1.
Clock
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A clock is an instrument to measure, keep, and indicate time. The word clock is derived from the Celtic words clagan and clocca meaning bell, a silent instrument missing such a striking mechanism has traditionally been known as a timepiece. In general usage today a clock refers to any device for measuring and displaying the time, Watches and other timepieces that can be carried on ones person are often distinguished from clocks. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units, the day, the lunar month. Devices operating on several physical processes have been used over the millennia, a sundial shows the time by displaying the position of a shadow on a flat surface. There is a range of duration timers, an example being the hourglass. Water clocks, along with the sundials, are possibly the oldest time-measuring instruments, spring-driven clocks appeared during the 15th century. During the 15th and 16th centuries, clockmaking flourished, the next development in accuracy occurred after 1656 with the invention of the pendulum clock. A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation, the electric clock was patented in 1840. The development of electronics in the 20th century led to clocks with no clockwork parts at all, the timekeeping element in every modern clock is a harmonic oscillator, a physical object that vibrates or oscillates at a particular frequency. This object can be a pendulum, a fork, a quartz crystal. Analog clocks usually indicate time using angles, Digital clocks display a numeric representation of time. Two numeric display formats are used on digital clocks, 24-hour notation. Most digital clocks use electronic mechanisms and LCD, LED, or VFD displays, for convenience, distance, telephony or blindness, auditory clocks present the time as sounds. There are also clocks for the blind that have displays that can be read by using the sense of touch, some of these are similar to normal analog displays, but are constructed so the hands can be felt without damaging them. The evolution of the technology of clocks continues today, the study of timekeeping is known as horology. The apparent position of the Sun in the sky moves over the course of a day, shadows cast by stationary objects move correspondingly, so their positions can be used to indicate the time of day. A sundial shows the time by displaying the position of a shadow on a flat surface, sundials can be horizontal, vertical, or in other orientations
2.
Long Now Foundation
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The Long Now Foundation, established in 1996, is a public, non-profit organization based in San Francisco that seeks to become the seed of a very long-term cultural institution. It aims to provide a counterpoint to what it views as todays faster/cheaper mindset, the Long Now Foundation hopes to creatively foster responsibility in the framework of the next 10,000 years, and so uses 5-digit dates to address the Year 10,000 problem. The organisations logo, a capital X with an overline, is a representation of 10,000 in Roman numerals, the purpose of the Clock of the Long Now is to construct a timepiece that will operate with minimum human intervention for ten millennia. Its power source should be renewable but similarly unlootable, a prototype of a potential final clock candidate was activated on December 31,1999, and is currently on display at the Science Museum at London. The Foundation hopes to construct the finished Clock at Mount Washington south of Great Basin National Park near Ely, the Rosetta Project is an effort to preserve all languages that have a high likelihood of extinction over the period from 2000 to 2100. These include many languages whose native speakers number in the thousands or fewer, other languages with many more speakers are considered by the project to be endangered because of the increasing importance of English as an international language of commerce and culture. Samples of such languages are to be inscribed onto a disc of nickel alloy three inches across, a Version 1.0 of the disc was completed on November 3,2008. The Long Bet Project was created by the Long Now Foundation to propose and keep track of bets on long-term events, the Long Now Foundation describes The Long Bet Project as a public arena for enjoyably competitive predictions, of interest to society, with philanthropic money at stake. One example bet would be on whether people will fly on pilotless aircraft by 2030. Bets that were resolved in 2010 included predictions about peak oil, print on demand, modem obsolescence, in November 2003, the Long Now Foundation began a series of monthly Seminars About Long-term Thinking with a lecture by Brian Eno. The seminars are held in the San Francisco Bay Area and have focused on policy and thinking, scenario planning, singularity. The seminars are available for download in various formats from the Long Now Foundation and they are intended to nudge civilization toward making long-term thinking automatic and common. As part of the Seminar series, there are debates on areas of long term concern. The point of Long Now debates is not win-lose, the point is public clarity and deep understanding, leading to action graced with nuance and built-in adaptivity, with long-term responsibility in mind. In operation, There are two debaters, Alice and Bob, Alice takes the podium, makes her argument. Then Bob takes her place, but before he can present his counter-argument, he must summarize Alice’s argument to her satisfaction — a demonstration of respect and good faith. Only when Alice agrees that Bob has got it right is he permitted to proceed with his own argument — and then, mutual understanding is enforced by a reciprocal requirement to describe the others argument to their satisfaction, with the goal being more understanding after the event than there was beforehand. Opened in June 2014, The Interval was designed as social space in the foundations Fort Mason facility in San Francisco
3.
Danny Hillis
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William Daniel Danny Hillis is an American inventor, engineer, mathematician, entrepreneur, and author. He co-founded Thinking Machines Corporation, a company developed the Connection Machine. He is also co-founder of the Long Now Foundation, Applied Minds, Metaweb Technologies, Applied Proteomics and he is Judge Widney Professor of Engineering and Medicine at the University of Southern California. Hillis was born in Baltimore, Maryland in 1956 and his father, William Hillis, was a US Air Force epidemiologist studying hepatitis in Africa and relocated with his family through Rwanda, Burundi, Republic of the Congo, and Kenya. He spent a part of his childhood in Calcutta, India when his father was a visiting faculty at ISI. During these years the young Hillis was home schooled by his mother Argye Briggs Hillis, a biostatistician, and developed an early appreciation for mathematics and biology. His younger brother is David Hillis, a professor of biology at the University of Texas at Austin, and his sister is Argye E. Hillis. In 1978 Hillis graduated from MIT with a BS degree in mathematics, followed in 1981 with an MS degree in Electrical Engineering and Computer Science, during this time Hillis worked at the MIT Logo Laboratory developing computer hardware and software for children. He designed computer-oriented toys and games for the Milton Bradley Company and he also built a digital computer composed of Tinkertoys that is on display at the Museum of Science, Boston. Hillis major research, however, was parallel computing. Hillis designed the Connection Machine, a supercomputer, in 1983 Hillis co-founded Thinking Machines Corporation to produce. In 1985, continuing research, Hillis received a PhD in EECS from MIT under doctoral advisers Gerald Jay Sussman, Marvin Minsky. The thesis won the ACM Distinguished Dissertation prize for 1985, Hillis co-founded Thinking Machines Corporation in 1983 while doing his doctoral work at MIT. The company was to develop Hillis Connection Machine design into commercial parallel supercomputers, the information is in the connection between a lot of very simple parallel units working together. So if we built a computer that was more along that system of organization and he even recruited Richard Feynman to spend his summers there. In 1990, Thinking Machines was the leader in parallel supercomputers. During 1994, however, Thinking Machines filed for bankruptcy, I remember listening to Walt Disney on television describing the Imagineers who designed Disneyland. I decided then that someday I would be an Imagineer, later, I became interested in a different kind of magic – the magic of computers
4.
Chime (bell instrument)
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A carillon-like instrument with less than 23 bells is called a chime. American chimes usually have one to one and a half diatonic octaves, the first bell chime was created in 1487. Before 1900, chime bells typically lacked dynamic variation and the inner tuning required to permit the use of harmony, since then, chime bells produced in Belgium, the Netherlands, England, and America have inner tuning and can produce fully harmonized music. Some towers in England hung for full change ringing chime by an Ellacombe apparatus. The Arma Sifton bells at the International Peace Garden, North Dakota, the 14 bells by Gillett & Johnston were a gift from Central United Church of Brandon, Manitoba, in 1972. The tower was supplied by North Dakota Veterans and dedicated in 1976, the chimes of St. Peter the Apostle Parish in New Brunswick, New Jersey, United States. These nine bells were installed in 1870 by Meneely Bell Company of Watervliet, consists of 17 bells,10 of which were originally cast for St. Stephens Church in Cohasset Massachusetts, in the 1920s and 7 bells cast in 1997 by Royal Eijsbouts bell foundry in The Netherlands. The chime at the Highland Arts Theatre in Sydney, Nova Scotia still sings forth regularly and has become a part of the sound of the citys downtown and this is the original McShane Bell Foundry Ten Bell Chime, installed in the church as it was constructed in 1911. Follow this link to a page of videos of the chime being played, The Chimes ~ Christmas Music Campanology, Chimes
5.
Science Museum, London
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The Science Museum is a major museum on Exhibition Road in South Kensington, London. It was founded in 1857 and today is one of the major tourist attractions. Like other publicly funded museums in the United Kingdom, the Science Museum does not charge visitors for admission. Temporary exhibitions, however, may incur an admission fee and it is part of the Science Museum Group, having merged with the Museum of Science and Industry in Manchester in 2012. It included a collection of machinery which became the Museum of Patents in 1858, and this collection contained many of the most famous exhibits of what is now the Science Museum. In 1883, the contents of the Patent Office Museum were transferred to the South Kensington Museum, in 1885, the Science Collections were renamed the Science Museum and in 1893 a separate director was appointed. The Art Collections were renamed the Art Museum, which became the Victoria. When Queen Victoria laid the stone for the new building for the Art Museum, she stipulated that the museum be renamed after herself. On 26 June 1909 the Science Museum, as an independent entity, the Science Museums present quarters, designed by Sir Richard Allison, were opened to the public in stages over the period 1919–28. This building was known as the East Block, construction of which began in 1913, as the name suggests it was intended to be the first building of a much larger project, which was never realized. It also contains hundreds of interactive exhibits, a recent addition is the IMAX 3D Cinema showing science and nature documentaries, most of them in 3-D, and the Wellcome Wing which focuses on digital technology. Entrance has been free since 1 December 2001, the museum houses some of the many objects collected by Henry Wellcome around a medical theme. The fourth floor exhibit is called Glimpses of Medical History, with reconstructions, the fifth floor gallery is called Science and the Art of Medicine, with exhibits of medical instruments and practices from ancient days and from many countries. The collection is strong in clinical medicine, biosciences and public health, the museum is a member of the London Museums of Health & Medicine. The Science Museum has a library, and until the 1960s was Britains National Library for Science, Medicine. It holds runs of periodicals, early books and manuscripts, and is used by scholars worldwide and it was, for a number of years, run in conjunction with the Library of Imperial College, but in 2007 the Library was divided over two sites. The Imperial College library catalogue search system now informs searchers that volumes formerly held there are Available at Science Museum Library Swindon Currently unavailable, a new Research Centre with library facilities is promised for late 2015 but is unlikely to have book stacks nearby. The Science Museums medical collections have a scope and coverage
6.
Fort Mason
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Fort Mason served as an Army post for more than 100 years, initially as a coastal defense site and subsequently as a military port facility. During World War II, it was the port for the Pacific campaign. Today it is part of the Golden Gate National Recreation Area and it is a National Historic Landmark District with over 49 buildings of historic significance, spread over 1,200 acres. Fort Mason can be split into two distinct areas, the upper area, sometimes called Fort Mason, is situated on a headland and was the site of the original coastal fortifications. The lower area, Fort Mason Center, is situated close to level to the west of Upper Fort Mason. The Marina Green lies to the west of Fort Mason, while Aquatic Park is to the east, the nucleus of Fort Mason was a private property owned by John C. Frémont, the explorer of the US west, who also spearheaded the conquest of California from Mexico. Appointed a Major General in the Union army at the start of the Civil War, in 1863, the government seized the property without payment, by executive order of Lincoln, on the grounds it was needed for the war effort. Frémont would again contest the US presidency in 1864, running as the candidate of radical Republicans, the 1968 lawsuit was perhaps the last shot of a century-long legal struggle to obtain compensation for the seized realty. In 1870, the government returned property to 49 parties in the vicinity, but not to Frémont, but in 1968 the Frémont heirs complained it had failed to carry out this direction, with John Frémont then recently dead and his widow Jessie nearly 70 years old. The Civil War prompted the construction of coastal defense batteries located inside the Golden Gate. A breast-high wall of brick and mounts for six 10-inch Rodman cannons, excavation in the early 1980s uncovered the well-preserved remains of the western-half of the temporary battery, and it has now been restored to its condition during the Civil War. The fort was named Fort Mason in 1882, after Richard Barnes Mason, President Grover Cleveland established the Endicott Board in 1885 for the purpose of modernizing the nations coastal fortifications. Chaired by Secretary of War William Endicott, the board recommended new defenses at 22 U. S. seaports, as a result, an extensive series of forts, batteries, and guns were built on the harbor, including Fort Mason. The piers and sheds of Lower Fort Mason were originally built from 1912 to warehouse army supplies, by this time, the US Army began to build new bases in Hawaii, the Philippines, and various other Pacific islands. Most of the materiel for those bases was shipped through San Francisco, by 1915, the three piers together with their associated warehouse had been completed, and Fort Mason Tunnel driven under Upper Fort Mason to connect with the railroad network along the Embarcadero. With these new facilities, Fort Mason was transformed from a defense post into a logistical. The Army ferry USAT General Frank M. Coxe provided scheduled transportation from Fort Mason to the center at Fort McDowell on Angel Island up to eight times per day during the war
7.
