4.5%. No numbers of distillations will ever result in a distillate that exceeds the azeotropic ratio. Likewise, when distilling a mixture of ethanol and water that is richer in ethanol than the azeotrope, the distillate (contrary to intuition) will be poorer in ethanol than the original but still richer than the azeotrope.  Distillation is one of the primary tools that chemists and chemical engineers use to separate mixtures into their constituents. Because distillation cannot separate the constituents of an azeotrope, the separation of azeotropic mixtures (also called "azeotrope breaking") is a topic of considerable interest. Indeed, this difficulty led some early investigators to believe that azeotropes were actually compounds of their constituents. But there are two reasons for believing that this is not the case. One is that the molar ratio of the constituents of an azeotrope is not generally the ratio of small integers. For example, the azeotrope formed by water and acetonitrile contains 2.253 moles ( or 9/4 with a relative error of just 2% ) of acetonitrile for each mole of water. A more compelling reason for believing that azeotropes are not compounds is, as discussed in the last section, that the composition of an azeotrope can be affected by pressure. Contrast that with a true compound, carbon dioxide for example, which is two moles of oxygen for each mole of carbon no matter what pressure the gas is observed at. That azeotropic composition can be affected by pressure suggests a means by which such a mixture can be separated.

Pressure swing distillation.

A hypothetical azeotrope of constituents X and Y is shown in the adjacent diagram. Two sets of curves on a phase diagram one at an arbitrarily chosen low pressure and another at an arbitrarily chosen, but higher, pressure. The composition of the azeotrope is substantially different between the high- and low-pressure plots – higher in X for the high-pressure system. The goal is to separate X in as high a concentration as possible starting from point A. At the low pressure, it is possible by progressive distillation to reach a distillate at the point, B, which is on the same side of the azeotrope as A. Note that successive distillation steps near the azeotropic composition exhibit very little difference in boiling temperature. If this distillate is now exposed to the high pressure, it boils at point C. From C, by progressive distillation it is possible to reach a distillate at the point D, which is on the same side of the high-pressure azeotrope as C. If that distillate is then exposed again to the low pressure, it boils at point E, which is on the "opposite" side of the low-pressure azeotrope to A. So, by means of the pressure swing, it is possible to cross over the low-pressure azeotrope.  When the solution is boiled at point E, the distillate is poorer in X than the residue at point E. This means that the residue is richer in X than the distillate at point E. Indeed, progressive distillation can produce a residue as rich in X as is required.  In summary: 1. Low-pressure rectification (A to B) 2. High-pressure rectification (C to D) 3. Low-pressure stripping (E to target purity)  Note that both azeotropes above are of the "positive", or "minimum boiling" type; care must be taken to ensure that the correct component of the separation step is retained, i.e. the binary phase-envelope diagram (boiling-point curve) must be correctly read.  A mixture of 5% water with 95% tetrahydrofuran is an example of an azeotrope that can be economically separated using a pressure swing – a swing in this case between 1 atm and 8 atm. By contrast the composition of the water to ethanol azeotrope discussed earlier is not affected enough by pressure to be easily separated using pressure swings and instead, an entrainer may be added that "either" modifies the azeotropic composition and exhibits immiscibility with one of the components, "or" extractive distillation may be used.

Azeotropic distillation.

Other methods of separation involve introducing an additional agent, called an "entrainer", that will affect the volatility of one of the azeotrope constituents more than another. When an entrainer is added to a binary azeotrope to form a ternary azeotrope, and the resulting mixture distilled, the method is called azeotropic distillation. The best known example is adding benzene or cyclohexane to the water/ethanol azeotrope. With cyclohexane as the entrainer, the ternary azeotrope is 7% water, 17% ethanol, and 76% cyclohexane, and boils at 62.1 °C. Just enough cyclohexane is added to the water/ethanol azeotrope to engage all of the water into the ternary azeotrope. When the mixture is then boiled, the azeotrope vaporizes leaving a residue composed almost entirely of the excess ethanol.

Chemical action separation.

Another type of entrainer is one that has a strong chemical affinity for one of the constituents. Using again the example of the water/ethanol azeotrope, the liquid can be shaken with calcium oxide, which reacts strongly with water to form the nonvolatile compound, calcium hydroxide. Nearly all of the calcium hydroxide can be separated by filtration and the filtrate redistilled to obtain 100% pure ethanol.  A more extreme example is the azeotrope of 1.2% water with 98.8% diethyl ether. Ether holds the last bit of water so tenaciously that only a very powerful desiccant such as sodium metal added to the liquid phase can result in completely dry ether.  Anhydrous calcium chloride is used as a desiccant for drying a wide variety of solvents since it is inexpensive and does not react with most nonaqueous solvents. Chloroform is an example of a solvent that can be effectively dried using calcium chloride.

Distillation using a dissolved salt.

When a salt is dissolved in a solvent, it always has the effect of raising the boiling point of that solvent – that is it decreases the volatility of the solvent. When the salt is readily soluble in one constituent of a mixture but not in another, the volatility of the constituent in which it is soluble is decreased and the other constituent is unaffected. In this way, for example, it is possible to break the water/ethanol azeotrope by dissolving potassium acetate in it and distilling the result.

Extractive distillation.

Extractive distillation is similar to azeotropic distillation, except in this case the entrainer is less volatile than any of the azeotrope's constituents. For example, the azeotrope of 20% acetone with 80% chloroform can be broken by adding water and distilling the result. The water forms a separate layer in which the acetone preferentially dissolves. The result is that the distillate is richer in chloroform than the original azeotrope.

