Hydrogen is a chemical element with symbol H and atomic number 1. With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table. Hydrogen is the most abundant chemical substance in the Universe, constituting 75% of all baryonic mass. Non-remnant stars are composed of hydrogen in the plasma state; the most common isotope of hydrogen, termed protium, has no neutrons. The universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, tasteless, non-toxic, nonmetallic combustible diatomic gas with the molecular formula H2. Since hydrogen forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays a important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a negative charge when it is known as a hydride, or as a positively charged species denoted by the symbol H+.
The hydrogen cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds are always more complex. As the only neutral atom for which the Schrödinger equation can be solved analytically, study of the energetics and bonding of the hydrogen atom has played a key role in the development of quantum mechanics. Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, that it produces water when burned, the property for which it was named: in Greek, hydrogen means "water-former". Industrial production is from steam reforming natural gas, less from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production for the fertilizer market. Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks.
Hydrogen gas is flammable and will burn in air at a wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol: 2 H2 + O2 → 2 H2O + 572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74% and with chlorine at 5–95%; the explosive reactions may be triggered by heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C. Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine, compared to the visible plume of a Space Shuttle Solid Rocket Booster, which uses an ammonium perchlorate composite; the detection of a burning hydrogen leak may require a flame detector. Hydrogen flames in other conditions are blue; the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin.
H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are potentially dangerous acids; the ground state energy level of the electron in a hydrogen atom is −13.6 eV, equivalent to an ultraviolet photon of 91 nm wavelength. The energy levels of hydrogen can be calculated accurately using the Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity; because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, therefore only certain allowed energies. A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation, Dirac equation or the Feynman path integral formulation to calculate the probability density of the electron around the proton.
The most complicated treatments allow for the small effects of special relativity and vacuum polarization. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at all—illustrating how the "planetary orbit" differs from electron motion. There exist two different spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei. In the orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1. At standard temperature and pressure, hydrogen gas contains about 25% of the para form and 75% of the ortho form known as the "normal form"; the equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but because the ortho form is an excited state and has a higher energy
Local Interstellar Cloud
The Local Interstellar Cloud known as the Local Fluff, is the interstellar cloud 30 light-years across through which the Solar System is moving. It is unknown if the Sun is embedded in the Local Interstellar Cloud, or in the region where the Local Interstellar Cloud is interacting with the neighboring G-Cloud; the Solar System is located within a structure called the Local Bubble, a low-density region of the galactic interstellar medium. Within this region is the Local Interstellar Cloud, an area of higher hydrogen density; the Sun is near the edge of the Local Interstellar Cloud. It is thought to have entered the region at some point between 44,000 and 150,000 years ago and is expected to remain within it for another 10,000 to 20,000 years; the cloud has a temperature of about 7,000 K, about the same temperature as the surface of the Sun. However, its specific heat capacity is low because it is not dense, with 0.3 atoms per cubic centimetre. This is less dense than the average for the interstellar medium in the Milky Way, though six times denser than the gas in the hot, low-density Local Bubble which surrounds the local cloud.
In comparison, Earth's atmosphere at the edge of space has around 1.2×1013 molecules per cubic centimeter, dropping to around 50 million at 450 km. The cloud is flowing outwards from the Scorpius–Centaurus Association, a stellar association, a star-forming region. In 2009, Voyager 2 data suggested that the magnetic strength of the local interstellar medium was much stronger than expected; the fact that the Local Interstellar Cloud is magnetized could explain its continued existence despite the pressures exerted upon it by the winds that blew out the Local Bubble. The Local Interstellar Cloud's potential effects on Earth are prevented by the solar wind and the Sun's magnetic field; this interaction with the heliosphere is under study by the Interstellar Boundary Explorer, a NASA satellite mapping the boundary between the Solar System and interstellar space. "A Breeze from the Stars" at NASA Science "Voyager Makes an Interstellar Discovery" at NASA Science "Local Chimney and Superbubbles" Anderson, Mark.