Jeff Bezos
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The company began as an Internet merchant of books and expanded to a wide variety of products and services, most recently video streaming and audio streaming. Amazon. com is currently the worlds largest Internet sales company on the World Wide Web, Bezos’s other diversified business interests include aerospace and newspapers. He is the founder and manufacturer of Blue Origin with test flights to space beginning in 2015, in 2013, Bezos purchased The Washington Post newspaper. A number of other investments are managed through Bezos Expeditions. Bezos is currently the second-richest person in the world, with a net worth of US$77.1 billion as of March 2017. His rise to this occurred after Amazon registered a 67% jump in share price. Bezos was born Jeffrey Preston Jorgensen to Jacklyn and Ted Jorgensen in Albuquerque and his maternal ancestors were settlers who lived in Texas, and over the generations acquired a 25, 000-acre ranch near Cotulla. As of March 2015, Bezos was among the largest landholders in Texas, Bezos’s maternal grandfather was a regional director of the U. S. He retired early to the ranch, where Bezos spent many summers as a youth, at an early age, he displayed mechanical aptitude—as a toddler, he even dismantled his crib with a screwdriver. Bezos’s mother Jacklyn was a teenager at the time of his birth and her marriage to Jorgensen lasted a little more than a year. In April 1968 she married her husband, Miguel Bezos. His family was originally from Valladolid, Miguel Bezos worked his way through the University of Albuquerque, married Jacklyn, and legally adopted his stepson Jeff, who changed his surname from Jorgensen to Bezos. After the wedding the family moved to Houston and Miguel worked as an engineer for Exxon, the young Jeff attended River Oaks Elementary School in Houston from fourth to sixth grade. As a child, he spent summers working on his grandfathers ranch in southern Texas, Bezos often displayed scientific interests and technological proficiency, he once rigged an electric alarm to keep his younger siblings out of his room. The family moved to Miami, Florida, where he attended Miami Palmetto High School, while in high school, he attended the Student Science Training Program at the University of Florida, receiving a Silver Knight Award in 1982. He was high school valedictorian and was a National Merit Scholar, Bezos graduated summa cum laude and Phi Beta Kappa from Princeton University with bachelor of science degrees in electrical engineering and computer science. While at Princeton, he was elected to Tau Beta Pi. He served as the president of the Princeton chapter of the Students for the Exploration, in 2016 Bezos played a Starfleet official in the movie Star Trek Beyond, later joining the cast and crew at a San Diego Comic-Con screening
8.
Stewart Brand
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Stewart Brand is an American writer, best known as editor of the Whole Earth Catalog. He founded a number of organizations, including The WELL, the Global Business Network, and he is the author of several books, most recently Whole Earth Discipline, An Ecopragmatist Manifesto. He studied biology at Stanford University, graduating in 1960 and he was married to Lois Jennings, an Ottawa Native American and mathematician. As a soldier in the U. S. Army, he was a parachutist and taught infantry skills, Brand has lived in California since the 1960s. He and his wife live on Mirene, a 64-foot -long working tugboat. Built in 1912, the boat is moored in a shipyard in Sausalito. He works in Mary Heartline, a fishing boat about 100 yards away. A favorite item of his is a table on which Otis Redding is said to have written “ The Dock of the Bay”, through engaging in scholarship and by visiting numerous Indian reservations, Brand familiarized himself with the Native Americans of the West. Native Americans have continued to be an important cultural interest, one which has continually re-emerged in Brands work in various ways, by the mid-1960s, Brand became associated with author Ken Kesey and the Merry Pranksters. With his partner Zach Stewart, he produced the Trips Festival in San Francisco and this was one of the first venues at which the Grateful Dead performed in San Francisco. About 10,000 hippies attended, and Haight-Ashbury soon emerged as a community, tom Wolfe describes Brand in the beginning of his 1968 book, The Electric Kool-Aid Acid Test. In 1966, Brand campaigned to have NASA release the satellite image of the entire Earth as seen from space. He sold and distributed buttons for 25 cents each asking, Why havent we seen a photograph of the whole Earth yet, during this campaign, Brand met Richard Buckminster Fuller, who offered to help Brand with his projects. In 1967, a satellite, ATS-3, took the photo, Brand thought the image of our planet would be a powerful symbol. It adorned the first edition of the Whole Earth Catalog, later in 1968, a NASA astronaut took an Earth photo, Earthrise, from Moon orbit, which became the front image of the spring 1969 edition of the Catalog. 1970 saw the first celebration of Earth Day, during a 2003 interview, Brand explained that the image gave the sense that Earth’s an island, surrounded by a lot of inhospitable space. And it’s so graphic, this blue, white, green. During the late 1960s and early 1970s about 10 million Americans were involved in living communally, Brand and his wife Lois travelled to communes in a 1963 Dodge truck known as the Whole Earth Truck Store, which moved to a storefront in Menlo Park, California
9.
Gemstone
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A gemstone is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks or organic materials that are not minerals are used for jewelry and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their luster or other properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone, apart from jewelry, from earliest antiquity engraved gems and hardstone carvings, such as cups, were major luxury art forms. A gem maker is called a lapidary or gemcutter, a worker is a diamantaire. The carvings of Carl Fabergé are significant works in this tradition, the traditional classification in the West, which goes back to the ancient Greeks, begins with a distinction between precious and semi-precious, similar distinctions are made in other cultures. In modern usage the precious stones are diamond, ruby, sapphire, other stones are classified by their color, translucency and hardness. Another unscientific term for semi-precious gemstones used in art history and archaeology is hardstone, in modern times gemstones are identified by gemologists, who describe gems and their characteristics using technical terminology specific to the field of gemology. The first characteristic a gemologist uses to identify a gemstone is its chemical composition, for example, diamonds are made of carbon and rubies of aluminium oxide. Next, many gems are crystals which are classified by their crystal system such as cubic or trigonal or monoclinic, another term used is habit, the form the gem is usually found in. For example, diamonds, which have a crystal system, are often found as octahedrons. Gemstones are classified into different groups, species, and varieties, for example, ruby is the red variety of the species corundum, while any other color of corundum is considered sapphire. Other examples are the Emerald, aquamarine, red beryl, goshenite, heliodor, and morganite, gems are characterized in terms of refractive index, dispersion, specific gravity, hardness, cleavage, fracture, and luster. They may exhibit pleochroism or double refraction and they may have luminescence and a distinctive absorption spectrum. Material or flaws within a stone may be present as inclusions, gemstones may also be classified in terms of their water. This is a grading of the gems luster, transparency. Very transparent gems are considered first water, while second or third water gems are those of a lesser transparency, there is no universally accepted grading system for gemstones. Diamonds are graded using a system developed by the Gemological Institute of America in the early 1950s, historically, all gemstones were graded using the naked eye
10.
Bronze Age
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The Bronze Age is a historical period characterized by the use of bronze, proto-writing, and other early features of urban civilization. The Bronze Age is the principal period of the three-age Stone-Bronze-Iron system, as proposed in modern times by Christian Jürgensen Thomsen. An ancient civilization is defined to be in the Bronze Age either by smelting its own copper and alloying with tin, arsenic, or other metals, or by trading for bronze from production areas elsewhere. Copper-tin ores are rare, as reflected in the fact there were no tin bronzes in Western Asia before trading in bronze began in the third millennium BC. Worldwide, the Bronze Age generally followed the Neolithic period, with the Chalcolithic serving as a transition, although the Iron Age generally followed the Bronze Age, in some areas, the Iron Age intruded directly on the Neolithic. Bronze Age cultures differed in their development of the first writing, according to archaeological evidence, cultures in Mesopotamia and Egypt developed the earliest viable writing systems. The overall period is characterized by use of bronze, though the place and time of the introduction. Human-made tin bronze technology requires set production techniques, tin must be mined and smelted separately, then added to molten copper to make bronze alloy. The Bronze Age was a time of use of metals. The dating of the foil has been disputed, the Bronze Age in the ancient Near East began with the rise of Sumer in the 4th millennium BC. Societies in the region laid the foundations for astronomy and mathematics, the usual tripartite division into an Early, Middle and Late Bronze Age is not used. Instead, a division based on art-historical and historical characteristics is more common. The cities of the Ancient Near East housed several tens of thousands of people, ur in the Middle Bronze Age and Babylon in the Late Bronze Age similarly had large populations. The earliest mention of Babylonia appears on a tablet from the reign of Sargon of Akkad in the 23rd century BC, the Amorite dynasty established the city-state of Babylon in the 19th century BC. Over 100 years later, it took over the other city-states. Babylonia adopted the written Semitic Akkadian language for official use, by that time, the Sumerian language was no longer spoken, but was still in religious use. Elam was an ancient civilization located to the east of Mesopotamia, in the Old Elamite period, Elam consisted of kingdoms on the Iranian plateau, centered in Anshan, and from the mid-2nd millennium BC, it was centered in Susa in the Khuzestan lowlands. Its culture played a role in the Gutian Empire and especially during the Achaemenid dynasty that succeeded it
11.
Nuclear power
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Nuclear power is the use of nuclear reactions that release nuclear energy to generate heat, which most frequently is then used in steam turbines to produce electricity in a nuclear power plant. The term includes nuclear fission, nuclear decay and nuclear fusion, since all electricity supplying technologies use cement, etc. during construction, emissions are yet to be brought to zero. Each result is contrasted with coal and fossil gas at 820 and 490 g CO2 eq/kWh, there is a social debate about nuclear power. Proponents, such as the World Nuclear Association and Environmentalists for Nuclear Energy, contend that nuclear power is a safe, opponents, such as Greenpeace International and NIRS, contend that nuclear power poses many threats to people and the environment. These include the Chernobyl disaster which occurred in 1986, the Fukushima Daiichi nuclear disaster, there have also been some nuclear submarine accidents. Energy production from coal, petroleum, natural gas and hydroelectricity has caused a number of fatalities per unit of energy generated due to air pollution. In 2015, Ten new reactors were connected to the grid, seven reactors were permanently shut down. 441 reactors had a net capacity of 382,855 megawatts of electricity. 67 new nuclear reactors were under construction, Most of the new activity is in China where there is an urgent need to control pollution from coal plants. In October 2016, Watts Bar 2 became the first new United States reactor to enter commercial operation since 1996. The same year, his doctoral student James Chadwick discovered the neutron, further work by Enrico Fermi in the 1930s focused on using slow neutrons to increase the effectiveness of induced radioactivity. Experiments bombarding uranium with neutrons led Fermi to believe he had created a new, transuranic element and they determined that the relatively tiny neutron split the nucleus of the massive uranium atoms into two roughly equal pieces, contradicting Fermi. Numerous scientists, including Leó Szilárd, who was one of the first, recognized that if fission reactions released additional neutrons, a self-sustaining nuclear chain reaction could result. In the United States, where Fermi and Szilárd had both emigrated, this led to the creation of the first man-made reactor, known as Chicago Pile-1, which achieved criticality on December 2,1942. In 1945, the pocketbook The Atomic Age heralded the untapped atomic power in everyday objects and depicted a future where fossil fuels would go unused. One science writer, David Dietz, wrote that instead of filling the gas tank of a car two or three times a week, people travel for a year on a pellet of atomic energy the size of a vitamin pill. The United Kingdom, Canada, and the USSR proceeded over the course of the late 1940s, electricity was generated for the first time by a nuclear reactor on December 20,1951, at the EBR-I experimental station near Arco, Idaho, which initially produced about 100 kW. Work was also researched in the US on nuclear marine propulsion
12.
Solar power
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Solar power is the conversion of energy from sunlight into electricity, either directly using photovoltaics, or indirectly using concentrated solar power. Concentrated solar power systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam, Photovoltaic cells convert light into an electric current using the photovoltaic effect. Most solar installations would be in China and India, Solar PV is rapidly becoming an inexpensive, low-carbon technology to harness renewable energy from the Sun. The current largest photovoltaic power station in the world is the 850 MW Longyangxia Dam Solar Park, in Qinghai, commercial concentrated solar power plants were first developed in the 1980s. The 392 MW Ivanpah installation is the largest concentrating solar power plant in the world, long distance transmission allows remote renewable energy resources to displace fossil fuel consumption. Solar power plants use one of two technologies, Photovoltaic systems use solar panels, either on rooftops or in ground-mounted solar farms, concentrated solar power plants use solar thermal energy to make steam, that is thereafter converted into electricity by a turbine. A solar cell, or photovoltaic cell, is a device that converts light into electric current using the photovoltaic effect, the first solar cell was constructed by Charles Fritts in the 1880s. The German industrialist Ernst Werner von Siemens was among those who recognized the importance of this discovery, following the work of Russell Ohl in the 1940s, researchers Gerald Pearson, Calvin Fuller and Daryl Chapin created the silicon solar cell in 1954. These early solar cells cost 286 USD/watt and reached efficiencies of 4. 5–6%, the array of a photovoltaic power system, or PV system, produces direct current power which fluctuates with the sunlights intensity. For practical use this usually requires conversion to certain desired voltages or alternating current, multiple solar cells are connected inside modules. Modules are wired together to form arrays, then tied to an inverter, which produces power at the voltage, and for AC. Many residential PV systems are connected to the grid wherever available, in these grid-connected PV systems, use of energy storage is optional. In certain applications such as satellites, lighthouses, or in developing countries, batteries or additional power generators are often added as back-ups, such stand-alone power systems permit operations at night and at other times of limited sunlight. Concentrated solar power, also called concentrated solar thermal, uses lenses or mirrors, contrary to photovoltaics – which converts light directly into electricity – CSP uses the heat of the suns radiation to generate electricity from conventional steam-driven turbines. A wide range of concentrating technologies exists, among the best known are the trough, the compact linear Fresnel reflector, the Stirling dish. Various techniques are used to track the sun and focus light, in all of these systems a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage. Thermal storage efficiently allows up to 24-hour electricity generation, a parabolic trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflectors focal line. The receiver is a tube positioned right above the middle of the mirror and is filled with a working fluid
13.