Pervaporation and other membrane methods.

The pervaporation method uses a membrane that is more permeable to the one constituent than to another to separate the constituents of an azeotrope as it passes from liquid to vapor phase. The membrane is rigged to lie between the liquid and vapor phases. Another membrane method is vapor permeation, where the constituents pass through the membrane entirely in the vapor phase. In all membrane methods, the membrane separates the fluid passing through it into a permeate (that which passes through) and a retentate (that which is left behind). When the membrane is chosen so that is it more permeable to one constituent than another, then the permeate will be richer in that first constituent than the retentate.

Complex systems.

The rules for positive and negative azeotropes apply to all the examples discussed so far, but there are some examples that don't fit into the categories of positive or negative azeotropes. The best known of these is the ternary azeotrope formed by 30% acetone, 47% chloroform, and 23% methanol, which boils at 57.5 °C. Each pair of these constituents forms a binary azeotrope, but chloroform/methanol and acetone/methanol both form positive azeotropes while chloroform/acetone forms a negative azeotrope. The resulting ternary azeotrope is neither positive nor negative. Its boiling point falls "between" the boiling points of acetone and chloroform, so it is neither a maximum nor a minimum boiling point. This type of system is called a saddle azeotrope. Only systems of three or more constituents can form saddle azeotropes.  A rare type of complex binary azeotrope is one where the boiling point and condensation point curves touch at two points in the phase diagram. Such a system is called a double azeotrope, and will have two azeotropic compositions and boiling points. An example is water and "N"-methylethylenediamine.

An azeotrope () or a constant boiling point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation. This happens because when an azeotrope is boiled, the vapour has the same proportions of constituents as the unboiled mixture. Because their composition is unchanged by distillation, azeotropes are also called (especially in older texts) constant boiling point mixtures.

共沸(英语:Azeotrope),是指两组分或多组分的液体混合物以特定比例组成时,在恒定压力下沸腾,其蒸气组成比例与溶液相同的现象。这实际是表明,此时沸腾产生的蒸气与液体本身有着完全相同的组成。共沸物是不可能通过常规的蒸馏或分馏手段加以分离的。


Applications.

DICOM is used worldwide to store, exchange, and transmit medical images. DICOM has been central to the development of modern radiological imaging: DICOM incorporates standards for imaging modalities such as radiography, ultrasonography, computed tomography (CT), magnetic resonance imaging (MRI), and radiation therapy. DICOM includes protocols for image exchange (e.g., via portable media such as DVDs), image compression, 3-D visualization, image presentation, and results reporting.

Parts of the standard.

The DICOM standard is divided into related but independent parts.

History.

DICOM is a standard developed by American College of Radiology (ACR) and National Electrical Manufacturers Association (NEMA).  In the beginning of the 1980s, it was very difficult for anyone other than manufacturers of computed tomography or magnetic resonance imaging devices to decode the images that the machines generated. Radiologists and medical physicists wanted to use the images for dose-planning for radiation therapy. ACR and NEMA joined forces and formed a standard committee in 1983. Their first standard, ACR/NEMA 300, entitled "Digital Imaging and Communications", was released in 1985. Very soon after its release, it became clear that improvements were needed. The text was vague and had internal contradictions.  In 1988 the second version was released. This version gained more acceptance among vendors. The image transmission was specified as over a dedicated 2 pair cable (EIA-485). The first demonstration of ACR/NEMA V2.0 interconnectivity technology was held at Georgetown University, May 21–23, 1990. Six companies participated in this event, DeJarnette Research Systems, General Electric Medical Systems, Merge Technologies, Siemens Medical Systems, Vortech (acquired by Kodak that same year) and 3M. Commercial equipment supporting ACR/NEMA 2.0 was presented at the annual meeting of the Radiological Society of North America (RSNA) in 1990 by these same vendors. Many soon realized that the second version also needed improvement. Several extensions to ACR/NEMA 2.0 were created, like Papyrus (developed by the University Hospital of Geneva, Switzerland) and SPI (Standard Product Interconnect), driven by Siemens Medical Systems and Philips Medical Systems.  The first large-scale deployment of ACR/NEMA technology was made in 1992 by the US Army and Air Force, as part of the MDIS (Medical Diagnostic Imaging Support) program based at Ft. Detrick, Maryland. Loral Aerospace and Siemens Medical Systems led a consortium of companies in deploying the first US military PACS (Picture Archiving and Communications System) at all major Army and Air Force medical treatment facilities and teleradiology nodes at a large number of US military clinics. DeJarnette Research Systems and Merge Technologies provided the modality gateway interfaces from third party imaging modalities to the Siemens SPI network. The Veterans Administration and the Navy also purchased systems from this contract.  In 1993 the third version of the standard was released. Its name was then changed to "Digital Imaging and Communications in Medicine", abbreviated DICOM. New service classes were defined, network support added and the Conformance Statement was introduced. Initially the DICOM standard was referred to as "DICOM 3.0" to distinguish it from its predecessors. DICOM has been constantly updated and extended since 1993, with the intent that changes are backward compatible, except in rare cases where the earlier specification was incorrect or ambiguous. Officially there is no "version" of the standard except the current standard, hence the "3.0" version number is no longer used. There are no "minor" versions to the standard (e.g., no such thing as "DICOM 3.1") and there are no current plans to develop a new, incompatible, version of the standard (i.e., no "DICOM 4.0"). The standard should be referenced without specification of the date of release of a particular published edition, except when specific conformance requirements are invoked that depend on a retired feature that is no longer documented in the current standard.  While the DICOM standard has achieved a near universal level of acceptance amongst medical imaging equipment vendors and healthcare IT organizations, the standard has its limitations. DICOM is a standard directed at addressing technical interoperability issues in medical imaging. It is not a framework or architecture for achieving a useful clinical workflow. The Integrating the Healthcare Enterprise (IHE) initiative layered on top of DICOM (and HL-7) defines profiles to select features from these standards to implement transactions for specific medical imaging interoperability use cases.  Though always Internet compatible and based on transport over TCP, over time there has been an increasing need to support port 80 HTTP transport to make use easier within the web browser. Most recently, a family of DICOM RESTful web services have been defined to allow mobile device friendly access to DICOM objects and services, which include WADO-RS, STOW-RS and QIDO-RS, which together constitute the DICOMweb initiative.