"Don't stop till you get to the Fluff". New Scientist. 193: 26–30. Doi:10.1016/S0262-407960043-8
In nuclear chemistry, nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles. The difference in mass between the reactants and products is manifested as either the release or absorption of energy; this difference in mass arises due to the difference in atomic "binding energy" between the atomic nuclei before and after the reaction. Fusion is other high magnitude stars. A fusion process that produces a nucleus lighter than iron-56 or nickel-62 will yield a net energy release; these elements have the smallest mass per nucleon and the largest binding energy per nucleon, respectively. Fusion of light elements toward these releases energy, while a fusion producing nuclei heavier than these elements will result in energy retained by the resulting nucleons, the resulting reaction is endothermic; the opposite is true for nuclear fission. This means that the lighter elements, such as helium, are in general more fusible.
The extreme astrophysical event of a supernova can produce enough energy to fuse nuclei into elements heavier than iron. In 1920, Arthur Eddington suggested hydrogen-helium fusion could be the primary source of stellar energy. Quantum tunneling was discovered by Friedrich Hund in 1929, shortly afterwards Robert Atkinson and Fritz Houtermans used the measured masses of light elements to show that large amounts of energy could be released by fusing small nuclei. Building on the early experiments in nuclear transmutation by Ernest Rutherford, laboratory fusion of hydrogen isotopes was accomplished by Mark Oliphant in 1932. In the remainder of that decade, the theory of the main cycle of nuclear fusion in stars were worked out by Hans Bethe. Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project. Fusion was accomplished in 1951 with the Greenhouse Item nuclear test. Nuclear fusion on a large scale in an explosion was first carried out on 1 November 1952, in the Ivy Mike hydrogen bomb test.
Research into developing controlled thermonuclear fusion for civil purposes began in earnest in the 1940s, it continues to this day. The release of energy with the fusion of light elements is due to the interplay of two opposing forces: the nuclear force, which combines together protons and neutrons, the Coulomb force, which causes protons to repel each other. Protons are positively charged and repel each other by the Coulomb force, but they can nonetheless stick together, demonstrating the existence of another, short-range, force referred to as nuclear attraction. Light nuclei are sufficiently small and proton-poor allowing the nuclear force to overcome repulsion; this is because the nucleus is sufficiently small that all nucleons feel the short-range attractive force at least as as they feel the infinite-range Coulomb repulsion. Building up nuclei from lighter nuclei by fusion releases the extra energy from the net attraction of particles. For larger nuclei, however, no energy is released, since the nuclear force is short-range and cannot continue to act across longer atomic length scales.
Thus, energy is not released with the fusion of such nuclei. Fusion powers stars and produces all elements in a process called nucleosynthesis; the Sun is a main-sequence star, and, as such, generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses 620 million metric tons of hydrogen and makes 606 million metric tons of helium each second; the fusion of lighter elements in stars releases the mass that always accompanies it. For example, in the fusion of two hydrogen nuclei to form helium, 0.7% of the mass is carried away in the form of kinetic energy of an alpha particle or other forms of energy, such as electromagnetic radiation. It takes considerable energy to force nuclei to fuse those of the lightest element, hydrogen; when accelerated to high enough speeds, nuclei can overcome this electrostatic repulsion and brought close enough such that the attractive nuclear force is greater than the repulsive Coulomb force. The strong force grows once the nuclei are close enough, the fusing nucleons can "fall" into each other and the result is fusion and net energy produced.
The fusion of lighter nuclei, which creates a heavier nucleus and a free neutron or proton releases more energy than it takes to force the nuclei together. Energy released in most nuclear reactions is much larger than in chemical reactions, because the binding energy that holds a nucleus together is greater than the energy that holds electrons to a nucleus. For example, the ionization energy gained by adding an electron to a hydrogen nucleus is 13.6 eV—less than one-millionth of the 17.6 MeV released in the deuterium–tritium reaction shown in the adjacent diagram. The complete conversion of one gram of matter would release 9×1013 joules of energy. Fusion reactions have an energy density many times greater than nuclear fission. Only direct conversion of mass into energy, such as that caused by the annihilatory collision of matter and antimatter, is more energetic per unit of mass than nuclear fusion. Research into using fusion for the p
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons. Occurring tritium is rare on Earth, where trace amounts are formed by the interaction of the atmosphere with cosmic rays, it can be produced by irradiating lithium metal or lithium-bearing ceramic pebbles in a nuclear reactor. Tritium is used as a radioactive tracer, in radioluminescent light sources for watches and instruments, along with deuterium, as a fuel for nuclear fusion reactions with applications in energy generation and weapons; the name of this isotope is derived from Greek, Modern τρίτος, meaning'third'. While tritium has several different experimentally determined values of its half-life, the National Institute of Standards and Technology lists 4,500 ± 8 days, it decays into helium-3 by beta decay as in this nuclear equation: and it releases 18.6 keV of energy in the process. The electron's kinetic energy varies, with an average of 5.7 keV, while the remaining energy is carried off by the nearly undetectable electron antineutrino.