Pendulum
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A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, when released, the restoring force combined with the pendulums mass causes it to oscillate about the equilibrium position, swinging back and forth. The time for one cycle, a left swing and a right swing, is called the period. The period depends on the length of the pendulum and also to a degree on the amplitude. Pendulums are also used in instruments such as accelerometers and seismometers. Historically they were used as gravimeters to measure the acceleration of gravity in geophysical surveys, the word pendulum is new Latin, from the Latin pendulus, meaning hanging. The simple gravity pendulum is a mathematical model of a pendulum. This is a weight on the end of a massless cord suspended from a pivot, when given an initial push, it will swing back and forth at a constant amplitude. Real pendulums are subject to friction and air drag, so the amplitude of their swings declines and it is independent of the mass of the bob. For small swings the period of swing is approximately the same for different size swings, that is and this property, called isochronism, is the reason pendulums are so useful for timekeeping. Successive swings of the pendulum, even if changing in amplitude, for larger amplitudes, the period increases gradually with amplitude so it is longer than given by equation. For example, at an amplitude of θ0 = 23° it is 1% larger than given by, the period increases asymptotically as θ0 approaches 180°, because the value θ0 = 180° is an unstable equilibrium point for the pendulum. The length L used to calculate the period of the simple pendulum in eq. above is the distance from the pivot point to the center of mass of the bob. Any swinging rigid body free to rotate about a horizontal axis is called a compound pendulum or physical pendulum. The appropriate equivalent length L for calculating the period of any such pendulum is the distance from the pivot to the center of oscillation. This point is located under the center of mass at a distance from the pivot traditionally called the radius of oscillation, if most of the mass is concentrated in a relatively small bob compared to the pendulum length, the center of oscillation is close to the center of mass. Substituting this expression in above, the period T of a pendulum is given by T =2 π I m g R for sufficiently small oscillations. For example, a rigid rod of length L pivoted about one end has moment of inertia I = m L2 /3
14.
Wear
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Wear is related to interactions between surfaces and specifically the removal and deformation of material on a surface as a result of mechanical action of the opposite surface. In materials science, wear is erosion or sideways displacement of material from its derivative, Wear of metals occurs by the plastic displacement of surface and near-surface material and by the detachment of particles that form wear debris. The size of the particles may vary from millimeter range down to an ion range. This process may occur by contact with metals, nonmetallic solids, flowing liquids. Secondary stage or mid-age process, where a rate of ageing is in motion. Most of the operational life is comprised in this stage. Tertiary stage or old-age period, where the components are subjected to rapid failure due to a rate of ageing. The secondary stage is shortened with increasing severity of conditions such as higher temperatures, strain rates, stress. Note that, wear rate is influenced by the operating conditions. Specifically, normal loads and sliding speeds play a role in determining wear rate. In addition, tribo-chemical reaction is important in order to understand the wear behavior. Different oxide layers are developed during the sliding motion, the layers are originated from complex interaction among surface, lubricants, and environmental molecules. In general, a plot, namely wear map. Demonstrating wear rate under different loading condition is used for operation and this graph also represents dominating wear modes under different loading conditions. Surface engineering and treatments are used to wear and extend the components working life. The study of the processes of wear is part of the discipline of tribology, the complex nature of wear has delayed its investigations and resulted in isolated studies towards specific wear mechanisms or processes. Impact-, cavitation-, diffusive- and corrosive- wear are all such examples and these wear mechanisms, however, do not necessarily act independently and wear mechanisms are not mutually exclusive. Industrial Wear are commonly described as incidence of multiple wear mechanisms occurring in unison, another way to describe Industrial Wear is to define clear distinctions in how different friction mechanisms operate, for example distinguish between mechanisms with high or low energy density
15.
Torsion spring
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A torsion spring is a spring that works by torsion or twisting, that is, a flexible elastic object that stores mechanical energy when it is twisted. When it is twisted, it exerts a force in the opposite direction, a torsion bar is a straight bar of metal or rubber that is subjected to twisting about its axis by torque applied at its ends. A more delicate form used in instruments, called a torsion fiber consists of a fiber of silk, glass, or quartz under tension. This terminology can be confusing because in a torsion spring the forces acting on the wire are actually bending stresses. It is analogous to the constant of a linear spring. The negative sign indicates that the direction of the torque is opposite to the direction of twist. Other uses are in the large, coiled torsion springs used to counterbalance the weight of doors. Small, coiled torsion springs are used to operate pop-up doors found on small consumer goods like digital cameras. It absorbs road shocks as the wheel goes over bumps and rough road surfaces, torsion-bar suspensions are used in many modern cars and trucks, as well as military vehicles. The sway bar used in vehicle suspension systems also uses the torsion spring principle. The torsion pendulum used in pendulum clocks is a wheel-shaped weight suspended from its center by a wire torsion spring. The weight rotates about the axis of the spring, twisting it, the force of the spring reverses the direction of rotation, so the wheel oscillates back and forth, driven at the top by the clocks gears. The balance spring or hairspring in mechanical watches is a fine, spiral-shaped torsion spring that pushes the wheel back toward its center position as it rotates back. The balance wheel and spring function similarly to the torsion pendulum above in keeping time for the watch, the DArsonval movement used in mechanical pointer-type meters to measure electric current is a type of torsion balance. A coil of wire attached to the twists in a magnetic field against the resistance of a torsion spring. Hookes law ensures that the angle of the pointer is proportional to the current, a DMD or digital micromirror device chip is at the heart of many video projectors. It uses hundreds of thousands of mirrors on tiny torsion springs fabricated on a silicon surface to reflect light onto the screen. The torsion balance consists of a bar suspended from its middle by a thin fiber, the fiber acts as a very weak torsion spring
16.
Balance wheel
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A balance wheel, or balance, is the timekeeping device used in mechanical watches and some clocks, analogous to the pendulum in a pendulum clock. It is a wheel that rotates back and forth, being returned toward its center position by a spiral torsion spring. It is driven by the escapement, which transforms the motion of the watch gear train into impulses delivered to the balance wheel. Each swing of the wheel allows the train to advance a set amount. From its invention in the 14th century until tuning fork and quartz movements became available in the 1960s, virtually every portable timekeeping device used some form of balance wheel. Modern watch balance wheels are made of Glucydur, a low thermal expansion alloy of beryllium, copper and iron. The two alloys are matched so their residual temperature responses cancel out, resulting in lower temperature error. The wheels are smooth, to air friction, and the pivots are supported on precision jewel bearings. Older balance wheels used weight screws around the rim to adjust the poise, Balance wheels rotate about 1½ turns with each swing, that is, about 270° to each side of their center equilibrium position. The rate of the wheel is adjusted with the regulator. This holds the part of the spring behind the slit stationary, moving the lever slides the slit up and down the balance spring, changing its effective length, and thus the resonant vibration rate of the balance. Since the regulator interferes with the action, chronometers and some precision watches have ‘free sprung’ balances with no regulator. Their rate is adjusted by weight screws on the balance rim, a balances vibration rate is traditionally measured in beats per hour, or BPH, although beats per second and Hz are also used. The length of a beat is one swing of the balance wheel, balances in precision watches are designed with faster beats, because they are less affected by motions of the wrist. Alarm clocks and kitchen timers often have a rate of 4 beats per second, Watches made prior to the 1970s usually had a rate of 5 beats per second. Current watches have rates of 6,8 and a few have 10 beats per second, during WWII, Elgin produced a very precise stopwatch that ran at 40 beats per second, earning it the nickname Jitterbug. Audemars Piguet currently produces a movement that allows for a high balance vibration of 12 beats/s. The precision of the best balance wheel watches on the wrist is around a few seconds per day, the most accurate balance wheel timepieces made were marine chronometers, which by WWII had achieved accuracies of 0.1 second per day
17.
Water clock
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A water clock or clepsydra is any timepiece in which time is measured by the regulated flow of liquid into or out from a vessel where the amount is then measured. Water clocks, along with sundials and hourglasses, are likely to be the oldest time-measuring instruments, with the exceptions being the vertical gnomon. Where and when they were first invented is not known, the bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BCE. Other regions of the world, including India and China, also have evidence of water clocks. Some authors, however, claim that water clocks appeared in China as early as 4000 BCE, some modern timepieces are called water clocks but work differently from the ancient ones. Independently, the Chinese developed their own advanced water clocks, incorporating gears, escapement mechanisms, some water clock designs were developed independently and some knowledge was transferred through the spread of trade. These early water clocks were calibrated with a sundial, a water clock uses a flow of water to measure time. If viscosity is neglected, the physical principle required to study such clocks is Torricellis law, there are two types of water clocks, inflow and outflow. In an outflow water clock, a container is filled with water, and this container has markings that are used to show the passage of time. As the water leaves the container, an observer can see where the water is level with the lines, an inflow water clock works in basically the same way, except instead of flowing out of the container, the water is filling up the marked container. As the container fills, the observer can see where the water meets the lines, according to Callisthenes, the Persians were using water clocks in 328 BC to ensure a just and exact distribution of water from qanats to their shareholders for agricultural irrigation. The use of clocks in Iran, especially in Zibad. Later they were used to determine the exact holy days of pre-Islamic religions, such as the Nowruz, Chelah, or Yaldā - the shortest, longest. The water clocks used in Iran were one of the most practical ancient tools for timing the yearly calendar, -Persian water clocks were a practical and useful tool for the qanats shareholders to calculate the length of time they could divert water to their farm. The qanat was the water source for agriculture and irrigation so a just. The Fenjaan consisted of a pot full of water and a bowl with a small hole in the center. When the bowl became full of water, it would sink into the pot, and he would record the number of times the bowl sank by putting small stones into a jar. The place where the clock was situated, and its managers, were known as khaneh Fenjaan
18.
Pitch drop experiment
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The pitch drop experiment is a long-term experiment that measures the flow of a piece of pitch over many years. Pitch is the name for any of a number of highly viscous liquids that appear solid, at room temperature, tar pitch flows at a very low rate, taking several years to form a single drop. Parnell poured a heated sample of pitch into a sealed funnel, in 1930, the seal at the neck of the funnel was cut, allowing the pitch to start flowing. A glass dome covers the funnel and it is placed on display outside a lecture theatre, large droplets form and fall over a period of about a decade. The eighth drop fell on 28 November 2000, allowing experimenters to calculate that the pitch has a viscosity approximately 230 billion times that of water. This experiment is predated by two other scientific devices, the Oxford Electric Bell and the Beverly Clock, but each of these has experienced brief interruptions since 1937. The experiment was not originally carried out any special controlled atmospheric conditions. Some time after the drop fell in 1988, air conditioning was added to the location where the experiment takes place. The lower average temperature has lengthened each drops stretch before it separates from the rest of the pitch in the funnel. In October 2005, John Mainstone and the late Thomas Parnell were awarded the Ig Nobel Prize in Physics, a parody of the Nobel Prize, for the pitch drop experiment. Professor Mainstone subsequently commented, I am sure that Thomas Parnell would have been flattered to know that Mark Henderson considers him worthy to become a recipient of an Ig Nobel prize, the experiment is monitored by a webcam but technical problems prevented the November 2000 drop from being recorded. The pitch drop experiment is on display on Level 2 of Parnell Building in the School of Mathematics and Physics at the St Lucia campus of the University of Queensland. Hundreds of thousands of Internet users check the live stream each year, Professor John Mainstone died on 23 August 2013, aged 78, following a stroke. Custodianship then passed to Professor Andrew White, the ninth drop touched the eighth drop on 17 April 2014. However, it was attached to the funnel. On 24 April 2014, Professor White decided to replace the beaker holding the previous eight drops before the ninth drop fused to them, while the bell jar was being lifted, the wooden base wobbled and the ninth drop snapped away from the funnel. Timeline for the University of Queensland experiment, A After the 7th drop, air conditioning was installed, B11 July 2013, 9th drop touched 8th drop,24 Apr 2014, 9th drop separated from funnel during beaker change. This experiment, like the one at Queensland University, was set up to demonstrate the high viscosity of pitch, in April 2013, about a decade after the previous pitch drop, physicists at Trinity College noticed that another drip was forming
19.