Derivations.

There are some derivations from the DICOM standard into other application areas. These include DICONDE ("Digital Imaging and Communication in Nondestructive Evaluation") that was established in 2004 as a way for nondestructive testing manufacturers and users to share image data, and DICOS ("Digital Imaging and Communication in Security") that was established in 2009 to be used for image sharing in airport security.

Data format.

DICOM groups information into data sets. For example, a file of a chest x-ray image may contain the patient ID within the file, so that the image can never be separated from this information by mistake. This is similar to the way that image formats such as JPEG can also have embedded tags to identify and otherwise describe the image.  A DICOM data object consists of a number of attributes, including items such as name, ID, etc., and also one special attribute containing the image pixel data (i.e. logically, the main object has no "header" as such, being merely a list of attributes, including the pixel data). A single DICOM object can have only one attribute containing pixel data. For many modalities, this corresponds to a single image. However, the attribute may contain multiple "frames", allowing storage of cine loops or other multi-frame data. Another example is NM data, where an NM image, by definition, is a multi-dimensional multi-frame image. In these cases, three- or four-dimensional data can be encapsulated in a single DICOM object. Pixel data can be compressed using a variety of standards, including JPEG, lossless JPEG, JPEG 2000, and run-length encoding (RLE). LZW (zip) compression can be used for the whole data set (not just the pixel data), but this has rarely been implemented.  DICOM uses three different data element encoding schemes. With explicit value representation (VR) data elements, for VRs that are not OB, OW, OF, SQ, UT, or UN, the format for each data element is: GROUP (2 bytes) ELEMENT (2 bytes) VR (2 bytes) LengthInByte (2 bytes) Data (variable length). For the other explicit data elements or implicit data elements, see section 7.1 of Part 5 of the DICOM Standard.  The same basic format is used for all applications, including network and file usage, but when written to a file, usually a true "header" (containing copies of a few key attributes and details of the application which wrote it) is added.

Image display.

To promote identical grayscale image display on different monitors and consistent hard-copy images from various printers, the DICOM committee developed a lookup table to display digitally assigned pixel values. To use the DICOM grayscale standard display function (GSDF), images must be viewed (or printed) on devices that have this lookup curve or on devices that have been calibrated to the GSDF curve.

Value representations.

In addition to a value representation, each attribute also has a "value multiplicity" to indicate the number of data elements contained in the attribute. For character string value representations, if more than one data element is being encoded, the successive data elements are separated by the backslash character "\".

Services.

DICOM consists of services, most of which involve transmission of data over a network. The file format for offline media is a later addition to the standard.

Store.

The DICOM Store service is used to send images or other persistent objects (structured reports, etc.) to a picture archiving and communication system (PACS) or workstation.

Storage commitment.

The DICOM storage commitment service is used to confirm that an image has been permanently stored by a device (either on redundant disks or on backup media, e.g. burnt to a CD). The Service Class User (SCU: similar to a client), a modality or workstation, etc., uses the confirmation from the Service Class Provider (SCP: similar to a server), an archive station for instance, to make sure that it is safe to delete the images locally.

Query/retrieve.

This enables a workstation to find lists of images or other such objects and then retrieve them from a picture archiving and communication system.

Modality worklist.

The DICOM modality worklist service provides a list of imaging procedures that have been scheduled for performance by an image acquisition device (sometimes referred to as a modality system). The items in the worklist include relevant details about the subject of the procedure (patient ID, name, sex, and age), the type of procedure (equipment type, procedure description, procedure code) and the procedure order (referring physician, accession number, reason for exam). An image acquisition device, such as a CT scanner, queries a service provider, such as a RIS, to get this information which is then presented to the system operator and is used by the imaging device to populate details in the image metadata.  Prior to the use of the DICOM modality worklist service, the scanner operator was required to manually enter all the relevant details. Manual entry is slower and introduces the risk of misspelled patient names, and other data entry errors.

Modality performed procedure step.

A complementary service to modality worklist, this enables the modality to send a report about a performed examination including data about the images acquired, beginning time, end time, and duration of a study, dose delivered, etc. It helps give the radiology department a more precise handle on resource (acquisition station) use. Also known as MPPS, this service allows a modality to better coordinate with image storage servers by giving the server a list of objects to send before or while actually sending such objects.

Print.

The DICOM print service is used to send images to a DICOM printer, normally to print an "X-Ray" film. There is a standard calibration (defined in DICOM Part 14) to help ensure consistency between various display devices, including hard copy printout.

Off-line media (files).

