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Electric guitar

An electric guitar is a guitar that requires external amplification in order to be heard at typical performance volumes, unlike a standard acoustic guitar. It uses one or more pickups to convert the vibration of its strings into electrical signals, which ultimately are reproduced as sound by loudspeakers. The sound is sometimes shaped or electronically altered to achieve different timbres or tonal qualities from that of an acoustic guitar via amplifier settings or knobs on the guitar. Often, this is done through the use of effects such as reverb, distortion and "overdrive"; the latter is considered to be a key element of electric blues guitar music and jazz, rock and heavy-metal guitar-playing. Designs also exist combining attributes of the electric and acoustic guitars: the semi-acoustic and acoustic-electric guitars.

Invented in 1932, the electric guitar was adopted by jazz guitar players, who wanted to play single-note guitar solos in large big band ensembles. Early proponents of the electric guitar on record include Les Paul, Eddie Durham, George Barnes, Lonnie Johnson, Sister Rosetta Tharpe, T-Bone Walker, and Charlie Christian. During the 1950s and 1960s, the electric guitar became the most important instrument in popular music. It has evolved into an instrument that is capable of a multitude of sounds and styles in genres ranging from pop and rock to folk to country music, blues and jazz. It served as a major component in the development of electric blues, rock and roll, rock music, heavy metal music and many other genres of music.

Electric guitar design and construction varies greatly in the shape of the body and the configuration of the neck, bridge, and pickups. Guitars may have a fixed bridge or a spring-loaded hinged bridge, which lets players "bend" the pitch of notes or chords up or down, or perform vibrato effects. The sound of an electric guitar can be modified by new playing techniques such as string bending, tapping, and hammering-on, using audio feedback, or slide guitar playing.

There are several types of electric guitar. Early forms were hollow-body semi-acoustic guitars, while solid body guitars developed later. String configurations include the six-string guitar (the most common type), which is usually tuned E, A, D, G, B, E, from lowest to highest strings; the seven-string guitar, which typically adds a low B string below the low E; the eight-string guitar, which typically adds a low E or F# string below the low B; and the twelve-string guitar, which has six two-string courses similar to a mandolin.

In rock, the electric guitar is often used in two roles: as a rhythm guitar, which plays the chord sequences or progressions, and riffs, and sets the beat (as part of a rhythm section); and as a lead guitar, which provides instrumental melody lines, melodic instrumental fill passages, and solos. In a small group, such as a power trio, one guitarist may switch between both roles; in larger groups there is often a rhythm guitarist and a lead guitarist.

History
Many experiments with electrically amplifying the vibrations of a string instrument were made dating back to the early part of the 20th century. Patents from the 1910s show telephone transmitters were adapted and placed inside violins and banjos to amplify the sound. Hobbyists in the 1920s used carbon button microphones attached to the bridge; however, these detected vibrations from the bridge on top of the instrument, resulting in a weak signal.

Electric guitars were originally designed by acoustic guitar makers and instrument manufacturers. The demand for amplified guitars began during the big band era; as orchestras increased in size, guitar players soon realized the necessity in guitar amplification and electrification. The first electric guitars used in jazz were hollow archtop acoustic guitar bodies with electromagnetic transducers.

The first electrically amplified stringed instrument to be marketed commercially was a cast aluminium lap steel guitar nicknamed the "Frying Pan" designed in 1931 by George Beauchamp, the general manager of the National Stringed Instrument Corporation, with Paul Barth, who was vice president. George Beauchamp, along with Adolph Rickenbacker, invented the electromagnetic pickups. Coils that were wrapped around a magnet would create an electromagnetic field that converted the vibrations of the guitar strings into electrical signals, which could then be amplified. Commercial production began in late summer of 1932 by the Ro-Pat-In Corporation (Electro-Patent-Instrument Company), in Los Angeles, a partnership of Beauchamp, Adolph Rickenbacker (originally Rickenbacher), and Paul Barth.

In 1934, the company was renamed the Rickenbacker Electro Stringed Instrument Company. In that year Beauchamp applied for a United States patent for an Electrical Stringed Musical Instrument and the patent was later issued in 1937. By the time it was patented, other manufacturers were already making their own electric guitar designs. Early electric guitar manufacturers include Rickenbacker in 1932; Dobro in 1933; National, AudioVox and Volu-tone in 1934; Vega, Epiphone (Electrophone and Electar), and Gibson in 1935 and many others by 1936.

By early-mid 1935, Electro String Instrument Corporation had achieved success with the "Frying Pan", and set out to capture a new audience through its release of the Electro-Spanish Model B and the Electro-Spanish Ken Roberts, which was the first full 25-inch scale electric guitar ever produced. The Electro-Spanish Ken Roberts was revolutionary for its time, providing players a full 25-inch scale, with easy access to 17 frets free of the body. Unlike other lap-steel electrified instruments produced during the time, the Electro-Spanish Ken Roberts was designed to play while standing upright with the guitar on a strap, as with acoustic guitars. The Electro-Spanish Ken Roberts was also the first instrument to feature a hand-operated vibrato as a standard arrangement, a device called the "Vibrola", invented by Doc Kauffman. It is estimated that fewer than 50 Electro-Spanish Ken Roberts were constructed between 1933 and 1937; fewer than 10 are known to survive today.

The solid-body electric guitar is made of solid wood, without functionally resonating air spaces. The first solid-body Spanish standard guitar was offered by Vivi-Tone no later than 1934. This model featured a guitar-shaped body of a single sheet of plywood affixed to a wood frame. Another early, substantially solid Spanish electric guitar, called the Electro Spanish, was marketed by the Rickenbacker guitar company in 1935 and made of Bakelite. By 1936, the Slingerland company introduced a wooden solid-body electric model, the Slingerland Songster 401 (and a lap steel counterpart, the Songster 400).

Gibson's first production electric guitar, marketed in 1936, was the ES-150 model ("ES" for "Electric Spanish", and "150" reflecting the $150 price of the instrument, along with matching amplifier). The ES-150 guitar featured a single-coil, hexagonally shaped "bar" pickup, which was designed by Walt Fuller. It became known as the "Charlie Christian" pickup (named for the jazz guitarist who was among the first to perform with the ES-150 guitar). The ES-150 achieved some popularity but suffered from unequal loudness across the six strings.

A functioning solid-body electric guitar was designed and built in 1940 by Les Paul from an Epiphone acoustic archtop as an experiment. His "log guitar" — a wood post with a neck attached and two hollow-body halves attached to the sides for appearance only — shares nothing in common for design or hardware with the solid-body Gibson Les Paul, designed by Ted McCarty and introduced in 1952.

The feedback associated with amplified hollow-bodied electric guitars was understood long before Paul's "log" was created in 1940; Gage Brewer's Ro-Pat-In of 1932 had a top so heavily reinforced that it essentially functioned as a solid-body instrument.

Solid-body
Unlike acoustic guitars, solid-body electric guitars have no vibrating soundboard to amplify string vibration. Instead, solid-body instruments depend on electric pickups, and an amplifier ("amp") and speaker. The solid body ensures that the amplified sound reproduces the string vibration alone, thus avoiding the wolf tones and unwanted feedback associated with amplified acoustic guitars. These guitars are generally made of hardwood covered with a hard polymer finish, often polyester or lacquer. In large production facilities, the wood is stored for three to six months in a wood-drying kiln before being cut to shape. Premium custom-built guitars are frequently made with much older, hand-selected wood.

One of the first solid-body guitars was invented by Les Paul. Gibson did not present their Gibson Les Paul guitar prototypes to the public, as they did not believe the solid-body style would catch on. Another early solid-body Spanish style guitar, resembling what would become Gibson's Les Paul guitar a decade later, was developed in 1941 by O.W. Appleton, of Nogales, Arizona. Appleton made contact with both Gibson and Fender but was unable to sell the idea behind his "App" guitar to either company. In 1946, Merle Travis commissioned steel guitar builder Paul Bigsby to build him a solid-body Spanish-style electric. Bigsby delivered the guitar in 1948. The first mass-produced solid-body guitar was Fender Esquire and Fender Broadcaster (later to become the Fender Telecaster), first made in 1950, five years after Les Paul made his prototype. The Gibson Les Paul appeared soon after to compete with the Broadcaster. Another notable solid-body design is the Fender Stratocaster, which was introduced in 1954 and became extremely popular among musicians in the 1960s and 1970s for its wide tonal capabilities and more comfortable ergonomics than other models. Different styles of guitar have different pick-up styles, the main being 2 or 3 "single-coil" pick-ups or a double humbucker, with the Stratocaster being a triple single-coil guitar.