Corrosion
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Corrosion is a natural process, which converts a refined metal to a more chemically-stable form, such as its oxide, hydroxide, or sulfide. It is the destruction of materials by chemical and/or electrochemical reaction with their environment. Corrosion engineering is the dedicated to controlling and stopping corrosion. In the most common use of the word, this means electrochemical oxidation of metal in reaction with an oxidant such as oxygen or sulfur, rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. This type of damage typically produces oxide or salt of the original metal, corrosion can also occur in materials other than metals, such as ceramics or polymers, although in this context, the term degradation is more common. Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids, many structural alloys corrode merely from exposure to moisture in air, but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, because corrosion is a diffusion-controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the surface, such as passivation and chromate conversion. However, some corrosion mechanisms are less visible and less predictable, in a galvanic couple, the more active metal corrodes at an accelerated rate and the more noble metal corrodes at a slower rate. When immersed separately, each metal corrodes at its own rate, what type of metal to use is readily determined by following the galvanic series. For example, zinc is used as a sacrificial anode for steel structures. Galvanic corrosion is of major interest to the industry and also anywhere water contacts pipes or metal structures. Factors such as size of anode, types of metal. The surface area ratio of the anode and cathode directly affects the corrosion rates of the materials, galvanic corrosion is often prevented by the use of sacrificial anodes. In any given environment, one metal will be more noble or more active than others. Two metals in electrical contact share the same electrons, so that the tug-of-war at each surface is analogous to competition for free electrons between the two materials. Using the electrolyte as a host for the flow of ions in the same direction, the resulting mass flow or electric current can be measured to establish a hierarchy of materials in the medium of interest. This hierarchy is called a series and is useful in predicting and understanding corrosion
20.
Diffusion
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Diffusion is the net movement of molecules or atoms from a region of high concentration to a region of low concentration. This is also referred to as the movement of a substance down a concentration gradient, a gradient is the change in the value of a quantity with the change in another variable. The word diffusion derives from the Latin word, diffundere, which means to spread out, a distinguishing feature of diffusion is that it results in mixing or mass transport, without requiring bulk motion. Thus, diffusion should not be confused with convection, or advection, an example of a situation in which bulk flow and diffusion can be differentiated is the mechanism by which oxygen enters the body during external respiration. The lungs are located in the cavity, which expands as the first step in external respiration. This expansion leads to an increase in volume of the alveoli in the lungs and this creates a pressure gradient between the air outside the body and the alveoli. The air moves down the gradient through the airways of the lungs and into the alveoli until the pressure of the air. The air arriving in the alveoli has a concentration of oxygen than the “stale” air in the alveoli. The increase in oxygen concentration creates a concentration gradient for oxygen between the air in the alveoli and the blood in the capillaries that surround the alveoli, oxygen then moves by diffusion, down the concentration gradient, into the blood. The other consequence of the air arriving in alveoli is that the concentration of carbon dioxide in the alveoli decreases and this creates a concentration gradient for carbon dioxide to diffuse from the blood into the alveoli. The pumping action of the heart then transports the blood around the body, as the left ventricle of the heart contracts, the volume decreases, which increases the pressure in the ventricle. This creates a gradient between the heart and the capillaries, and blood moves through blood vessels by bulk flow. The concept of diffusion is widely used in, physics, chemistry, biology, sociology, economics, however, in each case, the object that is undergoing diffusion is “spreading out” from a point or location at which there is a higher concentration of that object. In the phenomenological approach, diffusion is the movement of a substance from a region of high concentration to a region of low concentration without bulk motion, according to Ficks laws, the diffusion flux is proportional to the negative gradient of concentrations. It goes from regions of higher concentration to regions of lower concentration, some time later, various generalizations of Ficks laws were developed in the frame of thermodynamics and non-equilibrium thermodynamics. From the atomistic point of view, diffusion is considered as a result of the walk of the diffusing particles. In molecular diffusion, the molecules are self-propelled by thermal energy. Random walk of small particles in suspension in a fluid was discovered in 1827 by Robert Brown, the theory of the Brownian motion and the atomistic backgrounds of diffusion were developed by Albert Einstein
21.
Tuning fork
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A tuning fork is an acoustic resonator in the form of a two-pronged fork with the prongs formed from a U-shaped bar of elastic metal. It resonates at a constant pitch when set vibrating by striking it against a surface or with an object. The pitch that a tuning fork generates depends on the length. It is frequently used as a standard of pitch to tune musical instruments, the tuning fork was invented in 1711 by British musician John Shore, Sergeant Trumpeter and Lutenist to the court. A tuning fork is an acoustic resonator used in many applications to produce a fixed tone. The reason for this is that the frequency of the first overtone is about 52/22 = 25/4 = 6 1⁄4 times the fundamental. When the tuning fork is struck, little of the energy goes into the overtone modes, they die out correspondingly faster. It is easier to other instruments with this pure tone. Another reason for using the shape is that, when it vibrates in its principal mode. There is a node at the base of each prong, the fork is usually struck, and then the handle is pressed against a wooden box resonator, or a table top. Without the resonator the sound of a fork is very faint. The reason for this is that the waves produced by each fork prong are 180° out of phase with the other, so at a distance from the fork they interfere. Commercial tuning forks are normally tuned to the pitch at the factory. They can be retuned by filing material off the prongs, filing the ends of the prongs raises the pitch, while filing the inside of the base of the prongs lowers it. Tuning forks used by orchestras between 1750 and 1820 mostly had a frequency of A =423.5 Hz, although there were many forks, standard tuning forks are available that vibrate at all the musical pitches within the central octave of the piano, and other pitches. Well-known manufacturers of tuning forks include Ragg and John Walker, both of Sheffield, England, the pitch of a tuning fork varies slightly with temperature, due mainly to a slight decrease in the modulus of elasticity of steel with increasing temperature. A change in frequency of one vibration in 21,000 per °F change is typical for a tuning fork. The frequency decreases with increasing temperature, tuning forks are manufactured to have their correct pitch at a standard temperature
22.
Pressure vessel
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A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. The pressure differential is dangerous, and fatal accidents have occurred in the history of pressure vessel development, consequently, pressure vessel design, manufacture, and operation are regulated by engineering authorities backed by legislation. The preferred test is hydrostatic testing because its a much safer method of testing as it much less energy if fracture were to occur. Today vessels in the USA require BPVC stamping but the BPVC is not just a domestic code, there are, however, other official codes in some countries, Japan, Australia, Canada, Britain, and Europe have their own codes. Regardless of the nearly all recognize the inherent potential hazards of pressure vessels. Pressure vessels can theoretically be almost any shape, but shapes made of sections of spheres, cylinders, a common design is a cylinder with end caps called heads. Head shapes are frequently either hemispherical or dished, more complicated shapes have historically been much harder to analyze for safe operation and are usually far more difficult to construct. Theoretically, a pressure vessel has approximately twice the strength of a cylindrical pressure vessel with the same wall thickness. However, a shape is difficult to manufacture, and therefore more expensive. Smaller pressure vessels are assembled from a pipe and two covers, many pressure vessels are made of steel. To manufacture a cylindrical or spherical pressure vessel, rolled and possibly forged parts would have to be welded together, some mechanical properties of steel, achieved by rolling or forging, could be adversely affected by welding, unless special precautions are taken. In addition to adequate strength, current standards dictate the use of steel with a high impact resistance. In applications where carbon steel would suffer corrosion, special corrosion resistant material should also be used, some pressure vessels are made of composite materials, such as filament wound composite using carbon fibre held in place with a polymer. Due to the high tensile strength of carbon fibre these vessels can be very light. The composite material may be wound around a metal liner, forming a composite overwrapped pressure vessel, other very common materials include polymers such as PET in carbonated beverage containers and copper in plumbing. Pressure vessels may be lined with metals, ceramics, or polymers to prevent leaking. This liner may also carry a significant portion of the pressure load, Pressure Vessels may also be constructed from concrete or other materials which are weak in tension. Cabling, wrapped around the vessel or within the wall or the vessel itself, a leakproof steel thin membrane lines the internal wall of the vessel
23.
Governor (device)
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A governor, or speed limiter, is a device used to measure and regulate the speed of a machine, such as an engine. Centrifugal governors were used to regulate the distance and pressure between millstones in windmills since the 17th century, early steam engines employed a purely reciprocating motion, and were used for pumping water – an application that could tolerate variations in the working speed. It was not until the Scottish engineer James Watt introduced the steam engine, for driving factory machinery. Between the years 1775 and 1800, Watt, in partnership with industrialist Matthew Boulton, the theoretical basis for the operation of governors was described by James Clerk Maxwell in 1868 in his seminal paper On Governors. Building on Watt’s design was American engineer Willard Gibbs who in 1872 theoretically analyzed Watt’s conical pendulum governor from an energy balance perspective. Gibbs theorized that, analogous to the equilibrium of the simple Watt governor, the first is the heat energy supplied to the intermediate substance, and the second is the work energy performed by the intermediate substance. In this case, the substance is steam. These sorts of theoretical investigations culminated in the 1876 publication of the Gibbs famous work On the Equilibrium of Heterogeneous Substances, Governors can be used to limit the top speed for vehicles, and for some classes of vehicle such devices are a legal requirement. They can more generally be used to limit the speed of the internal combustion engine or protect the engine from damage due to excessive rotational speed. Today, BMW, Audi, Volkswagen and Mercedes-Benz limit their cars to 250 kilometres per hour. Certain Quattro GmbH and AMG cars, and the Mercedes/McLaren SLR is an exception, the BMW Rolls-Royces are limited to 240 kilometres per hour. Jaguars, although British, also have a limiter, as do the Swedish Saab and this was done to reduce the political desire to introduce a legal speed limit. Ferrari, Lamborghini, Maserati, Porsche, Aston Martin and Bentley also do not limit their cars, the Chrysler 300C SRT8 is limited to 270 km/h. Most Japanese domestic market vehicles are limited to only 180 kilometres per hour or 190 kilometres per hour, the top speed is a strong sales argument, though speeds above about 300 kilometres per hour are not likely reachable on public roads. Many performance cars are limited to a speed of 250 kilometres per hour to limit insurance costs of the vehicle, mopeds in the United Kingdom have had to have a 30 mph speed limiter since 1977. Most other European countries have similar rules, public service vehicles often have a legislated top speed. Scheduled coach services in the United kingdom are limited to 65 mph, urban public buses often have speed governors which are typically set to between 65 kilometres per hour and 100 kilometres per hour. All heavy vehicles in Europe and New Zealand have law/by-law governors that limits their speeds to 90 kilometres per hour or 100 kilometres per hour, fire engines and other emergency vehicles are exempt from this requirement
24.
Atomic clock
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The principle of operation of an atomic clock is not based on nuclear physics, but rather on atomic physics, it uses the microwave signal that electrons in atoms emit when they change energy levels. Early atomic clocks were based on masers at room temperature, currently, the most accurate atomic clocks first cool the atoms to near absolute zero temperature by slowing them with lasers and probing them in atomic fountains in a microwave-filled cavity. An example of this is the NIST-F1 atomic clock, one of the primary time. The accuracy of an atomic clock depends on two factors, the first factor is temperature of the sample atoms—colder atoms move much more slowly, allowing longer probe times. The second factor is the frequency and intrinsic width of the electronic transition, higher frequencies and narrow lines increase the precision. National standards agencies in many countries maintain a network of atomic clocks which are intercompared and these clocks collectively define a continuous and stable time scale, International Atomic Time. For civil time, another time scale is disseminated, Coordinated Universal Time, UTC is derived from TAI, but approximately synchronised, by using leap seconds, to UT1, which is based on actual rotation of the Earth with respect to the solar time. The idea of using atomic transitions to measure time was suggested by Lord Kelvin in 1879, magnetic resonance, developed in the 1930s by Isidor Rabi, became the practical method for doing this. In 1945, Rabi first publicly suggested that atomic beam magnetic resonance might be used as the basis of a clock, the first atomic clock was an ammonia maser device built in 1949 at the U. S. National Bureau of Standards. It was less accurate than existing quartz clocks, but served to demonstrate the concept, calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ephemeris time. This led to the agreed definition of the latest SI second being based on atomic time. Equality of the ET second with the SI second has been verified to within 1 part in 1010, the SI second thus inherits the effect of decisions by the original designers of the ephemeris time scale, determining the length of the ET second. Since the beginning of development in the 1950s, atomic clocks have been based on the transitions in hydrogen-1, caesium-133. The first commercial atomic clock was the Atomichron, manufactured by the National Company, more than 50 were sold between 1956 and 1960. This bulky and expensive instrument was replaced by much smaller rack-mountable devices, such as the Hewlett-Packard model 5060 caesium frequency standard. In August 2004, NIST scientists demonstrated a chip-scale atomic clock, according to the researchers, the clock was believed to be one-hundredth the size of any other. It requires no more than 125 mW, making it suitable for battery-driven applications and this technology became available commercially in 2011. Ion trap experimental optical clocks are more precise than the current caesium standard, in March 2017, NASA plans to deploy the Deep Space Atomic Clock, a miniaturized, ultra-precise mercury-ion atomic clock, into outer space
25.