The format for off-line media files is specified in Part 10 of the DICOM Standard. Such files are sometimes referred to as "Part 10 files".  DICOM restricts the filenames on DICOM media to 8 characters (some systems wrongly use 8.3, but this does not conform to the standard). No information must be extracted from these names (PS3.10 Section 6.2.3.2). This is a common source of problems with media created by developers who did not read the specifications carefully. This is a historical requirement to maintain compatibility with older existing systems. It also mandates the presence of a media directory, the DICOMDIR file, which provides index and summary information for all the DICOM files on the media. The DICOMDIR information provides substantially greater information about each file than any filename could, so there is less need for meaningful file names.  DICOM files typically have a.dcm file extension if they are not part of a DICOM media (which requires them to be without extension).  The MIME type for DICOM files is defined by RFC 3240 as application/dicom.  The Uniform Type Identifier type for DICOM files is org.nema.dicom.  There is also an ongoing media exchange test and "connectathon" process for CD media and network operation that is organized by the IHE organization.

Application areas.

The core application of the DICOM standard is to capture, store and distribute medical images. The standard also provides services related to imaging such as managing imaging procedure worklists, printing images on film or digital media like DVDs, reporting procedure status like completion of an imaging acquisition, confirming successful archiving of images, encrypting datasets, removing patient identifying information from datasets, organizing layouts of images for review, saving image manipulations and annotations, calibrating image displays, encoding ECGs, encoding CAD results, encoding structured measurement data, and storing acquisition protocols.

Types of equipment.

The DICOM information object definitions encode the data produced by a wide variety of imaging device types, including, CT (computed tomography), MRI (magnetic resonance imaging), ultrasound, X-ray, fluoroscopy, angiography, mammography, breast tomosynthesis, PET (positron emission tomography), SPECT (single photon emission computed tomography), Endoscopy, microscopy, nd whole slide imaging, OCT (optical coherence tomography).  DICOM is also implemented by devices associated with images or imaging workflow including, PACS (picture archiving and communication systems), image viewers and display stations, CAD (computer-aided detection/diagnosis systems), 3D visualization systems, clinical analysis applications, image printers, Film scanners, media burners (that export DICOM files onto CDs, DVDs, etc.), media importers (that import DICOM files from CDs, DVDs, USBs, etc.), RIS (radiology information systems), VNA (vendor-neutral archives), EMR (electronic medical record) systems, and radiology reporting systems

Fields of medicine.

Many fields of medicine have a dedicated Working Group within DICOM, and DICOM is applicable to any field of medicine in which imaging is prevalent, including:, radiology, cardiology, oncology, nuclear medicine, radiotherapy, neurology, orthopedics, obstetrics, gynecology, ophthalmology, dentistry, maxillofacial surgery, dermatology, pathology, clinical trials, veterinary medicine, and medical/clinical photography

Port numbers over IP.

DICOM have reserved the following TCP and UDP port numbers by the Internet Assigned Numbers Authority (IANA): 104 well-known port for DICOM over Transmission Control Protocol (TCP) or User Datagram Protocol (UDP). Since 104 is in the reserved subset, many operating systems require special privileges to use it; 2761 registered port for DICOM using Integrated Secure Communication Layer (ISCL) over TCP or UDP; 2762 registered port for DICOM using Transport Layer Security (TLS) over TCP or UDP; 11112 registered port for DICOM using standard, open communication over TCP or UDP. The standard recommends but does not require the use of these port numbers.

Disadvantages.

According to a paper presented at an international symposium in 2008, the DICOM standard has problems related to data entry. "A major disadvantage of the DICOM Standard is the possibility for entering probably too many optional fields. This disadvantage is mostly showing in inconsistency of filling all the fields with the data. Some image objects are often incomplete because some fields are left blank and some are filled with incorrect data."  On the other hand, the file format admits executable code and may contain malware.

Related standards and SDOs.

DVTk is an Open Source project for testing, validating and diagnosing communication protocols and scenarios in medical environments. It supports DICOM, HL7 and IHE integration profiles.  Health Level 7 is a non-profit organization involved in the development of international healthcare informatics interoperability standards. HL7 and DICOM manage a joint Working Group to harmonize areas where the two standards overlap and address imaging integration in the electronic medical record.  Integrating the Healthcare Enterprise (IHE) is an industry sponsored non-profit organization that profiles the use of standards to address specific healthcare use cases. DICOM is incorporated in a variety of imaging related IHE profiles.  Systematized Nomenclature of Medicine (SNOMED) is a systematic, computer-processable collection of medical terms, in human and veterinary medicine, to provide codes, terms, synonyms and definitions which cover anatomy, diseases, findings, procedures, microorganisms, substances, etc. DICOM data makes use of SNOMED to encode relevant concepts.  XnView supports codice_1 / codice_2 for MIME type codice_3

Standards used by DICOM.

The best known standards and protocols used by DICOM are:

Standards that use DICOM.

The DICOM standard is used in a wide variety of resources (IHE, HL7... a) that are related to images.  The ISO12052: 2017 and CEN 12052 standards refer to the DICOM standard.

Digital Imaging and Communications in Medicine (DICOM) is the standard for the communication and management of medical imaging information and related data. DICOM is most commonly used for storing and transmitting medical images enabling the integration of medical imaging devices such as scanners, servers, workstations, printers, network hardware, and picture archiving and communication systems (PACS) from multiple manufacturers. It has been widely adopted by hospitals and is making inroads into smaller applications like dentists' and doctors' offices.