The history of electric guitars has been summarized by Guitar World magazine, and the earliest electric guitar on their top 10 list is the Ro-Pat-In Electro A-25 "Frying Pan" (1932) described as "The first-fully functioning solid-body electric guitar to be manufactured and sold". It was the first electric guitar used in a publicly promoted performance, performed by Gage Brewer in Wichita, Kansas in October 1932. The most recent electric guitar on this list was the Ibanez Jem (1987) which featured "24 frets", an impossibly thin neck" and was "designed to be the ultimate shredder machine". Numerous other important electric guitars are on the list, including Gibson ES-150 (1936), Fender Telecaster (1951), Gibson Les Paul (1952), Gretsch 6128 Duo Jet (1953), Fender Stratocaster (1954), Rickenbacker 360/12 (1964), Van Halen Frankenstrat (1975), Paul Reed Smith Custom (1985) many of these guitars were "successors" to earlier designs. Electric guitar designs eventually became culturally important and visually iconic, with various model companies selling miniature model versions of particularly famous electric guitars, for example, the Gibson SG used by Angus Young from the group AC/DC.

Chambered-body
Some otherwise solid-bodied guitars, such as the Gibson Les Paul Supreme, the PRS Singlecut, and the Fender Telecaster Thinline, are built with hollow chambers in the body. These chambers are designed to not interfere with the critical bridge and string anchor point on the solid body. In the case of Gibson and PRS, these are called chambered bodies. The motivation for this may be to reduce weight, to achieve a semi-acoustic tone (see below) or both.

Semi-acoustic
Semi-acoustic guitars have a hollow body similar to an acoustic guitar and electromagnetic pickups mounted directly into the body. They work in a similar way to solid-body electric guitars except that because the hollow body also vibrates, the pickups convert a combination of string and body vibration into an electrical signal. Many models, known as semi-hollow bodies, have a solid block running through the middle of the soundbox designed to reduce acoustic feedback. They do not provide enough acoustic volume for live performance, but they can be used unplugged for quiet practice. Semi-acoustic guitars are noted for being able to provide a sweet, plaintive, or funky tone. They are used in many genres, including jazz, blues, funk, sixties pop, and indie rock. They generally have cello-style F-shaped sound holes, which can be blocked off to further reduce feedback. Whereas chambered guitars are made, like solid-body guitars, from a single block of wood, semi-acoustic guitar bodies are made from multiple pieces of wood in an archtop form, a method of construction different from the typical steel string acoustic guitar. The top is formed from a moderately thick piece of wood which is then carved into a thin outward-curving shape, whereas conventional acoustic guitars have a thin, flat top.

Electric acoustic
Some steel-string acoustic guitars include a built-in system to electrically amplify their output without altering their tone as an alternative to using a separate microphone. The system may consist of piezoelectric pickups mounted under the bridge, or a low-mass microphone (usually a condenser mic) inside the body of the guitar that converts the vibrations in the body into electronic signals. Combinations of these types of pickups may be used, with an integral mixer/preamp/graphic equalizer. Such instruments are called electric acoustic guitars. They are regarded as acoustic guitars rather than electric guitars because the pickups do not produce a signal directly from the vibration of the strings, but rather from the vibration of the guitar top or body, and the amplification of the sound merely increases volume, not alters tone.

Construction
[[File:Electric Guitar (Superstrat based on ESP KH - vertical) - with hint lines and numbers.png|thumb|1. [[Headstock]]

1.1 machine heads

1.2 truss rod cover

1.3 string guide

1.4 nut

2. Neck

2.1 fretboard

2.2 inlay fret markers

2.3 frets

2.4 neck joint

3. Body

3.1 "neck" pickup

3.2 "bridge" pickup

3.3 saddles

3.4 bridge

3.5 fine tuners and tailpiece assembly

3.6 whammy bar (vibrato arm)

3.7 pickup selector switch

3.8 volume and tone control knobs

3.9 output connector (output jack)(TS)

3.10 strap buttons

4. Strings

4.1 bass strings

4.2 treble strings]]

Electric guitar design and construction vary greatly in the shape of the body and the configuration of the neck, bridge, and pickups. However, some features are present on most guitars. The photo below shows the different parts of an electric guitar. The headstock (1) contains the metal machine heads (1.1), which use a worm gear for tuning. The nut (1.4)—a thin fret-like strip of metal, plastic, graphite, or bone—supports the strings at the headstock end of the instrument. The frets (2.3) are thin metal strips that stop the string at the correct pitch when the player pushes a string against the fingerboard. The truss rod (1.2) is a metal rod (usually adjustable) that counters the tension of the strings to keep the neck straight. Position markers (2.2) provide the player with a reference to the playing position on the fingerboard.

The neck and fretboard (2.1) extend from the body. At the neck joint (2.4), the neck is either glued or bolted to the body. The body (3) is typically made of wood with a hard, polymerized finish. Strings vibrating in the magnetic field of the pickups (3.1, 3.2) produce an electric current in the pickup winding that passes through the tone and volume controls (3.8) to the output jack. Some guitars have piezo pickups, in addition to or instead of magnetic pickups.

Some guitars have a fixed bridge (3.4). Others have a spring-loaded hinged bridge called a vibrato bar, tremolo bar, or whammy bar, which lets players bend notes or chords up or down in pitch or perform a vibrato embellishment. A plastic pickguard on some guitars protects the body from scratches or covers the control cavity, which holds most of the wiring. The degree to which the choice of woods and other materials in the solid-guitar body (3) affects the sonic character of the amplified signal is disputed. Many believe it is highly significant, while others think the difference between woods is subtle. In acoustic and archtop guitars, wood choices more clearly affect tone.

Woods typically used in solid-body electric guitars include alder (brighter, but well rounded), swamp ash (similar to alder, but with more pronounced highs and lows), mahogany (dark, bassy, warm), poplar (similar to alder), and basswood (very neutral). Maple, a very bright tonewood, is also a popular body wood but is very heavy. For this reason, it is often placed as a "cap" on a guitar made primarily of another wood. Cheaper guitars are often made of cheaper woods, such as plywood, pine, or agathis—not true hardwoods—which can affect durability and tone. Though most guitars are made of wood, any material may be used. Materials such as plastic, metal, and even cardboard have been used in some instruments.

The guitar output jack typically provides a monaural signal. Many guitars with active electronics use a jack with an extra contact normally used for stereo. These guitars use the extra contact to break the ground connection to the on-board battery to preserve battery life when the guitar is unplugged. These guitars require a mono plug to close the internal switch and connect the battery to ground. Standard guitar cables use a high-impedance 1/4 in mono plug. These have a tip and sleeve configuration referred to as a TS phone connector. The voltage is usually around 1 to 9 millivolts.

A few guitars, such as Rickenbacker guitars equipped with Rick-O-Sound, feature stereo output. There are a variety of ways the "stereo" effect may be implemented. Commonly, but not exclusively, stereo guitars route the neck and bridge pickups to separate output buses on the guitar. A stereo cable then routes each pickup to its signal chain or amplifier. For these applications, the most popular connector is a high-impedance 1/4 in plug with a tip, ring, and sleeve configuration, also known as a TRS phone connector. Some studio instruments, notably certain Gibson Les Paul models, incorporate a low-impedance three-pin XLR connector for balanced audio. Many exotic arrangements and connectors exist that support features such as midi and hexaphonic pickups.

Bridge and tailpiece systems
The bridge and tailpiece, while serving separate purposes, work closely together to affect playing style and tone. There are four basic types of bridge and tailpiece systems on electric guitars. Within these four types are many variants.

A hard-tail guitar bridge anchors the strings at or directly behind the bridge and is fastened securely to the top of the instrument. These are common on carved-top guitars, such as the Gibson Les Paul and the Paul Reed Smith models, and on slab-body guitars, such as the Music Man Albert Lee and Fender guitars that are not equipped with a vibrato arm.

A floating or trapeze tailpiece (similar to a violin's) fastens to the body at the base of the guitar. These appear on Rickenbackers, Gretsches, Epiphones, a wide variety of archtop guitars, particularly jazz guitars, and the 1952 Gibson Les Paul.

Pictured is a tremolo arm or vibrato tailpiece-style bridge and tailpiece system, often called a whammy bar or trem. It uses a lever ("vibrato arm") attached to the bridge that can temporarily slacken or tighten the strings to alter the pitch. A player can use this to create a vibrato or a portamento effect. Early vibrato systems were often unreliable and made the guitar go out of tune easily. They also had a limited pitch range. Later Fender designs were better, but Fender held the patent on these, so other companies used older designs for many years.

With the expiration of the Fender patent on the Stratocaster-style vibrato, various improvements on this type of internal, multi-spring vibrato system are now available. Floyd Rose introduced one of the first improvements on the vibrato system in many years when, in the late 1970s, he experimented with "locking" nuts and bridges that prevent the guitar from losing tuning, even under heavy vibrato bar use.