Crystal oscillator
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A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a precise frequency. Quartz crystals are manufactured for frequencies from a few tens of kilohertz to hundreds of megahertz, more than two billion crystals are manufactured annually. Most are used for devices such as wristwatches, clocks, radios, computers. Quartz crystals are found inside test and measurement equipment, such as counters, signal generators. A crystal oscillator is an oscillator circuit that uses a piezoelectric resonator. Crystal is the term used in electronics for the frequency-determining component. A more accurate term for it is piezoelectric resonator, Crystals are also used in other types of electronic circuits, such as crystal filters. Piezoelectric resonators are sold as components for use in crystal oscillator circuits. An example is shown in the picture and they are also often incorporated in a single package with the crystal oscillator circuit, shown on the righthand side. Piezoelectricity was discovered by Jacques and Pierre Curie in 1880, paul Langevin first investigated quartz resonators for use in sonar during World War I. Cady built the first quartz oscillator in 1921. Other early innovators in quartz crystal oscillators include G. W. Pierce, Quartz crystal oscillators were developed for high-stability frequency references during the 1920s and 1930s. Prior to crystals, radio stations controlled their frequency with tuned circuits, since broadcast stations were assigned frequencies only 10 kHz apart, interference between adjacent stations due to frequency drift was a common problem. In 1928, Warren Marrison of Bell Telephone Laboratories developed the first quartz-crystal clock, with accuracies of up to 1 second in 30 years, quartz clocks replaced precision pendulum clocks as the worlds most accurate timekeepers until atomic clocks were developed in the 1950s. Using the early work at Bell Labs, AT&T eventually established their Frequency Control Products division, later spun off, a number of firms started producing quartz crystals for electronic use during this time. Using what are now considered primitive methods, about 100,000 crystal units were produced in the United States during 1939, through World War II crystals were made from natural quartz crystal, virtually all from Brazil. By the 1970s virtually all crystals used in electronics were synthetic, although crystal oscillators still most commonly use quartz crystals, devices using other materials are becoming more common, such as ceramic resonators. A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, almost any object made of an elastic material could be used like a crystal, with appropriate transducers, since all objects have natural resonant frequencies of vibration
26.
Radioactive decay
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A material containing such unstable nuclei is considered radioactive. Certain highly excited short-lived nuclear states can decay through neutron emission, or more rarely, however, for a collection of atoms, the collections expected decay rate is characterized in terms of their measured decay constants or half-lives. This is the basis of radiometric dating, the half-lives of radioactive atoms have no known upper limit, spanning a time range of over 55 orders of magnitude, from nearly instantaneous to far longer than the age of the universe. A radioactive nucleus with spin can have no defined orientation. If there are multiple particles produced during a single decay, as in decay, their relative angular distribution. Such a parent process could be a previous decay, or a nuclear reaction, the decaying nucleus is called the parent radionuclide, and the process produces at least one daughter nuclide. Except for gamma decay or internal conversion from an excited state. When the number of changes, an atom of a different chemical element is created. The first decay processes to be discovered were alpha decay, beta decay, alpha decay occurs when the nucleus ejects an alpha particle. This is the most common process of emitting nucleons, but highly excited nuclei can eject single nucleons, or in the case of cluster decay, specific light nuclei of other elements. Beta decay occurs when the nucleus emits an electron or positron, the nucleus may capture an orbiting electron, causing a proton to convert into a neutron in a process called electron capture. All of these result in a well-defined nuclear transmutation. By contrast, there are radioactive decay processes that do not result in a nuclear transmutation, another type of radioactive decay results in products that vary, appearing as two or more fragments of the original nucleus with a range of possible masses. For a summary table showing the number of stable and radioactive nuclides in each category, there are 29 naturally occurring chemical elements on Earth that are radioactive. They are those that contain 34 radionuclides that date before the time of formation of the solar system, well-known examples are uranium and thorium, but also included are naturally occurring long-lived radioisotopes, such as potassium-40. Radioactivity was discovered in 1896 by the French scientist Henri Becquerel and these materials glow in the dark after exposure to light, and he suspected that the glow produced in cathode ray tubes by X-rays might be associated with phosphorescence. He wrapped a photographic plate in black paper and placed various phosphorescent salts on it, all results were negative until he used uranium salts. The uranium salts caused a blackening of the plate in spite of the plate being wrapped in black paper and these radiations were given the name Becquerel Rays
27.
Tidal force
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The tidal force is a force that is the secondary effect of the force of gravity, it is responsible for the phenomenon of tides. It arises because the force exerted by one body on another is not constant across it. Thus, the force is differential. Consider the gravitational attraction of the Moon on the oceans nearest to the Moon, the solid Earth, there is a mutual attraction between the Moon and the solid Earth, which can be considered to act on its centre of mass. However, the oceans are more strongly attracted and, especially since they are fluid, they approach the Moon slightly. The far oceans are attracted less, viewing the Earth as a whole, we see that all its mass experiences a mutual attraction with that of the Moon, but the near oceans more so than the far oceans, leading to a separation of the two. When a body is acted on by the gravity of another body, Figure 2 shows the differential force of gravity on a spherical body exerted by another body. These so-called tidal forces cause strains on both bodies and may distort them or even, in cases, break one or the other apart. These strains would not occur if the field were uniform, because a uniform field only causes the entire body to accelerate together in the same direction. In the case of a small elastic sphere, the effect of a tidal force is to distort the shape of the body without any change in volume. The sphere becomes an ellipsoid with two bulges, pointing towards and away from the other body, larger objects distort into an ovoid, and are slightly compressed, which is what happens to the Earths oceans under the action of the Moon. The Earth and Moon rotate about their center of mass or barycenter. To an observer on the Earth, very close to this barycenter, all parts of the Earth are subject to the Moons gravitational forces, causing the water in the oceans to redistribute, forming bulges on the sides near the Moon and far from the Moon. When a body rotates while subject to forces, internal friction results in the gradual dissipation of its rotational kinetic energy as heat. In the case for the Earth, and Earths Moon, the loss of kinetic energy results in a gain of about 2 milliseconds per century. If the body is enough to its primary, this can result in a rotation which is tidally locked to the orbital motion. Tidal heating produces dramatic volcanic effects on Jupiters moon Io, stresses caused by tidal forces also cause a regular monthly pattern of moonquakes on Earths Moon. Tidal forces contribute to ocean currents, which moderate global temperatures by transporting heat energy toward the poles and it has been suggested that in addition to other factors, harmonic beat variations in tidal forcing may contribute to climate changes
28.
Inertial frame of reference
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In classical physics and special relativity, an inertial frame of reference is a frame of reference that describes time and space homogeneously, isotropically, and in a time-independent manner. The physics of a system in an inertial frame have no causes external to the system, all inertial frames are in a state of constant, rectilinear motion with respect to one another, an accelerometer moving with any of them would detect zero acceleration. Measurements in one frame can be converted to measurements in another by a simple transformation. In general relativity, in any region small enough for the curvature of spacetime and tidal forces to be negligible, systems in non-inertial frames in general relativity dont have external causes because of the principle of geodesic motion. Physical laws take the form in all inertial frames. For example, a ball dropped towards the ground does not go straight down because the Earth is rotating. Someone rotating with the Earth must account for the Coriolis effect—in this case thought of as a force—to predict the horizontal motion, another example of such a fictitious force associated with rotating reference frames is the centrifugal effect, or centrifugal force. The motion of a body can only be described relative to something else—other bodies, observers and these are called frames of reference. If the coordinates are chosen badly, the laws of motion may be more complex than necessary, for example, suppose a free body that has no external forces on it is at rest at some instant. In many coordinate systems, it would begin to move at the next instant, however, a frame of reference can always be chosen in which it remains stationary. Similarly, if space is not described uniformly or time independently, indeed, an intuitive summary of inertial frames can be given as, In an inertial reference frame, the laws of mechanics take their simplest form. In an inertial frame, Newtons first law, the law of inertia, is satisfied, Any free motion has a constant magnitude, the force F is the vector sum of all real forces on the particle, such as electromagnetic, gravitational, nuclear and so forth. The extra terms in the force F′ are the forces for this frame. The first extra term is the Coriolis force, the second the centrifugal force, also, fictitious forces do not drop off with distance. For example, the force that appears to emanate from the axis of rotation in a rotating frame increases with distance from the axis. All observers agree on the forces, F, only non-inertial observers need fictitious forces. The laws of physics in the frame are simpler because unnecessary forces are not present. In Newtons time the stars were invoked as a reference frame
29.
Continental drift
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Continental drift is the movement of the Earths continents relative to each other, thus appearing to drift across the ocean bed. The speculation that continents might have drifted was first put forward by Abraham Ortelius in 1596, the concept was independently and more fully developed by Alfred Wegener in 1912, but his theory was rejected by some for lack of a mechanism and others because of prior theoretical commitments. The idea of continental drift has been subsumed by the theory of plate tectonics, W. J. Kious described Ortelius thoughts in this way, Abraham Ortelius in his work Thesaurus Geographicus. Suggested that the Americas were torn away from Europe and Africa, by earthquakes and floods and went on to say, The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three. In his Manual of Geology,1863, Dana says The continents, Dana was enormously influential in America – his Manual of Mineralogy is still in print in revised form – and the theory became known as Permanence theory. This suggested that the oceans were a permanent feature of the earths surface and this led Mantovani to propose an Expanding Earth theory which has since been shown to be incorrect. Continental drift without expansion was proposed by Frank Bursley Taylor, who suggested in 1908 that the continents were moved into their present positions by a process of continental creep. Wegener said that of all theories, Taylors, although not fully developed, had the most similarities to his own. In the mid-20th century, the theory of drift was referred to as the Taylor-Wegener hypothesis. Alfred Wegener first presented his hypothesis to the German Geological Society on January 6,1912 and his hypothesis was that the continents had once formed a single landmass, called Pangea, before breaking apart and drifting to their present locations. Wegener was the first to use the continental drift and formally publish the hypothesis that the continents had somehow drifted apart. Although he presented evidence for continental drift, he was unable to provide a convincing explanation for the physical processes which might have caused this drift. The Polflucht hypothesis was studied by Paul Sophus Epstein in 1920. The theory of drift was not accepted for many years. One problem was that a driving force was missing. A second problem was that Wegeners estimate of the velocity of continental motion,250 cm/year, was implausibly high, and it did not help that Wegener was not a geologist. Other geologists also believed that the evidence that Wegener had provided was not sufficient, the British geologist Arthur Holmes championed the theory of continental drift at a time when it was deeply unfashionable. He proposed in 1931 that the Earths mantle contained convection cells that dissipated radioactive heat and his Principles of Physical Geology, ending with a chapter on continental drift, was published in 1944
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Orbit
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In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet about a star or a natural satellite around a planet. Normally, orbit refers to a regularly repeating path around a body, to a close approximation, planets and satellites follow elliptical orbits, with the central mass being orbited at a focal point of the ellipse, as described by Keplers laws of planetary motion. For ease of calculation, in most situations orbital motion is adequately approximated by Newtonian Mechanics, historically, the apparent motions of the planets were described by European and Arabic philosophers using the idea of celestial spheres. This model posited the existence of perfect moving spheres or rings to which the stars and it assumed the heavens were fixed apart from the motion of the spheres, and was developed without any understanding of gravity. After the planets motions were accurately measured, theoretical mechanisms such as deferent. Originally geocentric it was modified by Copernicus to place the sun at the centre to help simplify the model, the model was further challenged during the 16th century, as comets were observed traversing the spheres. The basis for the understanding of orbits was first formulated by Johannes Kepler whose results are summarised in his three laws of planetary motion. Second, he found that the speed of each planet is not constant, as had previously been thought. Third, Kepler found a relationship between the orbital properties of all the planets orbiting the Sun. For the planets, the cubes of their distances from the Sun are proportional to the squares of their orbital periods. Jupiter and Venus, for example, are respectively about 5.2 and 0.723 AU distant from the Sun, their orbital periods respectively about 11.86 and 0.615 years. The proportionality is seen by the fact that the ratio for Jupiter,5. 23/11.862, is equal to that for Venus,0. 7233/0.6152. Idealised orbits meeting these rules are known as Kepler orbits, isaac Newton demonstrated that Keplers laws were derivable from his theory of gravitation and that, in general, the orbits of bodies subject to gravity were conic sections. Newton showed that, for a pair of bodies, the sizes are in inverse proportion to their masses. Where one body is more massive than the other, it is a convenient approximation to take the center of mass as coinciding with the center of the more massive body. Lagrange developed a new approach to Newtonian mechanics emphasizing energy more than force, in a dramatic vindication of classical mechanics, in 1846 le Verrier was able to predict the position of Neptune based on unexplained perturbations in the orbit of Uranus. This led astronomers to recognize that Newtonian mechanics did not provide the highest accuracy in understanding orbits, in relativity theory, orbits follow geodesic trajectories which are usually approximated very well by the Newtonian predictions but the differences are measurable. Essentially all the evidence that can distinguish between the theories agrees with relativity theory to within experimental measurement accuracy
31.