医疗数码影像传输协定(英语:DICOM,英语:Digital Imaging and Communications in Medicine)是一组通用的标准协定,在对于医学影像的处理、储存、打印、传输上。它包含了档案格式的定义及网络通信协定。DICOM是以TCP/IP为基础的应用协定,并以TCP/IP联系各个系统。两个能接受DICOM格式的医疗仪器间,可借由DICOM格式的档案,来接收与交换影像及病人资料。


History.

In 1824, the French physicist François Arago formulated the existence of rotating magnetic fields, termed Arago's rotations. By manually turning switches on and off, Walter Baily demonstrated this in 1879, effectively the first primitive induction motor.  The first commutator-free single-phase AC induction motor was invented by Hungarian engineer Ottó Bláthy; he used the single phase motor to propel his invention, the electricity meter.  The first AC commutator-free three-phase induction motors were independently invented by Galileo Ferraris and Nikola Tesla, a working motor model having been demonstrated by the former in 1885 and by the latter in 1887. Tesla applied for US patents in October and November 1887 and was granted some of these patents in May 1888. In April 1888, the "Royal Academy of Science of Turin" published Ferraris's research on his AC polyphase motor detailing the foundations of motor operation. In May 1888 Tesla presented the technical paper "A New System for Alternating Current Motors and Transformers" to the "American Institute of Electrical Engineers" (AIEE)  George Westinghouse, who was developing an alternating current power system at that time, licensed Tesla's patents in 1888 and purchased a US patent option on Ferraris' induction motor concept. Tesla was also employed for one year as a consultant. Westinghouse employee C. F. Scott was assigned to assist Tesla and later took over development of the induction motor at Westinghouse. Steadfast in his promotion of three-phase development, Mikhail Dolivo-Dobrovolsky invented the cage-rotor induction motor in 1889 and the three-limb transformer in 1890. Furthermore, he claimed that Tesla's motor was not practical because of two-phase pulsations, which prompted him to persist in his three-phase work. Although Westinghouse achieved its first practical induction motor in 1892 and developed a line of polyphase 60 hertz induction motors in 1893, these early Westinghouse motors were two-phase motors with wound rotors until B. G. Lamme developed a rotating bar winding rotor.  The General Electric Company (GE) began developing three-phase induction motors in 1891. By 1896, General Electric and Westinghouse signed a cross-licensing agreement for the bar-winding-rotor design, later called the squirrel-cage rotor. Arthur E. Kennelly was the first to bring out the full significance of complex numbers (using "j" to represent the square root of minus one) to designate the 90o rotation operator in analysis of AC problems. GE's Charles Proteus Steinmetz greatly developed application of AC complex quantities including an analysis model now commonly known as the induction motor Steinmetz equivalent circuit.  Induction motor improvements flowing from these inventions and innovations were such that a 100-horsepower induction motor currently has the same mounting dimensions as a 7.5-horsepower motor in 1897.

Principle of operation.

In both induction and synchronous motors, the AC power supplied to the motor's stator creates a magnetic field that rotates in synchronism with the AC oscillations. Whereas a synchronous motor's rotor turns at the same rate as the stator field, an induction motor's rotor rotates at a somewhat slower speed than the stator field. The induction motor stator's magnetic field is therefore changing or rotating relative to the rotor. This induces an opposing current in the induction motor's rotor, in effect the motor's secondary winding, when the latter is short-circuited or closed through an external impedance. The rotating magnetic flux induces currents in the windings of the rotor, in a manner similar to currents induced in a transformer's secondary winding(s).  The induced currents in the rotor windings in turn create magnetic fields in the rotor that react against the stator field. The direction of the magnetic field created will be such as to oppose the change in current through the rotor windings, in agreement with Lenz's Law. The cause of induced current in the rotor windings is the rotating stator magnetic field, so to oppose the change in rotor-winding currents the rotor will start to rotate in the direction of the rotating stator magnetic field. The rotor accelerates until the magnitude of induced rotor current and torque balances the applied mechanical load on the rotation of the rotor. Since rotation at synchronous speed would result in no induced rotor current, an induction motor always operates slightly slower than synchronous speed. The difference, or "slip," between actual and synchronous speed varies from about 0.5% to 5.0% for standard Design B torque curve induction motors. The induction motor's essential character is that it is created solely by induction instead of being separately excited as in synchronous or DC machines or being self-magnetized as in permanent magnet motors.  For rotor currents to be induced, the speed of the physical rotor must be lower than that of the stator's rotating magnetic field (formula_1); otherwise the magnetic field would not be moving relative to the rotor conductors and no currents would be induced. As the speed of the rotor drops below synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and creating more torque. The ratio between the rotation rate of the magnetic field induced in the rotor and the rotation rate of the stator's rotating field is called "slip". Under load, the speed drops and the slip increases enough to create sufficient torque to turn the load. For this reason, induction motors are sometimes referred to as "asynchronous motors".  An induction motor can be used as an induction generator, or it can be unrolled to form a linear induction motor which can directly generate linear motion. The generating mode for induction motors is complicated by the need to excite the rotor, which begins with only residual magnetization. In some cases, that residual magnetization is enough to self-excite the motor under load. Therefore, it is necessary to either snap the motor and connect it momentarily to a live grid or to add capacitors charged initially by residual magnetism and providing the required reactive power during operation. Similar is the operation of the induction motor in parallel with a synchronous motor serving as a power factor compensator. A feature in the generator mode in parallel to the grid is that the rotor speed is higher than in the driving mode. Then active energy is being given to the grid. Another disadvantage of induction motor generator is that it consumes a significant magnetizing current I = (20-35)%.

Synchronous speed.