The fourth type of system employs string-through body anchoring. The strings pass over the bridge saddles, then through holes through the top of the guitar body to the back. The strings are typically anchored in place at the back of the guitar by metal ferrules. Many believe this design improves a guitar's sustain and timbre. A few examples of string-through body guitars are the Fender Telecaster Thinline, the Fender Telecaster Deluxe, the B.C. Rich IT Warlock and Mockingbird, and the Schecter Omen 6 and 7 series.

Pickups

Compared to an acoustic guitar, which has a hollow body, electric guitars make much less audible sound when their strings are plucked, so electric guitars are normally plugged into a guitar amplifier and speaker. When an electric guitar is played, string movement produces a signal by generating (i.e., inducing) a small electric current in the magnetic pickups, which are magnets wound with coils of very fine wire. The signal passes through the tone and volume circuits to the output jack, and through a cable to an amplifier. The current induced is proportional to such factors as string density and the amount of movement over the pickups.

Because of their natural qualities, magnetic pickups tend to pick up ambient, usually unwanted electromagnetic interference or EMI. This mains hum results in a tone of 50 or 60 cycles per second depending on the powerline frequency of the local alternating current supply.

The resulting hum is particularly strong with single-coil pickups. Double-coil or "humbucker" pickups were invented as a way to reduce or counter the sound, as they are designed to "buck" (in the verb sense of oppose or resist) the hum, hence their name. The high combined inductance of the two coils also leads to the richer, "fatter" tone associated with humbucking pickups.

Necks
Electric guitar necks vary in composition and shape. The primary metric of guitar necks is the scale length, which is the vibrating length of the strings from nut to bridge. A typical Fender guitar uses a 25.5 in scale length, while Gibson uses a 24.75 in scale length in their Les Paul. While the scale length of the Les Paul is often described as 24.75 inches, it has varied through the years by as much as a half inch.

Frets are positioned proportionally to scale length—the shorter the scale length, the closer the fret spacing. Opinions vary regarding the effect of scale length on tone and feel. Popular opinion holds that longer scale length contributes to greater amplitude. Reports of playing feel are greatly complicated by the many factors involved in this perception. String gauge and design, neck construction and relief, guitar setup, playing style, and other factors contribute to the subjective impression of playability or feel. Necks are described as bolt-on, set-in, or neck-through, depending on how they attach to the body. Set-in necks are glued to the body at the factory. This is the traditional type of joint. Leo Fender pioneered bolt-on necks on electric guitars to facilitate easy adjustment and replacement. Neck-through instruments extend the neck to the length of the instrument so that it forms the center of the body. While a set-in neck can be carefully unglued by a skilled luthier, and a bolt-on neck can simply be unscrewed, a neck-through design is difficult or even impossible to repair, depending on the damage. Historically, the bolt-on style has been more popular for ease of installation and adjustment. Since bolt-on necks can be easily removed, there is an after-market in replacement bolt-on necks from companies such as Warmoth and Mighty Mite. Some instruments—notably most Gibson models—continue to use set-in glued necks. Neck-through bodies are somewhat more common in bass guitars.

Materials for necks are selected for dimensional stability and rigidity, and some allege that they influence tone. Hardwoods are preferred, with maple, mahogany, and ash topping the list. The neck and fingerboard can be made from different materials; for example, a guitar may have a maple neck with a rosewood or ebony fingerboard. Today there are expensive and budget guitars exploring other options for fretboard wood for instance Pau-Ferro, both for availability and cheap price while still maintaining quality. In the 1970s, designers began to use exotic human-made materials such as aircraft-grade aluminum, carbon fiber, and ebonol. Makers known for these unusual materials include John Veleno, Travis Bean, Geoff Gould, and Alembic. Aside from possible engineering advantages, some feel that with the rising cost of rare tonewoods, human-made materials may be economically preferable and more ecologically sensitive. However, wood remains popular in production instruments, though sometimes in conjunction with new materials. Vigier guitars, for example, use a wooden neck reinforced by embedding a light, carbon fiber rod in place of the usual heavier steel bar or adjustable steel truss rod. After-market necks made entirely from carbon fiber fit existing bolt-on instruments. Few, if any, extensive formal investigations have been widely published that confirm or refute claims over the effects of different woods or materials on the electric guitar sound. Several neck shapes appear on guitars, including shapes known as C necks, U necks, and V necks. These refer to the cross-sectional shape of the neck (especially near the nut). Several sizes of fret wire are available, with traditional players often preferring thin frets, and metal shredders liking thick frets. Thin frets are considered better for playing chords, while thick frets allow lead guitarists to bend notes with less effort.

An electric guitar with a folding neck called the "Foldaxe" was designed and built for Chet Atkins by Roger C. Field. Steinberger guitars developed a line of exotic, carbon fiber instruments without headstocks, with tuning done on the bridge instead.

Fingerboards vary as much as necks. The fingerboard surface usually has a cross-sectional radius that is optimized to accommodate finger movement for different playing techniques. Fingerboard radius typically ranges from nearly flat (a very large radius) to radically arched (a small radius). The vintage Fender Telecaster, for example, has a typical small radius of approximately 7.25 in. Some manufacturers have experimented with fret profile and material, fret layout, number of frets, and modifications of the fingerboard surface for various reasons. Some innovations were intended to improve playability by ergonomic means, such as Warmoth Guitars' compound radius fingerboard. Scalloped fingerboards added enhanced microtonality during fast legato runs. Fanned frets intend to provide each string with an optimal playing tension and enhanced musicality. Some guitars have no frets, while others, like the Gittler guitar, have no neck in the traditional sense.

title: Electric guitar


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Embryo drawing

Embryo drawing is the illustration of embryos in their developmental sequence. In plants and animals, an embryo develops from a zygote, the single cell that results when an egg and sperm fuse during fertilization. In animals, the zygote divides repeatedly to form a ball of cells, which then forms a set of tissue layers that migrate and fold to form an early embryo. Images of embryos provide a means of comparing embryos of different ages, and species. To this day, embryo drawings are made in undergraduate developmental biology lessons.

Comparing different embryonic stages of different animals is a tool that can be used to infer relationships between species, and thus biological evolution. This has been a source of quite some controversy, both now and in the past. Ernst Haeckel at the University of Basel pioneered in this field. By comparing different embryonic stages of different vertebrate species, he formulated the recapitulation theory. This theory states that an animal's embryonic development follows exactly the same sequence as the sequence of its evolutionary ancestors. Haeckel's work and the ensuing controversy linked the fields of developmental biology and comparative anatomy into comparative embryology. From a more modern perspective, Haeckel's drawings were the beginnings of the field of evolutionary developmental biology (evo-devo).

The study of comparative embryology aims to prove or disprove that vertebrate embryos of different classes (e.g. mammals vs. fish) follow a similar developmental path due to their common ancestry. Such developing vertebrates have similar genes, which determine the basic body plan. However, further development allows for the distinguishing of distinct characteristics as adults.

Ernst Haeckel (1834–1919)

Haeckel's illustrations show vertebrate embryos at different stages of development, which exhibit embryonic resemblance as support for evolution, recapitulation as evidence of the Biogenetic Law, and phenotypic divergence as evidence of von Baer's laws. The series of twenty-four embryos from the early editions of Haeckel's Anthropogenie remain the most famous. The different species are arranged in columns, and the different stages in rows. Similarities can be seen along the first two rows; the appearance of specialized characters in each species can be seen in the columns and a diagonal interpretation leads one to Haeckel's idea of recapitulation.

Haeckel's embryo drawings are primarily intended to express his theory of embryonic development, the Biogenetic Law, which in turn assumes (but is not crucial to) the evolutionary concept of common descent. His postulation of embryonic development coincides with his understanding of evolution as a developmental process. In and around 1800, embryology fused with comparative anatomy as the primary foundation of morphology. Ernst Haeckel, along with Karl von Baer and Wilhelm His, are primarily influential in forming the preliminary foundations of 'phylogenetic embryology' based on principles of evolution. Haeckel's 'Biogenetic Law' portrays the parallel relationship between an embryo's development and phylogenetic history. The term, 'recapitulation,' has come to embody Haeckel's Biogenetic Law, for embryonic development is a recapitulation of evolution. Haeckel proposes that all classes of vertebrates pass through an evolutionarily conserved "phylotypic" stage of development, a period of reduced phenotypic diversity among higher embryos. Only in later development do particular differences appear. Haeckel portrays a concrete demonstration of his Biogenetic Law through his Gastrea theory, in which he argues that the early cup-shaped gastrula stage of development is a universal feature of multi-celled animals. An ancestral form existed, known as the gastrea, which was a common ancestor to the corresponding gastrula.