Phase-locked loop
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A phase-locked loop or phase lock loop abbreviated as PLL is a control system that generates an output signal whose phase is related to the phase of an input signal. While there are several differing types, it is easy to visualize as an electronic circuit consisting of a variable frequency oscillator. The oscillator generates a signal, and the phase detector compares the phase of that signal with the phase of the input periodic signal. Bringing the output signal back toward the signal for comparison is called a feedback loop since the output is fed back toward the input forming a loop. Keeping the input and output phase in lock step also implies keeping the input and output frequencies the same, consequently, in addition to synchronizing signals, a phase-locked loop can track an input frequency, or it can generate a frequency that is a multiple of the input frequency. These properties are used for clock synchronization, demodulation. Phase-locked loops are widely employed in radio, telecommunications, computers, for a practical idea of what is going on, consider an auto race. There are many cars, and the driver of each of them wants to go around the track as fast as possible, each lap corresponds to a complete cycle, and each car will complete dozens of laps per hour. The number of laps per hour corresponds to an angular velocity, but the number of laps corresponds to a phase. During most of the race, each car is on its own and the driver of the car is trying to beat the driver of other car on the course. However, if there is an accident, a car comes out to set a safe speed. None of the cars are permitted to pass the pace car. While it is on the track, the car is a reference. Each driver will measure the difference between him and the pace car. If the driver is far away, he will increase his speed to close the gap. If hes too close to the car, he will slow down. The result is all the race cars lock on to the phase of the pace car, the cars travel around the track in a tight group that is a small fraction of a lap. Phase can be proportional to time, so a difference can be a time difference
32.
Lens (optics)
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A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a piece of transparent material, while a compound lens consists of several simple lenses. Lenses are made from such as glass or plastic, and are ground. A lens can focus light to form an image, unlike a prism, devices that similarly focus or disperse waves and radiation other than visible light are also called lenses, such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses. The word lens comes from the Latin name of the lentil, the genus of the lentil plant is Lens, and the most commonly eaten species is Lens culinaris. The lentil plant also gives its name to a geometric figure, the variant spelling lense is sometimes seen. While it is listed as a spelling in some dictionaries. The oldest lens artifact is the Nimrud lens, dating back 2700 years to ancient Assyria, david Brewster proposed that it may have been used as a magnifying glass, or as a burning-glass to start fires by concentrating sunlight. Another early reference to magnification dates back to ancient Egyptian hieroglyphs in the 8th century BC, the earliest written records of lenses date to Ancient Greece, with Aristophanes play The Clouds mentioning a burning-glass. Some scholars argue that the evidence indicates that there was widespread use of lenses in antiquity. Such lenses were used by artisans for fine work, and for authenticating seal impressions, both Pliny and Seneca the Younger described the magnifying effect of a glass globe filled with water. The Viking lenses were capable of concentrating enough sunlight to ignite fires, between the 11th and 13th century reading stones were invented. Often used by monks to assist in illuminating manuscripts, these were primitive plano-convex lenses initially made by cutting a sphere in half. As the stones were experimented with, it was understood that shallower lenses magnified more effectively. Lenses came into use in Europe with the invention of spectacles. Spectacle makers created improved types of lenses for the correction of vision based more on knowledge gained from observing the effects of the lenses. With the invention of the telescope and microscope there was a deal of experimentation with lens shapes in the 17th. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the figure of their surfaces
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Calendar
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A calendar is a system of organizing days for social, religious, commercial or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, a date is the designation of a single, specific day within such a system. A calendar is also a record of such a system. A calendar can also mean a list of planned events, such as a calendar or a partly or fully chronological list of documents. Periods in a calendar are usually, though not necessarily, synchronized with the cycle of the sun or the moon. The most common type of calendar was the lunisolar calendar. Latin calendarium meant account book, register, the Latin term was adopted in Old French as calendier and from there in Middle English as calender by the 13th century. The course of the Sun and the Moon are the most evident forms of timekeeping, nevertheless, the Roman calendar contained very ancient remnants of a pre-Etruscan 10-month solar year. The first recorded calendars date to the Bronze Age, dependent on the development of writing in the Ancient Near East, a larger number of calendar systems of the Ancient Near East becomes accessible in the Iron Age, based on the Babylonian calendar. This includes the calendar of the Persian Empire, which in turn gave rise to the Zoroastrian calendar as well as the Hebrew calendar, calendars in antiquity were lunisolar, depending on the introduction of intercalary months to align the solar and the lunar years. This was mostly based on observation, but there may have been attempts to model the pattern of intercalation algorithmically. The Roman calendar was reformed by Julius Caesar in 45 BC, the Julian calendar was no longer dependent on the observation of the new moon but simply followed an algorithm of introducing a leap day every four years. This created a dissociation of the month from the lunation. The Islamic calendar is based on the prohibition of intercalation by Muhammad and this resulted in an observationally based lunar calendar that shifts relative to the seasons of the solar year. The first calendar reform of the modern era was the Gregorian calendar. Such ideas are mooted from time to time but have failed to gain traction because of the loss of continuity, massive upheaval in implementation, a full calendar system has a different calendar date for every day. Thus the week cycle is by not a full calendar system. The simplest calendar system just counts time periods from a reference date and this applies for the Julian day or Unix Time
34.
Axial precession
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In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical bodys rotational axis. The term precession typically refers only to this largest part of the motion, other changes in the alignment of Earths axis—nutation and this term is still used in non-technical discussions, that is, when detailed mathematics are absent. Their combination was named general precession, instead of precession of the equinoxes, lunisolar precession is caused by the gravitational forces of the Moon and Sun on Earths equatorial bulge, causing Earths axis to move with respect to inertial space. Lunisolar precession is about 500 times greater than planetary precession, many references to the old terms exist in publications predating the change. Etymologically, precession and procession are both terms that relate to motion, Precession is derived from the Latin praecedere, to precede, to come before or earlier), while procession is derived from the Latin procedere, to march forward, to advance). Generally the term procession is used to describe a group of objects moving forward, the stars viewed from Earth are seen to proceed in a procession from east to west daily, due to the Earth’s diurnal motion, and yearly, due to the Earth’s revolution around the Sun. At the same time the stars can be observed to anticipate slightly such motion, at the rate of approximately 50 arc seconds per year, in describing this motion astronomers generally have shortened the term to simply precession. This issue is further obfuscated by the fact that many astronomers are physicists or astrophysicists, the precession of the Earths axis has a number of observable effects. First, the positions of the south and north celestial poles appear to move in circles against the backdrop of stars. Thus, while today the star Polaris lies approximately at the celestial pole, this will change over time. In approximately 3200 years, the star Gamma Cephei in the Cepheus constellation will succeed Polaris for this position, the south celestial pole currently lacks a bright star to mark its position, but over time precession also will cause bright stars to become south stars. As the celestial poles shift, there is a gradual shift in the apparent orientation of the whole star field. Secondly, the position of the Earth in its orbit around the Sun at the solstices, equinoxes, or other time defined relative to the seasons, slowly changes. For example, suppose that the Earths orbital position is marked at the summer solstice, in other words, the solstice occurred a little earlier in the orbit. Thus, the year, measuring the cycle of seasons, is about 20 minutes shorter than the sidereal year. For further details, see Changing pole stars and Polar shift and equinoxes shift, according to Ptolemys Almagest, Hipparchus measured the longitude of Spica and other bright stars. Comparing his measurements with data from his predecessors, Timocharis and Aristillus and he also compared the lengths of the tropical year and the sidereal year, and found a slight discrepancy. Virtually all of the writings of Hipparchus are lost, including his work on precession and they are mentioned by Ptolemy, who explains precession as the rotation of the celestial sphere around a motionless Earth
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Sidereal day
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Sidereal time /saɪˈdɪəriəl/ is a time-keeping system that astronomers use to locate celestial objects. Using sidereal time it is possible to point a telescope to the proper coordinates in the night sky. Briefly, sidereal time is a scale that is based on Earths rate of rotation measured relative to the fixed stars rather than the Sun. From a given point, a star found at one location in the sky will be found at the same location on another night at the same sidereal time. This is similar to how the time kept by a sundial can be used to find the location of the Sun, just as the Sun and Moon appear to rise in the east and set in the west due to the rotation of Earth, so do the stars. Both solar time and sidereal time make use of the regularity of Earths rotation about its polar axis, a mean sidereal day is 23 hours,56 minutes,4.0916 seconds, the time it takes Earth to make one rotation relative to the vernal equinox. The longer true sidereal period is called a day by the International Earth Rotation. It is also referred to as the period of rotation. Maps of the stars in the night sky use declination and right ascension as coordinates and these correspond to latitude and longitude respectively. In the sky, the meridian is the north to south line that goes through the point directly overhead. Solar time is measured by the apparent diurnal motion of the Sun, a mean solar day is the average time between local solar noons. Earth makes one rotation around its axis in a sidereal day, so after a sidereal day has passed, Earth still needs to rotate slightly more before the Sun reaches local noon according to solar time. A mean solar day is, therefore, nearly 4 minutes longer than a sidereal day, the stars are so far away that Earths movement along its orbit makes nearly no difference to their apparent direction, and so they return to their highest point in a sidereal day. Another way to see this difference is to notice that, relative to the stars, therefore, there is one fewer solar day per year than there are sidereal days. This makes a sidereal day approximately 365. 24/366.24 times the length of the 24-hour solar day, Earths rotation is not a simple rotation around an axis that would always remain parallel to itself. Earths rotational axis itself rotates about an axis, orthogonal to Earths orbit. This phenomenon is called the precession of the equinoxes, because of this precession, the stars appear to move around Earth in a manner more complicated than a simple constant rotation. In this reference frame, Earths rotation is close to constant and it is also in this reference frame that the tropical year, the year related to Earths seasons, represents one orbit of Earth around the Sun
36.
Precession
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Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, in other words, if the axis of rotation of a body is itself rotating about a second axis, that body is said to be precessing about the second axis. A motion in which the second Euler angle changes is called nutation, in physics, there are two types of precession, torque-free and torque-induced. In astronomy, precession refers to any of several changes in an astronomical bodys rotational or orbital parameters. An important example is the change in the orientation of the axis of rotation of the Earth. Torque-free precession implies that no moment is applied to the body. In torque-free precession, the momentum is a constant. What makes this possible is a moment of inertia, or more precisely. The inertia matrix is composed of the moments of inertia of a body calculated with respect to coordinate axes. If an object is asymmetric about its axis of rotation. The result is that the component of the velocities of the body about each axis will vary inversely with each axis moment of inertia. When an object is not perfectly solid, internal vortices will tend to damp torque-free precession, R new = exp R old for the skew-symmetric matrix ×. Torque-induced precession is the phenomenon in which the axis of a spinning object describes a cone in space when a torque is applied to it. The phenomenon is seen in a spinning toy top. If the speed of the rotation and the magnitude of the external torque are constant, the spin axis will move at right angles to the direction that would intuitively result from the external torque. In the case of a toy top, its weight is acting downwards from its center of mass and these two opposite forces produce a torque which causes the top to precess. The device depicted on the right is gimbal mounted, from inside to outside there are three axes of rotation, the hub of the wheel, the gimbal axis, and the vertical pivot. To distinguish between the two axes, rotation around the wheel hub will be called spinning, and rotation around the gimbal axis will be called pitching
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Zodiac
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The zodiac is an area of the sky centered upon the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year. The paths of the Moon and visible planets also remain close to the ecliptic, within the belt of the zodiac, in western astrology and astronomy, the zodiac is divided into twelve signs, each sign occupying 30° of celestial longitude. Because the signs are regular, they do not correspond exactly to the boundaries of the twelve constellations after which they are named, the English word zodiac derives from zōdiacus, the Latinized form of the Ancient Greek zōidiakòs kýklos, meaning circle of little animals. Zōidion is the diminutive of zōion, the name reflects the prominence of animals among the twelve signs. The construction of the zodiac is described in Ptolemys vast 2nd century AD work, the term zodiac may also refer to the region of the celestial sphere encompassing the paths of the planets corresponding to the band of about eight arc degrees above and below the ecliptic. The zodiac of a planet is the band that contains the path of that particular body, e. g. the zodiac of the Moon is the band of five degrees above. By extension, the zodiac of the comets may refer to the band encompassing most short-period comets, the division of the ecliptic into the zodiacal signs originates in Babylonian astronomy during the first half of the 1st millennium BC. The zodiac draws on stars in earlier Babylonian star catalogues, such as the MUL. APIN catalogue, around the end of the 5th century BC, Babylonian astronomers divided the ecliptic into twelve equal signs, by analogy to twelve schematic months of thirty days each. Each sign contained thirty degrees of longitude, thus creating the first known celestial coordinate system. Because the division was made into equal arcs, 30° each, in Babylonian astronomical diaries, a planet position was generally given with respect to a zodiacal sign alone, less often in specific degrees within a sign. When the degrees of longitude were given, they were expressed with reference to the 30° of the zodiacal sign, in astronomical ephemerides, the positions of significant astronomical phenomena were computed in sexagesimal fractions of a degree. For daily ephemerides, the positions of a planet were not as important as the astrologically significant dates when the planet crossed from one zodiacal sign to the next. The Babylonian star catalogs entered Greek astronomy in the 4th century BC, Babylonia or Chaldea in the Hellenistic world came to be so identified with astrology that Chaldean wisdom became among Greeks and Romans the synonym of divination through the planets and stars. Hellenistic astrology derived in part from Babylonian and Egyptian astrology, horoscopic astrology first appeared in Ptolemaic Egypt. The Dendera zodiac, a dating to ca.50 BC, is the first known depiction of the classical zodiac of twelve signs. The earliest extant Greek text using the Babylonian division of the zodiac into 12 signs of 30 equal degrees each is the Anaphoricus of Hypsicles of Alexandria. Particularly important in the development of Western horoscopic astrology was the astrologer and astronomer Ptolemy, whose work Tetrabiblos laid the basis of the Western astrological tradition. Under the Greeks, and Ptolemy in particular, the planets, Houses, Ptolemy lived in the 2nd century AD, three centuries after the discovery of the precession of the equinoxes by Hipparchus around 130 BC
38.