An AC motor's synchronous speed, formula_1, is the rotation rate of the stator's magnetic field,  where formula_4 is the frequency of the power supply, formula_5 is the number of magnetic poles, and formula_1 is the synchronous speed of the machine. For formula_4 in hertz and formula_1 in RPM, the formula becomes:  For example, for a four-pole, three-phase motor, formula_5 = 4 and formula_11 = 1,500 RPM (for formula_4 = 50 Hz) and 1,800 RPM (for formula_4 = 60 Hz) synchronous speed.  The two figures at right and left above each illustrate a 2-pole 3-phase machine consisting of three pole-pairs with each pole set 60° apart.

Slip.

Slip, formula_14, is defined as the difference between synchronous speed and operating speed, at the same frequency, expressed in rpm, or in percentage or ratio of synchronous speed. Thus  where formula_1 is stator electrical speed, formula_17 is rotor mechanical speed. Slip, which varies from zero at synchronous speed and 1 when the rotor is stalled, determines the motor's torque. Since the short-circuited rotor windings have small resistance, even a small slip induces a large current in the rotor and produces significant torque. At full rated load, slip varies from more than 5% for small or special purpose motors to less than 1% for large motors. These speed variations can cause load-sharing problems when differently sized motors are mechanically connected. Various methods are available to reduce slip, VFDs often offering the best solution.

Torque.

Standard torque.

The typical speed-torque relationship of a standard NEMA Design B polyphase induction motor is as shown in the curve at right. Suitable for most low performance loads such as centrifugal pumps and fans, Design B motors are constrained by the following typical torque ranges:  Over a motor's normal load range, the torque's slope is approximately linear or proportional to slip because the value of rotor resistance divided by slip, formula_18, dominates torque in a linear manner. As load increases above rated load, stator and rotor leakage reactance factors gradually become more significant in relation to formula_18 such that torque gradually curves towards breakdown torque. As the load torque increases beyond breakdown torque the motor stalls.

Starting.

There are three basic types of small induction motors: split-phase single-phase, shaded-pole single-phase, and polyphase.  In two-pole single-phase motors, the torque goes to zero at 100% slip (zero speed), so these require alterations to the stator such as shaded-poles to provide starting torque. A single phase induction motor requires separate starting circuitry to provide a rotating field to the motor. The normal running windings within such a single-phase motor can cause the rotor to turn in either direction, so the starting circuit determines the operating direction.  In certain smaller single-phase motors, starting is done by means of a copper wire turn around part of a pole; such a pole is referred to as a shaded pole. The current induced in this turn lags behind the supply current, creating a delayed magnetic field around the shaded part of the pole face. This imparts sufficient rotational field energy to start the motor. These motors are typically used in applications such as desk fans and record players, as the required starting torque is low, and the low efficiency is tolerable relative to the reduced cost of the motor and starting method compared to other AC motor designs.  Larger single phase motors are split-phase motors and have a second stator winding fed with out-of-phase current; such currents may be created by feeding the winding through a capacitor or having it receive different values of inductance and resistance from the main winding. In "capacitor-start" designs, the second winding is disconnected once the motor is up to speed, usually either by a centrifugal switch acting on weights on the motor shaft or a thermistor which heats up and increases its resistance, reducing the current through the second winding to an insignificant level. The "capacitor-run" designs keep the second winding on when running, improving torque. A "resistance start" design uses a starter inserted in series with the startup winding, creating reactance.  Self-starting polyphase induction motors produce torque even at standstill. Available squirrel cage induction motor starting methods include direct-on-line starting, reduced-voltage reactor or auto-transformer starting, star-delta starting or, increasingly, new solid-state soft assemblies and, of course, variable frequency drives (VFDs).  Polyphase motors have rotor bars shaped to give different speed-torque characteristics. The current distribution within the rotor bars varies depending on the frequency of the induced current. At standstill, the rotor current is the same frequency as the stator current, and tends to travel at the outermost parts of the cage rotor bars (by skin effect). The different bar shapes can give usefully different speed-torque characteristics as well as some control over the inrush current at startup.  Although polyphase motors are inherently self-starting, their starting and pull-up torque design limits must be high enough to overcome actual load conditions.  In wound rotor motors, rotor circuit connection through slip rings to external resistances allows change of speed-torque characteristics for acceleration control and speed control purposes.

Speed control.

Resistance.

Before the development of semiconductor power electronics, it was difficult to vary the frequency, and cage induction motors were mainly used in fixed speed applications. Applications such as electric overhead cranes used DC drives or wound rotor motors (WRIM) with slip rings for rotor circuit connection to variable external resistance allowing considerable range of speed control. However, resistor losses associated with low speed operation of WRIMs is a major cost disadvantage, especially for constant loads. Large slip ring motor drives, termed slip energy recovery systems, some still in use, recover energy from the rotor circuit, rectify it, and return it to the power system using a VFD.

Cascade.

The speed of a pair of slip-ring motors can be controlled by a cascade connection, or concatenation. The rotor of one motor is connected to the stator of the other. If the two motors are also mechanically connected, they will run at half speed. This system was once widely used in three-phase AC railway locomotives, such as FS Class E.333.

Variable-frequency drive.