Haeckel argues that certain features in embryonic development are conserved and palingenetic, while others are caenogenetic. Caenogenesis represents "the blurring of ancestral resemblances in development", which are said to be the result of certain adaptations to embryonic life due to environmental changes. In his drawings, Haeckel cites the notochord, pharyngeal arches and clefts, pronephros and neural tube as palingenetic features. However, the yolk sac, extra-embryonic membranes, egg membranes and endocardial tube are considered caenogenetic features. The addition of terminal adult stages and the telescoping, or driving back, of such stages to descendant's embryonic stages are likewise representative of Haeckelian embryonic development. In addressing his embryo drawings to a general audience, Haeckel does not cite any sources, which gives his opponents the freedom to make assumptions regarding the originality of his work.

Karl Ernst von Baer (1792–1876)
Haeckel was not the only one to create a series of drawings representing embryonic development. Karl E. von Baer and Haeckel both struggled to model one of the most complex problems facing embryologists at the time: the arrangement of general and special characters during development in different species of animals. In relation to developmental timing, von Baer's scheme of development differs from Haeckel's scheme. Von Baer's scheme of development need not be tied to developmental stages defined by particular characters, where recapitulation involves heterochrony. Heterochrony represents a gradual alteration in the original phylogenetic sequence due to embryonic adaptation. As well, von Baer early noted that embryos of different species could not be easily distinguished from one another as in adults.

Von Baer's laws governing embryonic development are specific rejections of recapitulation. As a response to Haeckel's theory of recapitulation, von Baer enunciates his most notorious laws of development. Von Baer's laws state that general features of animals appear earlier in the embryo than special features, where less general features stem from the most general, each embryo of a species departs more and more from a predetermined passage through the stages of other animals, and there is never a complete morphological similarity between an embryo and a lower adult. Von Baer's embryo drawings display that individual development proceeds from general features of the developing embryo in early stages through differentiation into special features specific to the species, establishing that linear evolution could not occur. Embryological development, in von Baer's mind, is a process of differentiation, "a movement from the more homogeneous and universal to the more heterogeneous and individual."

Von Baer argues that embryos will resemble each other before attaining characteristics differentiating them as part of a specific family, genus or species, but embryos are not the same as the final forms of lower organisms.

Wilhelm His (1831–1904)
Wilhelm His was one of Haeckel's most authoritative and primary opponents advocating physiological embryology. His Anatomie menschlicher Embryonen (Anatomy of human embryos) employs a series of his most important drawings chronicling developing embryos from the end of the second week through the end of the second month of pregnancy. In 1878, His begins to engage in serious study of the anatomy of human embryos for his drawings. During the 19th century, embryologists often obtained early human embryos from abortions and miscarriages, postmortems of pregnant women and collections in anatomical museums. In order to construct his series of drawings, His collected specimens which he manipulated into a form that he could operate with.

In His' Normentafel, he displays specific individual embryos rather than ideal types. His does not produce norms from aborted specimens, but rather visualizes the embryos in order to make them comparable and specifically subjects his embryo specimens to criticism and comparison with other cases. Ultimately, His' critical work in embryonic development comes with his production of a series of embryo drawings of increasing length and degree of development. His' depiction of embryological development strongly differs from Haeckel's depiction, for His argues that the phylogenetic explanation of ontogenetic events is unnecessary. His argues that all ontogenetic events are the "mechanical" result of differential cell growth. His' embryology is not explained in terms of ancestral history.

The debate between Haeckel and His ultimately becomes fueled by the description of an embryo that Wilhelm Krause propels directly into the ongoing feud between Haeckel and His. Haeckel speculates that the allantois is formed in a similar way in both humans and other mammals. His, on the other hand, accuses Haeckel of altering and playing with the facts. Although Haeckel is proven right about the allantois, the utilization of Krause's embryo as justification turns out to be problematic, for the embryo is that of a bird rather than a human. The underlying debate between Haeckel and His derives from differing viewpoints regarding the similarity or dissimilarity of vertebrate embryos. In response to Haeckel's evolutionary claim that all vertebrates are essentially identical in the first month of embryonic life as proof of common descent, His responds by insisting that a more skilled observer would recognize even sooner that early embryos can be distinguished. His also counteracts Haeckel's sequence of drawings in the Anthropogenie with what he refers to as "exact" drawings, highlighting specific differences. Ultimately, His goes so far as to accuse Haeckel of "faking" his embryo illustrations to make the vertebrate embryos appear more similar than in reality. His also accuses Haeckel of creating early human embryos that he conjured in his imagination rather than obtained through empirical observation. His completes his denunciation of Haeckel by pronouncing that Haeckel had "'relinquished the right to count as an equal in the company of serious researchers.'"

Controversy
The exactness of Ernst Haeckel's drawings of embryos has caused much controversy among Intelligent Design proponents recently and Haeckel's intellectual opponents in the past. Although the early embryos of different species exhibit similarities, Haeckel apparently exaggerated these similarities in support of his Recapitulation theory, sometimes known as the Biogenetic Law or "Ontogeny recapitulates phylogeny". Furthermore, Haeckel even proposed theoretical life-forms to accommodate certain stages in embryogenesis. A recent review concluded that the "biogenetic law is supported by several recent studies – if applied to single characters only".

Critics in the late 19th and early 20th centuries, Karl von Baer and Wilhelm His, did not believe that living embryos reproduce the evolutionary process and produced embryo drawings of their own which emphasized the differences in early embryological development. Late 20th and early 21st century critic Stephen Jay Gould has objected to the continued use of Haeckel's embryo drawings in textbooks.

On the other hand, Michael K. Richardson, Professor of Evolutionary Developmental Zoology, Leiden University, while recognizing that some criticisms of the drawings are legitimate (indeed, it was he and his co-workers who began the modern criticisms in 1998), has supported the drawings as teaching aids, and has said that "on a fundamental level, Haeckel was correct."

Opposition to Haeckel
Haeckel encountered numerous oppositions to his artistic depictions of embryonic development during the late nineteenth and early twentieth centuries. Haeckel's opponents believe that he de-emphasizes the differences between early embryonic stages in order to make the similarities between embryos of different species more pronounced.

Early opponents: Ludwig Rutimeyer, Theodor Bischoff and Rudolf Virchow
The first suggestion of fakery against Haeckel was made in late 1868 by Ludwig Rutimeyer in the Archiv für Anthropogenie. Rutimeyer was a professor of zoology and comparative anatomy at the University of Basel, who rejected natural selection as simply mechanistic and proposed an anti-materialist view of nature. Rutimeyer claimed that Haeckel "had taken to kinds of liberty with established truth". Rutimeyer claimed that Haeckel presented the same image three consecutive times as the embryo of the dog, the chicken, and the turtle.

Theodor Bischoff (1807–1882), was a strong opponent of Darwinism. As a pioneer in mammalian embryology, he was one of Haeckel's strongest critics. Although Bischoff's 1840 surveys depict how similar the early embryos of man are to other vertebrates, he later demanded that such hasty generalization was inconsistent with his recent findings regarding the dissimilarity between hamster embryos and those of rabbits and dogs. Nevertheless, Bischoff's main argument was in reference to Haeckel's drawings of human embryos, for Haeckel is later accused of miscopying the dog embryo from him. Throughout Haeckel's time, criticism of his embryo drawings was often due in part to his critics' belief in his representations of embryological development as "crude schemata".

Contemporary criticism of Haeckel: Michael Richardson and Stephen Jay Gould
Michael Richardson and his colleagues in a July 1997 issue of Anatomy and Embryology, demonstrated that Haeckel falsified his drawings in order to exaggerate the similarity of the phylotypic stage. In a March 2000 issue of Natural History, Stephen Jay Gould argued that Haeckel "exaggerated the similarities by idealizations and omissions." As well, Gould argued that Haeckel's drawings are simply inaccurate and falsified. On the other hand, one of those who criticized Haeckel's drawings, Michael Richardson, has argued that "Haeckel's much-criticized drawings are important as phylogenetic hypotheses, teaching aids, and evidence for evolution". But even Richardson admitted in Science Magazine in 1997 that his team's investigation of Haeckel's drawings were showing them to be "one of the most famous fakes in biology."

Some version of Haeckel's drawings can be found in many modern biology textbooks in discussions of the history of embryology, with clarification that these are no longer considered valid.

Haeckel's proponents (past and present)
Although Charles Darwin accepted Haeckel's support for natural selection, he was tentative in using Haeckel's ideas in his writings; with regard to embryology, Darwin relied far more on von Baer's work. Haeckel's work was published in 1866 and 1874, years after Darwin's "The Origin of Species" (1859).

Despite the numerous oppositions, Haeckel has influenced many disciplines in science in his drive to integrate such disciplines of taxonomy and embryology into the Darwinian framework and to investigate phylogenetic reconstruction through his Biogenetic Law. As well, Haeckel served as a mentor to many important scientists, including Anton Dohrn, Richard and Oscar Hertwig, Wilhelm Roux, and Hans Driesch.