Lunar phase
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The lunar phase or phase of the moon is the shape of the illuminated portion of the Moon as seen by an observer on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the positions of the Moon. The Moons rotation is locked by the Earths gravity, therefore the same lunar surface always faces Earth. This face is variously sunlit depending on the position of the Moon in its orbit, therefore, the portion of this hemisphere that is visible to an observer on Earth can vary from about 100% to 0%. The lunar terminator is the boundary between the illuminated and darkened hemispheres, each of the four intermediate lunar phases is roughly seven days but this varies slightly due to the elliptical shape of the Moons orbit. Aside from some craters near the lunar poles such as Shoemaker, all parts of the Moon see around 14.77 days of sunlight, in Western culture, the four principal lunar phases are new moon, first quarter, full moon, and third quarter. These are the instants when the Moons apparent geocentric celestial longitude minus the Suns apparent geocentric longitude is 0°, 90°, 180° and 270°. Each of these phases is instantaneous, lasting theoretically zero time, during the intervals between principal phases, the Moon appears either crescent-shaped or gibbous. These shapes, and the periods of time when the Moon shows them, are called the intermediate phases. They last, on average, one-quarter of a month, roughly 7.38 days, but their durations vary slightly because the Moons orbit is slightly elliptical. The descriptor waxing is used for a phase when the Moons apparent size is increasing, from new moon toward full moon. As the moon waxes, the lunar phases progress through new moon, crescent moon, first-quarter moon, gibbous moon, the moon is then said to wane as it passes through the gibbous moon, third-quarter moon, crescent moon and back to new moon. The terms old moon and new moon are not interchangeable, the old moon is a waning sliver until the moment it aligns with the sun and begins to wax, at which point it becomes new again. Half moon is used to mean the first- and third-quarter moons, while the term quarter refers to the extent of the moons cycle around the Earth. When a crescent Moon occurs, the phenomenon of earthshine may be apparent, in the Northern Hemisphere, if the left side of the Moon is dark then the light part is growing, and the Moon is referred to as waxing. If the right side of the Moon is dark then the part is shrinking. Assuming that the viewer is in the hemisphere, the right portion of the Moon is the part that is always growing. Nearer the Equator the Moon with its terminator will appear apparently horizontal during the morning and evening, the crescent Moon can open upward or downward, with the horns of the crescent pointing up or down, respectively
39.
Ephemeris
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In astronomy and celestial navigation, an ephemeris gives the positions of naturally occurring astronomical objects as well as artificial satellites in the sky at a given time or times. Historically, positions were given as printed tables of values, given at intervals of date. Modern ephemerides are often computed electronically from mathematical models of the motion of astronomical objects, the astronomical position calculated from an ephemeris is given in the spherical polar coordinate system of right ascension and declination. Ephemerides are used in navigation and astronomy. They are also used by some astrologers, 1st millennium BC — Ephemerides in Babylonian astronomy. 13th century — the Zīj-i Īlkhānī were compiled at the Maragheh observatory in Persia, 13th century — the Alfonsine Tables were compiled in Spain to correct anomalies in the Tables of Toledo, remaining the standard European ephemeris until the Prutenic Tables almost 300 years later. 1531 — Work of Johannes Stöffler is published posthumously at Tübingen,1551 — the Prutenic Tables of Erasmus Reinhold were published, based on Copernicuss theories. 1554 — Johannes Stadius published Ephemerides novae et auctae, the first major ephemeris computed according to Copernicus heliocentric model, one of the users of Stadiuss tables is Tycho Brahe. 1627 — the Rudolphine Tables of Johannes Kepler based on elliptical planetary motion became the new standard. 1679 — La Connaissance des Temps ou calendrier et éphémérides du lever & coucher du Soleil, de la Lune & des autres planètes, first published yearly by Jean Picard and still extent. According to Gingerich, the patterns are as distinctive as fingerprints. Typically, such ephemerides cover several centuries, past and future, nevertheless, there are secular phenomena which cannot adequately be considered by ephemerides. The greatest uncertainties in the positions of planets are caused by the perturbations of asteroids, most of whose masses and orbits are poorly known. Reflecting the continuing influx of new data and observations, NASAs Jet Propulsion Laboratory has revised its published ephemerides nearly every year for the past 20 years. Solar system ephemerides are essential for the navigation of spacecraft and for all kinds of observations of the planets, their natural satellites, stars. The equinox of the system must be given. It is, in all cases, either the actual equinox, or that of one of the standard equinoxes, typically J2000.0, B1950.0. Star maps almost always use one of the standard equinoxes, Ephemerides of the planet Saturn also sometimes contain the apparent inclination of its ring
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Gregorian calendar
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The Gregorian calendar is internationally the most widely used civil calendar. It is named after Pope Gregory XIII, who introduced it in October 1582, the calendar was a refinement to the Julian calendar involving a 0. 002% correction in the length of the year. The motivation for the reform was to stop the drift of the calendar with respect to the equinoxes and solstices—particularly the northern vernal equinox, transition to the Gregorian calendar would restore the holiday to the time of the year in which it was celebrated when introduced by the early Church. The reform was adopted initially by the Catholic countries of Europe, the last European country to adopt the reform was Greece, in 1923. Many countries that have used the Islamic and other religious calendars have come to adopt this calendar for civil purposes. The reform was a modification of a made by Aloysius Lilius. His proposal included reducing the number of years in four centuries from 100 to 97. Lilius also produced an original and practical scheme for adjusting the epacts of the moon when calculating the date of Easter. For example, the years 1700,1800, and 1900 are not leap years, but the years 1600 and 2000 are. The canonical Easter tables were devised at the end of the third century, when the vernal equinox fell either on 20 March or 21 March depending on the years position in the leap year cycle. As the rule was that the full moon preceding Easter was not to precede the equinox, the date was fixed at 21 March for computational purposes, the Gregorian calendar reproduced these conditions by removing ten days. To unambiguously specify a date, dual dating or Old Style, dual dating gives two consecutive years for a given date, because of differences in the starting date of the year, and/or to give both the Julian and the Gregorian dates. The Gregorian calendar continued to use the calendar era, which counts years from the traditional date of the nativity. This year-numbering system, also known as Dionysian era or Common Era, is the predominant international standard today, the Gregorian calendar is a solar calendar. A regular Gregorian year consists of 365 days, but as in the Julian calendar, in a leap year, in the Julian calendar a leap year occurs every 4 years, but the Gregorian calendar omits 3 leap days every 400 years. In the Julian calendar, this day was inserted by doubling 24 February. In the modern period, it has become customary to number the days from the beginning of the month, some churches, notably the Roman Catholic Church, delay February festivals after the 23rd by one day in leap years. Gregorian years are identified by consecutive year numbers, the cycles repeat completely every 146,097 days, which equals 400 years
41.
Electronics
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Electronics is the science of controlling electrical energy electrically, in which the electrons have a fundamental role. Commonly, electronic devices contain circuitry consisting primarily or exclusively of active semiconductors supplemented with passive elements, the science of electronics is also considered to be a branch of physics and electrical engineering. The ability of electronic devices to act as switches makes digital information processing possible, until 1950 this field was called radio technology because its principal application was the design and theory of radio transmitters, receivers, and vacuum tubes. Today, most electronic devices use semiconductor components to perform electron control and this article focuses on engineering aspects of electronics. Components are generally intended to be connected together, usually by being soldered to a circuit board. Components may be packaged singly, or in more complex groups as integrated circuits, some common electronic components are capacitors, inductors, resistors, diodes, transistors, etc. Components are often categorized as active or passive, vacuum tubes were among the earliest electronic components. They were almost solely responsible for the revolution of the first half of the Twentieth Century. They took electronics from parlor tricks and gave us radio, television, phonographs, radar, long distance telephony and they played a leading role in the field of microwave and high power transmission as well as television receivers until the middle of the 1980s. Since that time, solid state devices have all but completely taken over, vacuum tubes are still used in some specialist applications such as high power RF amplifiers, cathode ray tubes, specialist audio equipment, guitar amplifiers and some microwave devices. The 608 contained more than 3,000 germanium transistors, thomas J. Watson Jr. ordered all future IBM products to use transistors in their design. From that time on transistors were almost exclusively used for computer logic, circuits and components can be divided into two groups, analog and digital. A particular device may consist of circuitry that has one or the other or a mix of the two types, most analog electronic appliances, such as radio receivers, are constructed from combinations of a few types of basic circuits. Analog circuits use a range of voltage or current as opposed to discrete levels as in digital circuits. The number of different analog circuits so far devised is huge, especially because a circuit can be defined as anything from a single component, analog circuits are sometimes called linear circuits although many non-linear effects are used in analog circuits such as mixers, modulators, etc. Good examples of analog circuits include vacuum tube and transistor amplifiers, one rarely finds modern circuits that are entirely analog. These days analog circuitry may use digital or even microprocessor techniques to improve performance and this type of circuit is usually called mixed signal rather than analog or digital. Sometimes it may be difficult to differentiate between analog and digital circuits as they have elements of both linear and non-linear operation, an example is the comparator which takes in a continuous range of voltage but only outputs one of two levels as in a digital circuit
42.