In many industrial variable-speed applications, DC and WRIM drives are being displaced by VFD-fed cage induction motors. The most common efficient way to control asynchronous motor speed of many loads is with VFDs. Barriers to adoption of VFDs due to cost and reliability considerations have been reduced considerably over the past three decades such that it is estimated that drive technology is adopted in as many as 30–40% of all newly installed motors.<ref name="Motoring Ahead 1/11"></ref>  Variable frequency drives implement the scalar or vector control of an induction motor.  With scalar control, only the magnitude and frequency of the supply voltage are controlled without phase control (absent feedback by rotor position). Scalar control is suitable for application where the load is constant.  Vector control allows to independently control the speed and torque of the motor, hence, makes it possible to maintain a constant rotation speed at varying load torque. But vector control is more expensive in implementation because the sensor (not always) and more powerful controller are required.

Construction.

The stator of an induction motor consists of poles carrying supply current to induce a magnetic field that penetrates the rotor. To optimize the distribution of the magnetic field, windings are distributed in slots around the stator, with the magnetic field having the same number of north and south poles. Induction motors are most commonly run on single-phase or three-phase power, but two-phase motors exist; in theory, induction motors can have any number of phases. Many single-phase motors having two windings can be viewed as two-phase motors, since a capacitor is used to generate a second power phase 90° from the single-phase supply and feeds it to the second motor winding. Single-phase motors require some mechanism to produce a rotating field on startup. Cage induction motor rotor's conductor bars are typically skewed to avoid magnetic locking.  Standardized NEMA & IEC motor frame sizes throughout the industry result in interchangeable dimensions for shaft, foot mounting, general aspects as well as certain motor flange aspect. Since an open, drip proof (ODP) motor design allows a free air exchange from outside to the inner stator windings, this style of motor tends to be slightly more efficient because the windings are cooler. At a given power rating, lower speed requires a larger frame.

Rotation reversal.

The method of changing the direction of rotation of an induction motor depends on whether it is a three-phase or single-phase machine. In the case of three-phase, reversal is straightforwardly implemented by swapping connection of any two phase conductors.  In a single-phase split-phase motor, reversal is achieved by changing the connection between the primary winding and the start circuit. Some single-phase split-phase motors that are designed for specific applications may have the connection between the primary winding and the start circuit connected internally so that the rotation cannot be changed. Also, single-phase shaded-pole motors have a fixed rotation, and the direction cannot be changed except by disassembly of the motor and reversing the stator to face opposite relative to the original rotor direction.

Power factor.

The power factor of induction motors varies with load, typically from around 0.85 or 0.90 at full load to as low as about 0.20 at no-load, due to stator and rotor leakage and magnetizing reactances. Power factor can be improved by connecting capacitors either on an individual motor basis or, by preference, on a common bus covering several motors. For economic and other considerations, power systems are rarely power factor corrected to unity power factor. Power capacitor application with harmonic currents requires power system analysis to avoid harmonic resonance between capacitors and transformer and circuit reactances. Common bus power factor correction is recommended to minimize resonant risk and to simplify power system analysis.

Efficiency.

Full-load motor efficiency is around 85–97%, related motor losses being broken down roughly as follows:  Various regulatory authorities in many countries have introduced and implemented legislation to encourage the manufacture and use of higher efficiency electric motors. There is existing and forthcoming legislation regarding the future mandatory use of premium-efficiency induction-type motors in defined equipment. "For more information, see: Premium efficiency."

Steinmetz equivalent circuit.

Many useful motor relationships between time, current, voltage, speed, power factor, and torque can be obtained from analysis of the Steinmetz equivalent circuit (also termed T-equivalent circuit or IEEE recommended equivalent circuit), a mathematical model used to describe how an induction motor's electrical input is transformed into useful mechanical energy output. The equivalent circuit is a single-phase representation of a multiphase induction motor that is valid in steady-state balanced-load conditions.  The Steinmetz equivalent circuit is expressed simply in terms of the following components:  Paraphrasing from Alger in Knowlton, an induction motor is simply an electrical transformer the magnetic circuit of which is separated by an air gap between the stator winding and the moving rotor winding. The equivalent circuit can accordingly be shown either with equivalent circuit components of respective windings separated by an ideal transformer or with rotor components referred to the stator side as shown in the following circuit and associated equation and parameter definition tables.  The following rule-of-thumb approximations apply to the circuit:

Linear induction motor.

Linear induction motors, which work on the same general principles as rotary induction motors (frequently three-phase), are designed to produce straight line motion. Uses include magnetic levitation, linear propulsion, linear actuators, and liquid metal pumping.

An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor can therefore be made without electrical connections to the rotor. An induction motor's rotor can be either wound type or squirrel-cage type.

异步电动机(英语:asynchronous motor)也称为感应电动机(induction motor)是交流电动机的一种,由定子线圈利用电磁感应的方式使转子产生电流,让电动机产生力矩,这样就不用安装永久磁铁。异步电机可按转子绕组形式,分为绕线式和鼠笼式。绕线型的均为三相电动机,而鼠笼式的有三相电动机,也有单相电动机。鼠笼式感应电动机的转子可以不用连接电动机外部的电路。


Principles.