One of Haeckel's earliest proponents was Carl Gegenbaur at the University of Jena (1865–1873), during which both men were absorbing the impact of Darwin's theory. The two quickly sought to integrate their knowledge into an evolutionary program. In determining the relationships between "phylogenetic linkages" and "evolutionary laws of form," both Gegenbaur and Haeckel relied on a method of comparison. As Gegenbaur argued, the task of comparative anatomy lies in explaining the form and organization of the animal body in order to provide evidence for the continuity and evolution of a series of organs in the body. Haeckel then provided a means of pursuing this aim with his biogenetic law, in which he proposed to compare an individual's various stages of development with its ancestral line. Although Haeckel stressed comparative embryology and Gegenbaur promoted the comparison of adult structures, both believed that the two methods could work in conjunction to produce the goal of evolutionary morphology.

The philologist and anthropologist, Friedrich Müller, used Haeckel's concepts as a source for his ethnological research, involving the systematic comparison of the folklore, beliefs and practices of different societies. Müller's work relies specifically on theoretical assumptions that are very similar to Haeckel's and reflects the German practice to maintain strong connections between empirical research and the philosophical framework of science. Language is particularly important, for it establishes a bridge between natural science and philosophy. For Haeckel, language specifically represented the concept that all phenomena of human development relate to the laws of biology. Although Müller did not specifically have an influence in advocating Haeckel's embryo drawings, both shared a common understanding of development from lower to higher forms, for Müller specifically saw humans as the last link in an endless chain of evolutionary development.

Modern acceptance of Haeckel's Biogenetic Law, despite current rejection of Haeckelian views, finds support in the certain degree of parallelism between ontogeny and phylogeny. A. M. Khazen, on the one hand, states that "ontogeny is obliged to repeat the main stages of phylogeny." A. S. Rautian, on the other hand, argues that the reproduction of ancestral patterns of development is a key aspect of certain biological systems. Dr. Rolf Siewing acknowledges the similarity of embryos in different species, along with the laws of von Baer, but does not believe that one should compare embryos with adult stages of development. According to M. S. Fischer, reconsideration of the Biogenetic Law is possible as a result of two fundamental innovations in biology since Haeckel's time: cladistics and developmental genetics.

In defense of Haeckel's embryo drawings, the principal argument is that of "schematisation." Haeckel's drawings were not intended to be technical and scientific depictions, but rather schematic drawings and reconstructions for a specifically lay audience. Therefore, as R. Gursch argues, Haeckel's embryo drawings should be regarded as "reconstructions." Although his drawings are open to criticism, his drawings should not be considered falsifications of any sort. Although modern defense of Haeckel's embryo drawings still considers the inaccuracy of his drawings, charges of fraud are considered unreasonable. As Erland Nordenskiöld argues, charges of fraud against Haeckel are unnecessary. R. Bender ultimately goes so far as to reject His's claims regarding the fabrication of certain stages of development in Haeckel's drawings, arguing that Haeckel's embryo drawings are faithful representations of real stages of embryonic development in comparison to published embryos.

Use of embryo drawings in contemporary biology
Haeckel's embryo drawings, as comparative plates, were at first only copied into biology textbooks, rather than texts on the study of embryology. Even though Haeckel's program in comparative embryology virtually collapsed after the First World War, his embryo drawings have often been reproduced and redrawn with increased precision and accuracy in works that have kept the study of comparative embryology alive. Nevertheless, neither His-inspired human embryology nor developmental biology are concerned with the comparison of vertebrate embryos. Although Stephen Jay Gould's 1977 book Ontogeny and Phylogeny helps to reassess Haeckelian embryology, it does not address the controversy over Haeckel's embryo drawings. Nevertheless, new interest in evolution in and around 1977 inspired developmental biologists to look more closely at Haeckel's illustrations.

In current biology, fundamental research in developmental biology and evolutionary developmental biology is no longer driven by morphological comparisons between embryos, but more by molecular biology. This is partly because Haeckel's drawings were very inaccurate.

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title: Embryo drawing


text:
Enthalpy

Enthalpy is the sum of a thermodynamic system's internal energy and the product of its pressure and volume. It is a state function in thermodynamics used in many measurements in chemical, biological, and physical systems at a constant external pressure, which is conveniently provided by the large ambient atmosphere. The pressure–volume term expresses the work $$W$$ that was done against constant external pressure $$P_{ext}$$ to establish the system's physical dimensions from $$V_{system, initial}=0$$ to some final volume $$V_{system, final}$$ (as $$W=P_{ext}\Delta V$$), i.e. to make room for it by displacing its surroundings. The pressure-volume term is very small for solids and liquids at common conditions, and fairly small for gases. Therefore, enthalpy is a stand-in for energy in chemical systems; bond, lattice, solvation, and other chemical "energies" are actually enthalpy differences. As a state function, enthalpy depends only on the final configuration of internal energy, pressure, and volume, not on the path taken to achieve it.

In the International System of Units (SI), the unit of measurement for enthalpy is the joule. Other historical conventional units still in use include the calorie and the British thermal unit (BTU).

The total enthalpy of a system cannot be measured directly because the internal energy contains components that are unknown, not easily accessible, or are not of interest for the thermodynamic problem at hand. In practice, a change in enthalpy is the preferred expression for measurements at constant pressure, because it simplifies the description of energy transfer. When transfer of matter into or out of the system is also prevented and no electrical or mechanical (stirring shaft or lift pumping) work is done, at constant pressure the enthalpy change equals the energy exchanged with the environment by heat.

In chemistry, the standard enthalpy of reaction is the enthalpy change when reactants in their standard states ($p = 1$ bar ; usually $T = 298$ K) change to products in their standard states. This quantity is the standard heat of reaction at constant pressure and temperature, but it can be measured by calorimetric methods even if the temperature does vary during the measurement, provided that the initial and final pressure and temperature correspond to the standard state. The value does not depend on the path from initial to final state because enthalpy is a state function.

Enthalpies of chemical substances are usually listed for 1 bar pressure as a standard state. Enthalpies and enthalpy changes for reactions vary as a function of temperature, but tables generally list the standard heats of formation of substances at 25 °C. For endothermic (heat-absorbing) processes, the change $ΔH$ is a positive value; for exothermic (heat-releasing) processes it is negative.

The enthalpy of an ideal gas is independent of its pressure or volume, and depends only on its temperature, which correlates to its thermal energy. Real gases at common temperatures and pressures often closely approximate this behavior, which simplifies practical thermodynamic design and analysis.

The word "enthalpy" is derived from the Greek word enthalpein, which means to heat.

Definition
The enthalpy $H$ of a thermodynamic system is defined as the sum of its internal energy and the product of its pressure and volume:

where $U$ is the internal energy, $H = U + p V$ is pressure, and $p$ is the volume of the system; $V$ is sometimes referred to as the pressure energy $p V$$Ɛ$.

Enthalpy is an extensive property; it is proportional to the size of the system (for homogeneous systems). As intensive properties, the specific enthalpy, $p$, is referenced to a unit of mass $h = H⁄m$ of the system, and the molar enthalpy, $m$$H$ = $m$ , where $H⁄n$ is the number of moles. For inhomogeneous systems the enthalpy is the sum of the enthalpies of the component subsystems: $$ H = \sum_k H_k \; ,$$ where
 * $n$ is the total enthalpy of all the subsystems,
 * $H$ refers to the various subsystems,
 * $k$ refers to the enthalpy of each subsystem.

A closed system may lie in thermodynamic equilibrium in a static gravitational field, so that its pressure $Hk$ varies continuously with altitude, while, because of the equilibrium requirement, its temperature $p$ is invariant with altitude. (Correspondingly, the system's gravitational potential energy density also varies with altitude.) Then the enthalpy summation becomes an integral: $$ H = \int \left( \rho\, h \right) \, \mathrm{d}V \; ,$$ where
 * $T$ ("rho") is density (mass per unit volume),
 * $ρ$ is the specific enthalpy (enthalpy per unit mass),
 * $(ρh)$ represents the enthalpy density (enthalpy per unit volume),
 * $dV$ denotes an infinitesimally small element of volume within the system, for example, the volume of an infinitesimally thin horizontal layer.

The integral therefore represents the sum of the enthalpies of all the elements of the volume.

The enthalpy of a closed homogeneous system is its energy function $H( S, p )$, with its entropy $S[ p ]$ and its pressure $h$ as natural state variables which provide a differential relation for $dH$ of the simplest form, derived as follows. We start from the first law of thermodynamics for closed systems for an infinitesimal process:

$$ \mathrm{d}U = \mathrm{\delta}\, Q - \mathrm{\delta}\, W \; ,$$

where
 * $δQ$ is a small amount of heat added to the system,
 * $δW$ is a small amount of work performed by the system.