Hydraulics
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Hydraulics is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids or fluids. At a very basic level, hydraulics is the version of pneumatics. Fluid mechanics provides the foundation for hydraulics, which focuses on the applied engineering using the properties of fluids. In fluid power, hydraulics are used for the generation, control, hydraulic topics range through some parts of science and most of engineering modules, and cover concepts such as pipe flow, dam design, fluidics and fluid control circuitry, pumps. The principles of hydraulics are in use naturally in the body within the heart. Free surface hydraulics is the branch of hydraulics dealing with surface flow, such as occurring in rivers, canals, lakes, estuaries. Its sub-field open channel flow studies the flow in open channels, the word hydraulics originates from the Greek word ὑδραυλικός which in turn originates from ὕδωρ and αὐλός. Early uses of water power date back to Mesopotamia and ancient Egypt, other early examples of water power include the Qanat system in ancient Persia and the Turpan water system in ancient Central Asia. The Greeks constructed sophisticated water and hydraulic power systems, an example is the construction by Eupalinos, under a public contract, of a watering channel for Samos, the Tunnel of Eupalinos. An early example of the usage of hydraulic wheel, probably the earliest in Europe, is the Perachora wheel, notable is the construction of the first hydraulic automata by Ctesibius and Hero of Alexandria. Hero describes a number of working machines using hydraulic power, such as the force pump, in ancient China there was Sunshu Ao, Ximen Bao, Du Shi, Zhang Heng, and Ma Jun, while medieval China had Su Song and Shen Kuo. Du Shi employed a waterwheel to power the bellows of a blast furnace producing cast iron, Zhang Heng was the first to employ hydraulics to provide motive power in rotating an armillary sphere for astronomical observation. In ancient Sri Lanka, hydraulics were used in the ancient kingdoms of Anuradhapura. The discovery of the principle of the tower, or valve pit. By the first century AD, several irrigation works had been completed. The coral on the rock at the site includes cisterns for collecting water. They were among the first to use of the siphon to carry water across valleys. They used lead widely in plumbing systems for domestic and public supply, hydraulic mining was used in the gold-fields of northern Spain, which was conquered by Augustus in 25 BC
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Fluidics
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Fluidics, or fluidic logic, is the use of a fluid to perform analog or digital operations similar to those performed with electronics. The physical basis of fluidics is pneumatics and hydraulics, based on the foundation of fluid dynamics. The 1960s saw the application of fluidics to sophisticated control systems, a jet of fluid can be deflected by a weaker jet striking it at the side. This provides nonlinear amplification, similar to the used in electronic digital logic. It is used mostly in environments where electronic digital logic would be unreliable, nanotechnology considers fluidics as one of its instruments. In this domain, effects such as fluid-solid and fluid-fluid interface forces are highly significant. Fluidics have also used for military applications. Logic gates can be built that use water instead of electricity to power the gating function and these are reliant on being positioned in one orientation to perform correctly. An OR gate is simply two pipes being merged, and a NOT gate consists of A deflecting a supply stream to produce Ā, the AND and XOR gates are sketched in the diagram. An inverter could also be implemented with the XOR gate, as A XOR1 = Ā, bubble logic is another kind of fluidic logic. Bubble logic gates conserve the number of entering and exiting the device, since bubbles are neither produced nor destroyed in the logic operation. In a fluidic amplifier, A fluid supply, which may be air, water, or hydraulic fluid, pressure applied to the control ports C1 or C2 deflects the stream, so that it exits via either port O1 or O2. The stream entering the ports may be much weaker than the stream being deflected. Given this basic device, flip flops and other fluidic logic elements can be constructed, simple systems of digital logic can thus be built. Fluidic amplifiers typically have bandwidths in the low range, so systems built from them are quite slow compared to electronic devices. The fluidic triode is a device that uses a fluid to convey the signal. Although much studied in the laboratory they have few practical applications, many expect them to be key elements of nanotechnology. Fluidic triodes were used as the stage in the main Public Address system at the 1964 New York Worlds Fair
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Mechanics
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Mechanics is an area of science concerned with the behaviour of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment. The scientific discipline has its origins in Ancient Greece with the writings of Aristotle, during the early modern period, scientists such as Khayaam, Galileo, Kepler, and Newton, laid the foundation for what is now known as classical mechanics. It is a branch of physics that deals with particles that are either at rest or are moving with velocities significantly less than the speed of light. It can also be defined as a branch of science which deals with the motion of, historically, classical mechanics came first, while quantum mechanics is a comparatively recent invention. Classical mechanics originated with Isaac Newtons laws of motion in Philosophiæ Naturalis Principia Mathematica, both are commonly held to constitute the most certain knowledge that exists about physical nature. Classical mechanics has especially often been viewed as a model for other so-called exact sciences, essential in this respect is the relentless use of mathematics in theories, as well as the decisive role played by experiment in generating and testing them. Quantum mechanics is of a scope, as it encompasses classical mechanics as a sub-discipline which applies under certain restricted circumstances. According to the principle, there is no contradiction or conflict between the two subjects, each simply pertains to specific situations. The correspondence principle states that the behavior of systems described by quantum theories reproduces classical physics in the limit of quantum numbers. Quantum mechanics has superseded classical mechanics at the level and is indispensable for the explanation and prediction of processes at the molecular, atomic. However, for macroscopic processes classical mechanics is able to solve problems which are difficult in quantum mechanics and hence remains useful. Modern descriptions of such behavior begin with a definition of such quantities as displacement, time, velocity, acceleration, mass. Until about 400 years ago, however, motion was explained from a different point of view. He showed that the speed of falling objects increases steadily during the time of their fall and this acceleration is the same for heavy objects as for light ones, provided air friction is discounted. The English mathematician and physicist Isaac Newton improved this analysis by defining force and mass, for objects traveling at speeds close to the speed of light, Newton’s laws were superseded by Albert Einstein’s theory of relativity. For atomic and subatomic particles, Newton’s laws were superseded by quantum theory, for everyday phenomena, however, Newton’s three laws of motion remain the cornerstone of dynamics, which is the study of what causes motion. In analogy to the distinction between quantum and classical mechanics, Einsteins general and special theories of relativity have expanded the scope of Newton, the differences between relativistic and Newtonian mechanics become significant and even dominant as the velocity of a massive body approaches the speed of light. Relativistic corrections are also needed for quantum mechanics, although general relativity has not been integrated, the two theories remain incompatible, a hurdle which must be overcome in developing a theory of everything
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Gear train
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A transmission is a machine in a power transmission system, which provides controlled application of the power. Often the term refers simply to the gearbox that uses gears and gear trains to provide speed. In British English, the term refers to the whole drivetrain, including clutch, gearbox, prop shaft, differential. In American English, however, the term more specifically to the gearbox alone. The most common use is in vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels. Such engines need to operate at a high rotational speed, which is inappropriate for starting, stopping. The transmission reduces the engine speed to the slower wheel speed. Transmissions are also used on bicycles, fixed machines. Often, a transmission has multiple gear ratios with the ability to switch between them as speed varies and this switching may be done manually or automatically. Directional control may also be provided, single-ratio transmissions also exist, which simply change the speed and torque of motor output. The output of the transmission is transmitted via the driveshaft to one or more differentials, while a differential may also provide gear reduction, its primary purpose is to permit the wheels at either end of an axle to rotate at different speeds as it changes the direction of rotation. Conventional gear/belt transmissions are not the mechanism for speed/torque adaptation. Alternative mechanisms include torque converters and power transformation, automatic transmissions use a valve body to shift gears using fluid pressures in conjunction with an ecm. Early transmissions included the right-angle drives and other gearing in windmills, horse-powered devices, and steam engines, in support of pumping, milling, most modern gearboxes are used to increase torque while reducing the speed of a prime mover output shaft. This means that the shaft of a gearbox rotates at a slower rate than the input shaft. A gearbox can be set up to do the opposite and provide an increase in speed with a reduction of torque. Some of the simplest gearboxes merely change the rotational direction of power transmission. Many typical automobile transmissions include the ability to select one of several gear ratios, in this case, most of the gear ratios are used to slow down the output speed of the engine and increase torque
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Friction
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Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction, Dry friction resists relative lateral motion of two surfaces in contact. Dry friction is subdivided into static friction between non-moving surfaces, and kinetic friction between moving surfaces, fluid friction describes the friction between layers of a viscous fluid that are moving relative to each other. Lubricated friction is a case of fluid friction where a lubricant fluid separates two solid surfaces, skin friction is a component of drag, the force resisting the motion of a fluid across the surface of a body. Internal friction is the force resisting motion between the making up a solid material while it undergoes deformation. When surfaces in contact move relative to other, the friction between the two surfaces converts kinetic energy into thermal energy. This property can have consequences, as illustrated by the use of friction created by rubbing pieces of wood together to start a fire. Kinetic energy is converted to thermal energy whenever motion with friction occurs, another important consequence of many types of friction can be wear, which may lead to performance degradation and/or damage to components. Friction is a component of the science of tribology, Friction is not itself a fundamental force. Dry friction arises from a combination of adhesion, surface roughness, surface deformation. The complexity of interactions makes the calculation of friction from first principles impractical and necessitates the use of empirical methods for analysis. Friction is a non-conservative force - work done against friction is path dependent, in the presence of friction, some energy is always lost in the form of heat. Thus mechanical energy is not conserved, the Greeks, including Aristotle, Vitruvius, and Pliny the Elder, were interested in the cause and mitigation of friction. They were aware of differences between static and kinetic friction with Themistius stating in 350 A. D. that it is easier to further the motion of a moving body than to move a body at rest. The classic laws of sliding friction were discovered by Leonardo da Vinci in 1493, a pioneer in tribology and these laws were rediscovered by Guillaume Amontons in 1699. Amontons presented the nature of friction in terms of surface irregularities, the understanding of friction was further developed by Charles-Augustin de Coulomb. Coulomb further considered the influence of sliding velocity, temperature and humidity, the distinction between static and dynamic friction is made in Coulombs friction law, although this distinction was already drawn by Johann Andreas von Segner in 1758. Leslie was equally skeptical about the role of adhesion proposed by Desaguliers, in Leslies view, friction should be seen as a time-dependent process of flattening, pressing down asperities, which creates new obstacles in what were cavities before
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Differential analyser
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The differential analyser is a mechanical analogue computer designed to solve differential equations by integration, using wheel-and-disc mechanisms to perform the integration. It was one of the first advanced computing devices to be used operationally, the original machines could not add, but then it was noticed that if you turn the two wheels of a rear differential, the drive shaft will compute the average of the left and right wheels. A simple gear ratio of 1,2 then enables you to multiply by two, so addition are achieved, multiplication is just a special case of integration, namely integrating a constant function. One of the earliest practical uses of Thomsons concepts was a machine built by Kelvin starting in 1872-3. Italian mathematician Ernesto Pascal also developed integraphs for the integration of differential equations. However, the first widely practical general purpose differential analyser was constructed by Harold Locke Hazen and Vannevar Bush at MIT, 1928–1931, in the same year, Bush described this machine in a journal article as a continuous integraph. When he published an article on the device in 1931. In this article, Bush stated that present device incorporates the same idea of interconnection of integrating units as did. In detail, however, there is little resemblance to the earlier model, according to his 1970 autobiography, Bush was unaware of Kelvin’s work until after the first differential analyzer was operational. Claude Shannon was hired as a assistant in 1936 to run the differential analyzer in Bushs lab. During the next five years three more were added, at Cambridge University, Queens University Belfast, and the Royal Aircraft Establishment in Farnborough. One of the integrators from this proof of concept is on display in the History of Computing section of the Science Museum alongside a complete Manchester machine, in Norway, the locally built Oslo Analyser was finished during 1938, based on the same principles as the MIT machine. This machine had 12 integrators, and was the largest analyser built for a period of four years, the latter was used extensively in the computation of artillery firing tables prior to the invention of the ENIAC, which, in many ways, was modelled on the differential analyser. Also in the early 1940s, with Samuel H, in 1947, UCLA installed a differential analyser built for them by General Electric at a cost of $125,000. By 1950, this machine had joined by three more. It was later transferred to the Tokyo University of Science and has displayed at the school’s Museum of Science in Shinjuku Ward. Restored in 2014 is one of two still operational differential analyzers produced before the end of World War II. In Canada, a differential analyser was constructed at the University of Toronto in 1948 by Beatrice Helen Worsley, a differential analyser may have been used in the development of the bouncing bomb, used to attack German hydroelectric dams during World War II
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Cam
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A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice versa. It is often a part of a wheel or shaft that strikes a lever at one or more points on its circular path. The cam can be seen as a device that rotates from circular to reciprocating motion, certain cams can be characterized by their displacement diagrams, which reflect the changing position a roller follower would make as the cam rotates about an axis. These diagrams relate angular position, usually in degrees, to the radial displacement experienced at that position, displacement diagrams are traditionally presented as graphs with non-negative values. A simple displacement diagram illustrates the follower motion at a constant velocity rise followed by a return with a dwell in between as depicted in figure 2. The rise is the motion of the follower away from the cam center, dwell is the motion where the follower is at rest, however, the most common type is in the valve actuators in internal combustion engines. Here, the cam profile is symmetric and at rotational speeds generally met with. Ideally, a curve between the onset and maximum position of lift reduces acceleration, but this requires impractically large shaft diameters relative to lift. Thus, in practice, the points at which lift begins and this is continuous with a tangent to the tip circle. In designing the cam, the lift and the dwell angle θ are given, here, the follower moves in a plane perpendicular to the axis of rotation of the camshaft. The base circle is the smallest circle that can be drawn to the cam profile, a once common, but now outdated, application of this type of cam was automatic machine tool programming cams. Each tool movement or operation was controlled directly by one or more cams, instructions for producing programming cams and cam generation data for the most common makes of machine were included in engineering references well into the modern CNC era. This type of cam is used in many simple electromechanical appliance controllers, such as dishwashers and clothes washing machines, a cylindrical cam or barrel cam is a cam in which the follower rides on the surface of a cylinder. In the most common type, the rides in a groove cut into the surface of a cylinder. These cams are principally used to convert rotational motion to linear motion parallel to the axis of the cylinder. A cylinder may have several grooves cut into the surface and drive several followers, applications include machine tool drives, such as reciprocating saws, and shift control barrels in sequential transmissions, such as on most modern motorcycles. A special case of cam is constant lead, where the position of the follower is linear with rotation. The purpose and detail of implementation influence whether this application is called a cam or a thread, but in some cases