Beta particles from tritium can penetrate only about 6.0 mm of air, they are incapable of passing through the dead outermost layer of human skin. The unusually low energy released in the tritium beta decay makes the decay appropriate for absolute neutrino mass measurements in the laboratory; the low energy of tritium's radiation makes it difficult to detect tritium-labeled compounds except by using liquid scintillation counting. Tritium is produced in nuclear reactors by neutron activation of lithium-6; this is possible with neutrons of any energy, is an exothermic reaction yielding 4.8 MeV. In comparison, the fusion of deuterium with tritium releases about 17.6 MeV of energy. For applications in proposed fusion energy reactors, such as ITER, pebbles consisting of lithium bearing ceramics including Li2TiO3 and Li4SiO4, are being developed for tritium breeding within a helium cooled pebble bed known as a breeder blanket. High-energy neutrons can produce tritium from lithium-7 in an endothermic reaction, consuming 2.466 MeV.
This was discovered. High-energy neutrons irradiating boron-10 will occasionally produce tritium: A more common result of boron-10 neutron capture is 7Li and a single alpha particle. Tritium is produced in heavy water-moderated reactors whenever a deuterium nucleus captures a neutron; this reaction has a quite small absorption cross section, making heavy water a good neutron moderator, little tritium is produced. So, cleaning tritium from the moderator may be desirable after several years to reduce the risk of its escaping to the environment. Ontario Power Generation's "Tritium Removal Facility" processes up to 2,500 tonnes of heavy water a year, it separates out about 2.5 kg of tritium, making it available for other uses. Deuterium's absorption cross section for thermal neutrons is about 0.52 millibarns, whereas that of oxygen-16 is about 0.19 millibarns and that of oxygen-17 is about 240 millibarns. Tritium is an uncommon product of the nuclear fission of uranium-235, plutonium-239, uranium-233, with a production of about one atom per each 10,000 fissions.
The release or recovery of tritium needs to be considered in the operation of nuclear reactors in the reprocessing of nuclear fuels and in the storage of spent nuclear fuel. The production of tritium is not a goal, but rather a side-effect, it is discharged to the atmosphere in small quantities by some nuclear power plants. In June 2016 the Tritiated Water Task Force released a report on the status of tritium in tritiated water at Fukushima Daiichi nuclear plant, as part of considering options for final disposal of this water; this identified that the March 2016 holding of tritium on-site was 760 TBq in a total of 860000 m3 of stored water. This report identified the reducing concentration of tritium in the water extracted from the buildings etc. for storage, seeing a factor of ten decrease over the five years considered, 3.3 MBq/L to 0.3 MBq/L. According to a report by an expert panel considering the best approach to dealing with this issue, "Tritium could be separated theoretically, but there is no practical separation technology on an industrial scale.
Accordingly, a controlled environmental release is said to be the best way to treat low-tritium-concentration water." Tritium's decay product helium-3 has a large cross section for reacting with thermal neutrons, expelling a proton, hence it is converted back to tritium in nuclear reactors. Tritium occurs due to cosmic rays interacting with atmospheric gases. In the most important reaction for natural production, a fast neutron interacts with atmospheric nitrogen: Worldwide, the production of tritium from natural sources is 148 petabecquerels per year; the global equilibrium inventory of tritium created by natural sources remains constant at 2,590 petabecquerels. This is due to losses proportional to the inventory. According to a 1996 report from Institute for Energy and Environmental Research on the US Department of Energy, only 225 kg of tritium had been produced in the United States from 1955 to 1996. Since it continually de
Carl Edward Sagan was an American astronomer, astrophysicist, author, science popularizer, science communicator in astronomy and other natural sciences. He is best known for his work as a science communicator, his best known scientific contribution is research on extraterrestrial life, including experimental demonstration of the production of amino acids from basic chemicals by radiation. Sagan assembled the first physical messages sent into space: the Pioneer plaque and the Voyager Golden Record, universal messages that could be understood by any extraterrestrial intelligence that might find them. Sagan argued the now accepted hypothesis that the high surface temperatures of Venus can be attributed to and calculated using the greenhouse effect. Sagan published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books, he wrote many popular science books, such as The Dragons of Eden, Broca's Brain and Pale Blue Dot, narrated and co-wrote the award-winning 1980 television series Cosmos: A Personal Voyage.