In a homogeneous system in which only reversible processes or pure heat transfer are considered, the second law of thermodynamics gives $δQ = T dS$, with $p$ the absolute temperature and $dS$ the infinitesimal change in entropy $T$ of the system. Furthermore, if only $S$ work is done, $δW = p dV$. As a result,

$$\mathrm{d}U = T\,\mathrm{d}S - p\,\mathrm{d}V ~.$$

Adding $d(p V)$ to both sides of this expression gives

$$ \mathrm{d}U + \mathrm{d}(p\,V) = T\,\mathrm{d}S - p\,\mathrm{d}V + \mathrm{d}(p\,V) \; ,$$

or

$$\mathrm{d}(U + p\,V) = T\,\mathrm{d}S + V\,\mathrm{d}p ~.$$

So

$$ \mathrm{d}H(S,\,p) = T\,\mathrm{d}S + V\,\mathrm{d}p ~.$$

and the coefficients of the natural variable differentials $dS$ and $dp$ are just the single variables $p V$ and $T$.

Other expressions
The above expression of $dH$ in terms of entropy and pressure may be unfamiliar to some readers. There are also expressions in terms of more directly measurable variables such as temperature and pressure:

$$ \mathrm{d}H = C_\mathsf{p}\,\mathrm{d}T + V\,(1 - \alpha T)\,\mathrm{d}p ~.$$

Here $C$$V$ is the heat capacity at constant pressure and $p$ is the coefficient of (cubic) thermal expansion:

$$ \alpha = \frac{\,1\,}{V} \left( \frac{\partial V}{\,\partial T\,} \right)_\mathsf{p} ~.$$

With this expression one can, in principle, determine the enthalpy if $α$$C$ and $p$ are known as functions of $V$ and $p$. However the expression is more complicated than $$\;\mathrm{d}H = T\,\mathrm{d}S + V\,\mathrm{d}p\;$$ because $T$ is not a natural variable for the enthalpy $T$.

At constant pressure, $$\; \mathrm{d}P = 0 \;$$ so that $$\; \mathrm{d}H = C_\mathsf{p}\,\mathrm{d}T ~.$$ For an ideal gas, $$\;\mathrm{d}H\;$$ reduces to this form even if the process involves a pressure change, because $α T = 1$.

In a more general form, the first law describes the internal energy with additional terms involving the chemical potential and the number of particles of various types. The differential statement for $dH$ then becomes

$$ \mathrm{d}H = T\,\mathrm{d}S + V\,\mathrm{d}p + \sum_i \mu_i\,\mathrm{d}N_i \; ,$$

where $H$ is the chemical potential per particle for a type $μi$ particle, and $i$ is the number of such particles. The last term can also be written as $μi dni$ (with $dni 0$ the number of moles of component $Ni$ added to the system and, in this case, $i$ the molar chemical potential) or as $μi dmi$ (with $dmi$ the mass of component $μi$ added to the system and, in this case, $i$ the specific chemical potential).

Characteristic functions and natural state variables
The enthalpy, $H( S[p], p, \{Ni\} )$, expresses the thermodynamics of a system in the energy representation. As a function of state, its arguments include both one intensive and several extensive state variables. The state variables $S[p]$, $p$, and $\{Ni\}$ are said to be the natural state variables in this representation. They are suitable for describing processes in which they are determined by factors in the surroundings. For example, when a virtual parcel of atmospheric air moves to a different altitude, the pressure surrounding it changes, and the process is often so rapid that there is too little time for heat transfer. This is the basis of the so-called adiabatic approximation that is used in meteorology.

Conjugate with the enthalpy, with these arguments, the other characteristic function of state of a thermodynamic system is its entropy, as a function, $S[p]( H, p, \{Ni\} )$, of the same list of variables of state, except that the entropy, $S[p]$, is replaced in the list by the enthalpy, $μi$. It expresses the entropy representation. The state variables $H$, $H$, and $\{Ni\}$ are said to be the natural state variables in this representation. They are suitable for describing processes in which they are experimentally controlled. For example, $p$ and $H$ can be controlled by allowing heat transfer, and by varying only the external pressure on the piston that sets the volume of the system.

Physical interpretation
The $p$ term is the energy of the system, and the $U$ term can be interpreted as the work that would be required to "make room" for the system if the pressure of the environment remained constant. When a system, for example, $p V$ moles of a gas of volume $n$ at pressure $V$ and temperature $p$, is created or brought to its present state from absolute zero, energy must be supplied equal to its internal energy $T$ plus $U$, where $p V$ is the work done in pushing against the ambient (atmospheric) pressure.

In physics and statistical mechanics it may be more interesting to study the internal properties of a constant-volume system and therefore the internal energy is used. In chemistry, experiments are often conducted at constant atmospheric pressure, and the pressure–volume work represents a small, well-defined energy exchange with the atmosphere, so that $ΔH$ is the appropriate expression for the heat of reaction. For a heat engine, the change in its enthalpy after a full cycle is equal to zero, since the final and initial state are equal.

Relationship to heat
In order to discuss the relation between the enthalpy increase and heat supply, we return to the first law for closed systems, with the physics sign convention: $dU = δQ − δW$, where the heat $δQ$ is supplied by conduction, radiation, Joule heating. We apply it to the special case with a constant pressure at the surface. In this case the work is given by $p dV$ (where $p V$ is the pressure at the surface, $dV$ is the increase of the volume of the system). Cases of long range electromagnetic interaction require further state variables in their formulation, and are not considered here. In this case the first law reads: $$ \mathrm{d}U = \mathrm{\delta}\, Q - p \, \mathrm{d}V ~.$$

Now, $$ \mathrm{d}H = \mathrm{d}U + \mathrm{d}(p\,V) ~.$$

So $$\begin{align} \mathrm{d}H &= \mathrm{\delta}Q + V\,\mathrm{d}p + p \,\mathrm{d}V - p\,\mathrm{d}V\\ &= \mathrm{\delta}Q + V\,\mathrm{d}p ~. \end{align}$$

If the system is under constant pressure, $dp = 0$ and consequently, the increase in enthalpy of the system is equal to the heat added:

$$ \mathrm{d}H = \mathrm{\delta}\, Q$$ This is why the now-obsolete term heat content was used for enthalpy in the 19th century.

Applications
In thermodynamics, one can calculate enthalpy by determining the requirements for creating a system from "nothingness"; the mechanical work required, $p$, differs based upon the conditions that obtain during the creation of the thermodynamic system.

Energy must be supplied to remove particles from the surroundings to make space for the creation of the system, assuming that the pressure $p V$ remains constant; this is the $p$ term. The supplied energy must also provide the change in internal energy, $p V$, which includes activation energies, ionization energies, mixing energies, vaporization energies, chemical bond energies, and so forth. Together, these constitute the change in the enthalpy $U$. For systems at constant pressure, with no external work done other than the $U + p V$ work, the change in enthalpy is the heat received by the system.

For a simple system with a constant number of particles at constant pressure, the difference in enthalpy is the maximum amount of thermal energy derivable from an isobaric thermodynamic process.

Heat of reaction
The total enthalpy of a system cannot be measured directly; the enthalpy change of a system is measured instead. Enthalpy change is defined by the following equation: $$ \Delta H = H_\mathsf{f} - H_\mathsf{i} \, ,$$ where
 * $ΔH$ is the "enthalpy change",
 * $p V$$H$ is the final enthalpy of the system (in a chemical reaction, the enthalpy of the products or the system at equilibrium),
 * $f$$H$ is the initial enthalpy of the system (in a chemical reaction, the enthalpy of the reactants).

For an exothermic reaction at constant pressure, the system's change in enthalpy, $ΔH$, is negative due to the products of the reaction having a smaller enthalpy than the reactants, and equals the heat released in the reaction if no electrical or shaft work is done. In other words, the overall decrease in enthalpy is achieved by the generation of heat. Conversely, for a constant-pressure endothermic reaction, $ΔH$ is positive and equal to the heat absorbed in the reaction.

From the definition of enthalpy as $i$, the enthalpy change at constant pressure is $ΔH = ΔU + p ΔV$. However for most chemical reactions, the work term $p ΔV$ is much smaller than the internal energy change $ΔU$, which is approximately equal to $ΔH$. As an example, for the combustion of carbon monoxide 2 CO(g) + O$H = U + p V$(g) → 2 CO$2$(g), $ΔH = −566.0$ kJ and $ΔU = −563.5$ kJ. Since the differences are so small, reaction enthalpies are often described as reaction energies and analyzed in terms of bond energies.