The political purpose of Newspeak is to eliminate the expression of the shades of meaning inherent to ambiguity and nuance from Oldspeak (Standard English) in order to reduce the language's function of communication, by way of simplistic concepts of simple construction—pleasure and pain, happiness and sadness, "goodthink" and "crimethink"—the last one of these which linguistically reinforces the State's totalitarian dominance of the people of Oceania. In Newspeak, English root words function as both nouns and verbs, which reduce the vocabulary available for the speaker to communicate meaning. For example, "think" is both a noun and a verb, thus, the word "thought" is not functionally required to communicate the concepts of thought in Newspeak and therefore is not in the Newspeak vocabulary.  As personal communication, Newspeak is to be spoken in staccato rhythm, using words "with the stress equally distributed between the first syllable and the last" and that are easy to pronounce, which generates speech that is monotonic, physically automatic, and intellectually unconscious, thereby diminishing the possibility of critical thought occurring to the speaker. English words of comparative and superlative meanings and irregular spellings were simplified into regular spellings; thus, "better" becomes "gooder" and "best" becomes "goodest". The prefixes "plus-" and "doubleplus-" are used for emphasis (for example, "plusgood" meaning "very good" and "doubleplusgood" meaning "superlatively good"). Adjectives are formed by adding the suffix "–ful" to a root-word, e.g. "goodthinkful" means "Orthodox in thought."; while adverbs are formed by adding the suffix "–wise", e.g. "goodthinkwise" means "In an orthodox manner".

Grammar.

The grammar of Newspeak is greatly simplifed compared to English. It also has two "outstanding" characteristics: Almost completely interchangeable linguistic functions between the parts of speech, and inflectional regularity in the construction of usages and of words. This means that any word could function as a verb, noun, adjective, or adverb, and (for example) the preterite and the past participle constructions of verbs are alike, with both ending in "–ed." Hence, the Newspeak preterite of the English word "steal" is "stealed," and that of the word "think" is "thinked." Likewise, the past participles of "swim, give, bring, speak," and "take" were, respectively "swimmed, gived, bringed, speaked," and "taked," with all irregular forms (such as "swam, gave," and "brought") being eliminated. The auxiliaries (including "to be"), pronouns, demonstratives, and relatives still inflect irregularly. They mostly follow their use in English, but the word "whom" and the "shall" and "should" tenses were dropped, "whom" being replaced by "who" and "shall" and "should" by "will" and "would".

Suffixes.

In spoken and written Newspeak, suffixes are also used in the elimination of irregular conjugations:  Therefore, the Oldspeak sentence "He ran extremely quickly" would become "He runned doubleplusspeedwise".

Vocabulary.

As a controlled language, Newspeak limits the user's communications (thought, spoken, and written) with a vocabulary that diminishes the intellectual range allowed by Oldspeak (Standard English), which is realised by making root words function as both nouns and verbs; thus, the word "crimethink" denotes two things: (i) A thoughtcrime (noun), and (ii) the action "to commit thoughtcrime" (verb). The adjective is formed with the suffix "–ful" ("crimethinkful"), and the adverb is formed with the suffix "–wise" ("crimethinkwise").  Note: The novel says that the Ministry of Truth uses a jargon "not actually Newspeak, but consisting largely of Newspeak words" for its internal memos. As many of the words in this list (e.g. "bb", "upsub") come from such memos, it is not certain whether those words are actually Newspeak.

A, B, and C vocabularies.

The words of the A vocabulary describe the functional concepts of daily life (e.g. eating and drinking, working and cooking), mostly of Oldspeak words. The words of the B vocabulary are constructed to convey complex ideas; compound words (noun-verb) of political implication mean to impose upon and instill to the user the politically correct mental attitude required by the Party, e.g. the Newspeak word "goodthink" denotes "political orthodoxy", and is inflected according to the grammar of Standard English. The words of the C vocabulary are technical terms that supplement the linguistic functions of the A and B vocabularies. Distribution of the C vocabulary is limited, because the Party do not want the citizens of Oceania to know more than one way of life and techniques of production. Hence, the Oldspeak word "science" has no equivalent term in Newspeak, instead, there are specific technical words for speaking of technical fields.

Thought control.

The intellectual purpose of Newspeak is to express Ingsoc's worldview, and to attempt to make impossible all unorthodox (i.e. anti-Ingsoc) political thought. As constructed, the Newspeak vocabulary communicates the exact expression of sense and meaning that a member of the Party could wish to express, whilst excluding secondary denotations and connotations, eliminating the ways of indirect thinking that allow a word to have second and third meanings. The linguistic simplification of Oldspeak into Newspeak was realised with neologisms, the elimination of ideologically undesirable words, and the elimination of the politically unorthodox meanings of words.  The word "free" still existed in Newspeak, but only to communicate a lack of something, e.g. "The dog is free from lice" or "This field is free of weeds". The word could not denote free will, because intellectual freedom no longer exists in Oceania. The limitations of Newspeak's vocabulary enabled the Party to effectively control the population's minds, by allowing the user only a very narrow range of spoken and written thought; hence, words such as: "crimethink" (thought crime), "doublethink" (accepting contradictory beliefs), and "Ingsoc" (English Socialism) communicated only their surface meanings.  In the story of "Nineteen Eighty-Four", the lexicologist character Syme discusses his editorial work on the latest edition of the "Newspeak Dictionary":

See also.

Totalitarian regimes:  Fiction:

Newspeak is the fictional language of Oceania, a totalitarian superstate that is the setting of George Orwell's dystopian 1949 novel "Nineteen Eighty-Four." To meet the ideological requirements of English Socialism (Ingsoc) in Oceania, the ruling Party created Newspeak, a controlled language of simplified grammar and restricted vocabulary, meant to limit the freedom of thought—personal identity, self-expression, free will—that threatens the ideology of the régime of Big Brother and the Party, who have criminalised such concepts into thoughtcrime as contradictions of Ingsoc orthodoxy.

新语,又译为新话(英语:Newspeak)是乔治·奥威尔小说《一九八四》中设想的新人工语言,是大洋国的官方语言,被形容为「世界唯一会逐年减少词汇的语言」。