The most watched series in the history of American public television, Cosmos has been seen by at least 500 million people across 60 different countries. The book Cosmos was published to accompany the series, he wrote the science fiction novel Contact, the basis for a 1997 film of the same name. His papers, containing 595,000 items, are archived at The Library of Congress. Sagan advocated scientific skeptical inquiry and the scientific method, pioneered exobiology and promoted the Search for Extra-Terrestrial Intelligence, he spent most of his career as a professor of astronomy at Cornell University, where he directed the Laboratory for Planetary Studies. Sagan and his works received numerous awards and honors, including the NASA Distinguished Public Service Medal, the National Academy of Sciences Public Welfare Medal, the Pulitzer Prize for General Non-Fiction for his book The Dragons of Eden, regarding Cosmos: A Personal Voyage, two Emmy Awards, the Peabody Award, the Hugo Award, he had five children.
After suffering from myelodysplasia, Sagan died of pneumonia at the age of 62, on December 20, 1996. Carl Sagan was born in New York, his father, Samuel Sagan, was an immigrant garment worker from Kamianets-Podilskyi in the Russian Empire, in today's Ukraine. His mother, Rachel Molly Gruber, was a housewife from New York. Carl was named in honor of Rachel's biological mother, Chaiya Clara, in Sagan's words, "the mother she never knew", he had a sister and the family lived in a modest apartment near the Atlantic Ocean, in Bensonhurst, a Brooklyn neighborhood. According to Sagan, they were Reform Jews, the most liberal of North American Judaism's four main groups. Carl and his sister agreed that their father was not religious, but that their mother "definitely believed in God, was active in the temple. During the depths of the Depression, his father worked as a theater usher. According to biographer Keay Davidson, Sagan's "inner war" was a result of his close relationship with both of his parents, who were in many ways "opposites".
Sagan traced his analytical urges to his mother, a woman, poor as a child in New York City during World War I and the 1920s. As a young woman, she had held her own intellectual ambitions, but they were frustrated by social restrictions: her poverty, her status as a woman and a wife, her Jewish ethnicity. Davidson notes that she therefore "worshipped Carl, he would fulfill her unfulfilled dreams."However, he claimed that his sense of wonder came from his father, who in his free time gave apples to the poor or helped soothe labor-management tensions within New York's garment industry. Although he was awed by Carl's intellectual abilities, he took his son's inquisitiveness in stride and saw it as part of his growing up. In his years as a writer and scientist, Sagan would draw on his childhood memories to illustrate scientific points, as he did in his book Shadows of Forgotten Ancestors. Sagan describes his parents' influence on his thinking: My parents were not scientists, they knew nothing about science.
But in introducing me to skepticism and to wonder, they taught me the two uneasily cohabiting modes of thought that are central to the scientific method. Sagan recalls that one of his most defining moments was when his parents took him to the 1939 New York World's Fair when he was four years old; the exhibits became a turning point in his life. He recalled the moving map of the America of Tomorrow exhibit: "It showed beautiful highways and cloverleaves and little General Motors cars all carrying people to skyscrapers, buildings with lovely spires, flying buttresses—and it looked great!" At other exhibits, he remembered how a flashlight that shone on a photoelectric cell created a crackling sound, how the sound from a tuning fork became a wave on an oscilloscope. He witnessed the future media technology that would replace radio: television. Sagan wrote: Plainly, the world held wonders of a kind I had never guessed. How could a tone become a picture and light become a noise? He saw one of the Fair's most publicized events, the burial of a time capsule at Flushing Meadows, which contained mementos of the 1930s to be recovered by Earth's descendants in a future millennium.