Specific enthalpy
The specific enthalpy of a uniform system is defined as $2$ where $h = H⁄m$ is the mass of the system. The SI unit for specific enthalpy is joule per kilogram. It can be expressed in other specific quantities by $m$, where $h = u + p v$ is the specific internal energy, $u$ is the pressure, and $p$ is specific volume, which is equal to $1⁄ρ$, where $v$ is the density.

Enthalpy changes
An enthalpy change describes the change in enthalpy observed in the constituents of a thermodynamic system when undergoing a transformation or chemical reaction. It is the difference between the enthalpy after the process has completed, i.e. the enthalpy of the products assuming that the reaction goes to completion, and the initial enthalpy of the system, namely the reactants. These processes are specified solely by their initial and final states, so that the enthalpy change for the reverse is the negative of that for the forward process.

A common standard enthalpy change is the enthalpy of formation, which has been determined for a large number of substances. Enthalpy changes are routinely measured and compiled in chemical and physical reference works, such as the CRC Handbook of Chemistry and Physics. The following is a selection of enthalpy changes commonly recognized in thermodynamics.

When used in these recognized terms the qualifier change is usually dropped and the property is simply termed enthalpy of 'process'. Since these properties are often used as reference values it is very common to quote them for a standardized set of environmental parameters, or standard conditions, including: For such standardized values the name of the enthalpy is commonly prefixed with the term standard, e.g. standard enthalpy of formation.
 * A pressure of one atmosphere (1 atm or 1013.25 hPa) or 1 bar
 * A temperature of 25 °C or 298.15 K
 * A concentration of 1.0 M when the element or compound is present in solution
 * Elements or compounds in their normal physical states, i.e. standard state

Chemical properties
Enthalpy of reaction - is defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of substance reacts completely.

Enthalpy of formation - is defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of a compound is formed from its elementary antecedents.

Enthalpy of combustion - is defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of a substance burns completely with oxygen.

Enthalpy of hydrogenation - is defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of an unsaturated compound reacts completely with an excess of hydrogen to form a saturated compound.

Enthalpy of atomization - is defined as the enthalpy change required to separate one mole of a substance into its constituent atoms completely.

Enthalpy of neutralization - is defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of water is formed when an acid and a base react.

Standard Enthalpy of solution - is defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of a solute is dissolved completely in an excess of solvent, so that the solution is at infinite dilution.

Standard enthalpy of Denaturation (biochemistry) - is defined as the enthalpy change required to denature one mole of compound.

Enthalpy of hydration - is defined as the enthalpy change observed when one mole of gaseous ions are completely dissolved in water forming one mole of aqueous ions.

Physical properties
Enthalpy of fusion - is defined as the enthalpy change required to completely change the state of one mole of substance from solid to liquid.

Enthalpy of vaporization - is defined as the enthalpy change required to completely change the state of one mole of substance from liquid to gas.

Enthalpy of sublimation - is defined as the enthalpy change required to completely change the state of one mole of substance from solid to gas.

Lattice enthalpy - is defined as the energy required to separate one mole of an ionic compound into separated gaseous ions to an infinite distance apart (meaning no force of attraction).

Enthalpy of mixing - is defined as the enthalpy change upon mixing of two (non-reacting) chemical substances.

Open systems
In thermodynamic open systems, mass (of substances) may flow in and out of the system boundaries. The first law of thermodynamics for open systems states: The increase in the internal energy of a system is equal to the amount of energy added to the system by mass flowing in and by heating, minus the amount lost by mass flowing out and in the form of work done by the system:

$$ \mathrm{d}U = \mathrm{\delta}\, Q + \mathrm{d}U_\mathsf{in} - \mathrm{d}U_\mathsf{out} - \mathrm{\delta}\, W \,,$$

where $ρ$$U$ is the average internal energy entering the system, and $in$$U$ is the average internal energy leaving the system.

The region of space enclosed by the boundaries of the open system is usually called a control volume, and it may or may not correspond to physical walls. If we choose the shape of the control volume such that all flow in or out occurs perpendicular to its surface, then the flow of mass into the system performs work as if it were a piston of fluid pushing mass into the system, and the system performs work on the flow of mass out as if it were driving a piston of fluid. There are then two types of work performed: Flow work described above, which is performed on the fluid (this is also often called $out$ work), and mechanical work (shaft work), which may be performed on some mechanical device such as a turbine or pump.

These two types of work are expressed in the equation $$ \mathrm{\delta}\, W = \mathrm{d}(p_\mathsf{out} V_\mathsf{out}) - \mathrm{d}(p_\mathsf{in} V_\mathsf{in}) + \mathrm{\delta}\, W_\mathsf{shaft} ~.$$

Substitution into the equation above for the control volume (cv) yields:

$$ \mathrm{d}U_\mathsf{cv} = \mathrm{\delta}\, Q + \mathrm{d}U_\mathsf{in} + \mathrm{d}\left( p_\mathsf{in} V_\mathsf{in} \right) - \mathrm{d}U_\mathsf{out} - \mathrm{d}\left( p_\mathsf{out} V_\mathsf{out} \right) - \mathrm{\delta} W_\mathsf{shaft} ~.$$

The definition of enthalpy, $p V$, permits us to use this thermodynamic potential to account for both internal energy and $H$ work in fluids for open systems:

$$ \mathrm{d}U_\mathsf{cv} = \mathrm{\delta}\, Q + \mathrm{d}H_\mathsf{in} - \mathrm{d}H_\mathsf{out} - \mathrm{\delta}\, W_\mathsf{shaft} ~.$$

If we allow also the system boundary to move (e.g. due to moving pistons), we get a rather general form of the first law for open systems. In terms of time derivatives, using Newton's dot notation for time derivatives, it reads:

$$ \frac{ \mathrm{d}U }{\, \mathrm{d}\, t \,} = \sum_k \dot Q_k + \sum_k \dot H_k - \sum_k p_k\frac{\,\mathrm{d}V_k}{\, \mathrm{d}t \,} - P \, ,$$

with sums over the various places $p V$ where heat is supplied, mass flows into the system, and boundaries are moving. The $k$ terms represent enthalpy flows, which can be written as

$$ \dot H_k = h_k \dot m_k = H_\mathsf{m} \dot n_k \, ,$$

with $. Hk$ the mass flow and $. mk$ the molar flow at position $. nk$ respectively. The term $d Vk⁄d t$ represents the rate of change of the system volume at position $k$ that results in $k$ power done by the system. The parameter $p V$ represents all other forms of power done by the system such as shaft power, but it can also be, say, electric power produced by an electrical power plant.

Note that the previous expression holds true only if the kinetic energy flow rate is conserved between system inlet and outlet. Otherwise, it has to be included in the enthalpy balance. During steady-state operation of a device (see turbine, pump, and engine), the average $d U⁄d t$ may be set equal to zero. This yields a useful expression for the average power generation for these devices in the absence of chemical reactions:

$$ P = \sum_k \left\langle \dot Q_k \right\rangle + \sum_k \left\langle \dot H_k \right\rangle - \sum_k \left\langle p_k\frac{\, \mathrm{d}V_k }{\,\mathrm{d}t\,} \right\rangle \, ,$$

where the angle brackets denote time averages. The technical importance of the enthalpy is directly related to its presence in the first law for open systems, as formulated above.

Diagrams

The enthalpy values of important substances can be obtained using commercial software. Practically all relevant material properties can be obtained either in tabular or in graphical form. There are many types of diagrams, such as $T – s$ diagrams, which give the specific enthalpy as function of temperature for various pressures, and $T – s$ diagrams, which give $P$ as function of $U$ for various $kJ⁄kg$. One of the most common diagrams is the temperature–specific entropy diagram ($h – T$ diagram). It gives the melting curve and saturated liquid and vapor values together with isobars and isenthalps. These diagrams are powerful tools in the hands of the thermal engineer.

Some basic applications
The points $h – p$ through $T – s$ in the figure play a role in the discussion in this section.
 * {| class="wikitable" style="text-align:center"

!rowspan=2|Point ! $h$ !! $p$ !! $T$ !! $T$ Points $a$ and $h$ are saturated liquids, and point $a$ is a saturated gas.
 * - style="background:#EEEEEE;"
 * K || bar || $p$ || $s$
 * $b$ || 300 || 1 || 6.85 || 461
 * $c$ || 380 || 2 || 6.85 || 530
 * $d$ || 300 || 200 || 5.16 || 430
 * $e$ || 270 || 1 || 6.79 || 430
 * $f$ || 108 || 13 || 3.55 || 100
 * $g$ || 77.2 || 1 || 3.75 || 100
 * $h$ || 77.2 || 1 || 2.83 || 28
 * $e$ || 77.2 || 1 || 5.41 || 230
 * }
 * $g$ || 108 || 13 || 3.55 || 100
 * $h$ || 77.2 || 1 || 3.75 || 100
 * $h = 100$ || 77.2 || 1 || 2.83 || 28
 * $p = 13$ || 77.2 || 1 || 5.41 || 230
 * }
 * $T = 108$ || 77.2 || 1 || 5.41 || 230
 * }
 * }

Throttling

One of the simple applications of the concept of enthalpy is the so-called throttling process, also known as Joule–Thomson expansion. It concerns a steady adiabatic flow of a fluid through a flow resistance (valve, porous plug, or any other type of flow resistance) as shown in the figure. This process is very important, since it is at the heart of domestic refrigerators, where it is responsible for the temperature drop between ambient temperature and the interior of the refrigerator. It is also the final stage in many types of liquefiers.