"The time capsule thrilled Carl", writes Davidson. As an adult and his colleagues would create similar time capsules—capsules that would be sent out into the galaxy. During World War II Sa
Vernor Steffen Vinge is an American science fiction author and retired professor. He taught mathematics and computer science at San Diego State University, he is the originator of the technological singularity concept and the first to present a fictional "cyberspace". He has won the Hugo Award for his novels and novellas A Fire Upon the Deep, A Deepness in the Sky, Rainbows End, Fast Times at Fairmont High, The Cookie Monster. Vinge published his first short story, "Bookworm, Run!", in the March 1966 issue of Analog Science Fiction edited by John W. Campbell; the story explores the theme of artificially augmented intelligence by connecting the brain directly to computerised data sources. He became a moderately prolific contributor to SF magazines in early 1970s. In 1969, he expanded the story "Grimm's Story" into Grimm's World, his second novel, The Witling, was published in 1976. Vinge came to prominence in 1981 with his novella True Names the first story to present a fleshed-out concept of cyberspace, which would be central to cyberpunk stories by William Gibson, Neal Stephenson and others.
His next two novels, The Peace War and Marooned in Realtime, explore the spread of a future libertarian society, deal with the impact of a technology which can create impenetrable force fields called'bobbles'. These books built Vinge's reputation as an author who would explore ideas to their logical conclusions in inventive ways. Both books were nominated for the Hugo Award, but lost to novels by William Gibson and Orson Scott Card. Vinge won the Hugo Award with his 1992 novel, A Fire Upon the Deep. A Deepness in the Sky was a prequel to Fire, following competing groups of humans in The Slow Zone as they struggle over who has the rights to exploit a technologically emerging alien culture. Deepness won the Hugo Award for Best Novel in 2000, his novellas Fast Times at Fairmont High and The Cookie Monster won Hugo Awards in 2002 and 2004, respectively. Vinge's 2006 novel Rainbows End, set in the same universe and featuring some of the same characters as Fast Times at Fairmont High, won the 2007 Hugo Award for Best Novel.
In 2011, he released The Children of the Sky, a sequel to A Fire Upon the Deep set 10 years following the end of A Fire Upon the Deep. Vinge retired in 2000 from teaching at San Diego State University. Most years, since its inception in 1999, Vinge has been on the Free Software Foundation's selection committee for their Award for the Advancement of Free Software. Vernor Vinge was Writer Guest of Honor at ConJosé, the 60th World Science Fiction Convention in 2002, his former wife, Joan D. Vinge, is a science fiction author, they were married from 1972 to 1979. The Peace War ISBN 0-312-94342-3 — Hugo Award nominee, 1985 The Ungoverned - first published in Far Frontiers, Volume III, included in Across Realtime ISBN 0-671-72098-8 Marooned in Realtime ISBN 0-312-94295-8 — Prometheus Award winner, Hugo Award nominee, 1987 A Fire Upon the Deep — Hugo Award winner, 1993. "True Names" "The Peddler's Apprentice" "The Ungoverned" "Long Shot" Threats... and Other Promises ISBN 0-671-69790-0 "Apartness" "Conquest by Default" "The Whirligig of Time" "Gemstone" "Just Peace" "Original Sin" "The Blabber" True Names and the Opening of the Cyberspace Frontier ISBN 0-312-86207-5 The Collected Stories of Vernor Vinge ISBN 0-312-87373-5 or ISBN 0-312-87584-3 "Bookworm, Run!"
"The Accomplice" "The Peddler's Apprentice" "The Ungoverned" "Long Shot" "Apartness" "Conquest by Default" "The Whirligig of Time" "Bomb Scare" "The Science Fair" "Gemstone" "Just Peace" "Original Sin" "The Blabber" "Win a Nobel Prize!" "The Barbarian Princess" "Fast Times at Fairmont High" "The Coming Technological Singularity: How to Survive in the Post-Human Era", Whole Earth Review "2020 Computing: The creativity machine", Nature "A Dry Martini" "The Cookie Monster" "Synthetic Serendipity", IEEE Spectrum Online, 30 June 2004 "A Preliminary Assessment of the Drake Equation, Being an Excerpt from the Memoirs of Star Captain Y.-T. Lee" (Ga
Television, sometimes shortened to tele or telly, is a telecommunication medium used for transmitting moving images in monochrome, or in color, in two or three dimensions and sound. The term can refer to a television set, a television program, or the medium of television transmission. Television is a mass medium for advertising and news. Television became available in crude experimental forms in the late 1920s, but it would still be several years before the new technology would be marketed to consumers. After World War II, an improved form of black-and-white TV broadcasting became popular in the United States and Britain, television sets became commonplace in homes and institutions. During the 1950s, television was the primary medium for influencing public opinion. In the mid-1960s, color broadcasting was introduced in most other developed countries; the availability of multiple types of archival storage media such as Betamax, VHS tape, local disks, DVDs, flash drives, high-definition Blu-ray Discs, cloud digital video recorders has enabled viewers to watch pre-recorded material—such as movies—at home on their own time schedule.
For many reasons the convenience of remote retrieval, the storage of television and video programming now occurs on the cloud. At the end of the first decade of the 2000s, digital television transmissions increased in popularity. Another development was the move from standard-definition television to high-definition television, which provides a resolution, higher. HDTV may be transmitted in various formats: 1080p, 720p. Since 2010, with the invention of smart television, Internet television has increased the availability of television programs and movies via the Internet through streaming video services such as Netflix, Amazon Video, iPlayer and Hulu. In 2013, 79 % of the world's households owned; the replacement of early bulky, high-voltage cathode ray tube screen displays with compact, energy-efficient, flat-panel alternative technologies such as LCDs, OLED displays, plasma displays was a hardware revolution that began with computer monitors in the late 1990s. Most TV sets sold in the 2000s were flat-panel LEDs.
Major manufacturers announced the discontinuation of CRT, DLP, fluorescent-backlit LCDs by the mid-2010s. In the near future, LEDs are expected to be replaced by OLEDs. Major manufacturers have announced that they will produce smart TVs in the mid-2010s. Smart TVs with integrated Internet and Web 2.0 functions became the dominant form of television by the late 2010s. Television signals were distributed only as terrestrial television using high-powered radio-frequency transmitters to broadcast the signal to individual television receivers. Alternatively television signals are distributed by coaxial cable or optical fiber, satellite systems and, since the 2000s via the Internet; until the early 2000s, these were transmitted as analog signals, but a transition to digital television is expected to be completed worldwide by the late 2010s. A standard television set is composed of multiple internal electronic circuits, including a tuner for receiving and decoding broadcast signals. A visual display device which lacks a tuner is called a video monitor rather than a television.
The word television comes from Ancient Greek τῆλε, meaning'far', Latin visio, meaning'sight'. The first documented usage of the term dates back to 1900, when the Russian scientist Constantin Perskyi used it in a paper that he presented in French at the 1st International Congress of Electricity, which ran from 18 to 25 August 1900 during the International World Fair in Paris; the Anglicised version of the term is first attested in 1907, when it was still "...a theoretical system to transmit moving images over telegraph or telephone wires". It was "...formed in English or borrowed from French télévision." In the 19th century and early 20th century, other "...proposals for the name of a then-hypothetical technology for sending pictures over distance were telephote and televista." The abbreviation "TV" is from 1948. The use of the term to mean "a television set" dates from 1941; the use of the term to mean "television as a medium" dates from 1927. The slang term "telly" is more common in the UK; the slang term "the tube" or the "boob tube" derives from the bulky cathode ray tube used on most TVs until the advent of flat-screen TVs.
Another slang term for the TV is "idiot box". In the 1940s and throughout the 1950s, during the early rapid growth of television programming and television-set ownership in the United States, another slang term became used in that period and continues to be used today to distinguish productions created for broadcast on television from films developed for presentation in movie theaters; the "small screen", as both a compound adjective and noun, became specific references to television, while the "big screen" was used to identify productions made for theatrical release. Facsimile transmission systems for still photographs pioneered methods of mechanical scanning of images in the early 19th century. Alexander Bain introduced the facsimile machine between 1843 and 1846. Frederick Bakewell demonstrated a working laboratory version in 1851. Willoughby Smith discovered the photoconductivity of the element selenium in 1873; as a 23-year-old German university student, Paul Julius Gottlieb Nipkow proposed and patented the Nipkow disk in 1884.
This was a spinning disk with a spiral pattern of holes in it, so each hole scanned a line of the image. Although he never built a working model