For a steady state flow regime, the enthalpy of the system (dotted rectangle) has to be constant. Hence

$$ 0 = \dot m h_1 - \dot m h_2 ~.$$

Since the mass flow is constant, the specific enthalpies at the two sides of the flow resistance are the same:

$$ h_1 = h_2 \; ,$$

that is, the enthalpy per unit mass does not change during the throttling. The consequences of this relation can be demonstrated using the $h$ diagram above.

Example 1
Point c is at 200 bar and room temperature (300 K). A Joule–Thomson expansion from 200 bar to 1 bar follows a curve of constant enthalpy of roughly 425 $kJ⁄kg K$ (not shown in the diagram) lying between the 400 and 450 $kJ⁄kg$ isenthalps and ends in point d, which is at a temperature of about 270 K. Hence the expansion from 200 bar to 1 bar cools nitrogen from 300 K to 270 K. In the valve, there is a lot of friction, and a lot of entropy is produced, but still the final temperature is below the starting value.

Example 2
Point e is chosen so that it is on the saturated liquid line with $( hf )$$ṁ$. It corresponds roughly with $( hg )$ bar and $( xf )$ K. Throttling from this point to a pressure of 1 bar ends in the two-phase region (point f). This means that a mixture of gas and liquid leaves the throttling valve. Since the enthalpy is an extensive parameter, the enthalpy in f $( hh )$ is equal to the enthalpy in g $(1 − xf )$ multiplied by the liquid fraction in f $100 = xf × 28 + (1 − xf) × 230$ plus the enthalpy in h $xf = 0.64$ multiplied by the gas fraction in f $Ta$. So

$$ h_\mathbf\mathsf{f} = x_\mathbf\mathsf{f} h_\mathbf\mathsf{g} + (1 - x_\mathbf\mathsf{f})h_\mathsf\mathbf{h} ~.$$

With numbers:
 * $T – s$, so $Ta$.

This means that the mass fraction of the liquid in the liquid–gas mixture that leaves the throttling valve is 64%.

Compressors

A power $T − s$ is applied e.g. as electrical power. If the compression is adiabatic, the gas temperature goes up. In the reversible case it would be at constant entropy, which corresponds with a vertical line in the $( hc − ha ) − Ta( sc − sa )$ diagram. For example, compressing nitrogen from 1 bar (point a) to 2 bar (point b) would result in a temperature increase from 300 K to 380 K. In order to let the compressed gas exit at ambient temperature $T – s$, heat exchange, e.g. by cooling water, is necessary. In the ideal case the compression is isothermal. The average heat flow to the surroundings is $kJ⁄kg$. Since the system is in the steady state the first law gives

$$ 0 = -\dot Q + \dot m\, h_1 - \dot m\, h_2 + P ~.$$

The minimal power needed for the compression is realized if the compression is reversible. In that case the second law of thermodynamics for open systems gives

$$ 0 = -\frac{\, \dot Q \,}{T_\mathsf{a}} + \dot m \, s_1 - \dot m \, s_2 ~.$$

Eliminating $kJ⁄kg$ gives for the minimal power

$$ \frac{\, P_\mathsf{min} \,}{ \dot m } = h_2 - h_1 - T_\mathsf{a}\left( s_2 - s_1 \right) ~.$$

For example, compressing 1 kg of nitrogen from 1 bar to 200 bar costs at least :$(430 − 461) − 300 × (5.16 − 6.85) = 476$. With the data, obtained with the $dh = T ds + v dp$ diagram, we find a value of $ἔργον$$kJ⁄kg$.

The relation for the power can be further simplified by writing it as

$$ \frac{\, P_\mathsf{min} \,}{ \dot m } = \int_1^2 \left( \mathrm{d}h - T_\mathsf{a}\,\mathrm{d}s \right) ~.$$

With

this results in the final relation

$$\frac{\, P_\mathsf{min} }{ \dot m } = \int_1^2 v\,\mathrm{d}p ~.$$

History and etymology
The term enthalpy was coined relatively late in the history of thermodynamics, in the early 20th century. Energy was introduced in a modern sense by Thomas Young in 1802, while entropy by Rudolf Clausius in 1865. Energy uses the root of the Greek word $θάλπος$ (ergon), meaning "work", to express the idea of capacity to perform work. Entropy uses the Greek word τροπή (tropē) meaning transformation or turning. Enthalpy uses the root of the Greek word $dH$ (thalpos) "warmth, heat".

The term expresses the obsolete concept of heat content, as ᙭᙭᙭ refers to the amount of heat gained in a process at constant pressure only, but not in the general case when pressure is variable. J. W. Gibbs used the term "a heat function for constant pressure" for clarity.

Introduction of the concept of "heat content" $ṁ$ is associated with Benoît Paul Émile Clapeyron and Rudolf Clausius (Clausius–Clapeyron relation, 1850).

The term enthalpy first appeared in print in 1909. It is attributed to Heike Kamerlingh Onnes, who most likely introduced it orally the year before, at the first meeting of the Institute of Refrigeration in Paris. It gained currency only in the 1920s, notably with the Mollier Steam Tables and Diagrams, published in 1927.

Until the 1920s, the symbol $P$ was used, somewhat inconsistently, for "heat" in general. The definition of $Q̇$ as strictly limited to enthalpy or "heat content at constant pressure" was formally proposed by A. W. Porter in 1922.

title: Enthalpy


text:
Erdoğan Atalay

Erdoğan Atalay (born 22 September 1966) is a German actor. He is known for his role as police detective Semir Gerkhan in Alarm für Cobra 11 - Die Autobahnpolizei.

Early life and career
Atalay was born in Hanover, West Germany, to a Turkish father and German mother. He was a member of the Theater-AG at the IGS Garbsen. At age 18, he made his first appearance as a supporting role in "Aladdin and the Magic Lamp" at the National Theatre of Hanover before studying acting at the Hochschule für Musik und Theater Hamburg. Afterwards he took on guest roles in several German television series such as Music Groschenweise, Einsatz für Lohbeck, Doppelter Einsatz and Die Wache.

In March 1996, Action Concept cast Atalay in what would become his breakthrough role, starring as Semir Gerkhan, a police detective of Turkish origin. Atalay co-wrote the screenplay for one of the series' episodes, titled "Checkmate," and is a consulting producer for the series as of 2016. In 2005, Atalay published a short story, "Die Türkei ist da oben" ("Turkey is Up There"), in the German-Turkish anthology Was lebbt du?. In September 2012 he shot together with Ilka Bessin (Cindy from Marzahn) the short film "Alarm for Cindy 11", a parody of Alarm for Cobra 11. The short film was broadcast on September 15, 2012 in the program.

Personal life
Atalay's first marriage was to film and theatre actress Astrid Pollmann in 2004; the couple separated in late 2009. They have one daughter, Pauletta, who has also starred alongside her father in Alarm für Cobra 11 as Ayda Gerkhan, the middle child of Semir Gerkhan. His second child was born in mid-2012 to makeup artist and manager Katja Ohneck, with whom he is married as of 2017.

Filmography
Film
 * 1997: Sperling und der falsche Freund
 * 1998: Der Clown
 * 2000: Liebe Pur
 * 2002: Maximum Speed (film)
 * 2006: Hammer und Hart
 * 2011: Geister all inclusive
 * 2015: Macho Man
 * 2018: Asphaltgorillas

TV
 * 1990: Musik Groschenweise
 * 1994: Die Wache
 * 1995: Doppelter Einsatz
 * 1996–present: Alarm für Cobra 11 – Die Autobahnpolizei
 * 1999–2000: Hinter Gittern – Der Frauenknast (3 episodes)
 * 2003: Alarm für Cobra 11 – Einsatz für Team 2
 * 2010: C.I.S. – Chaoten im Sondereinsatz
 * 2012: SOKO 5113
 * 2012: Cindy aus Marzahn und die jungen Wilden
 * 2013: Mordkommission Istanbul
 * 2016: SOKO Stuttgart – Fluch des Geldes
 * since 2019: Die Martina Hill Show (7 episodes, Alarm für Mutti 11)

Theatre
 * 1984: Aladdin and the Magic Lamp (Staatstheater Hannover)

title: Erdoğan Atalay