Vacuum tube
In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as a thermionic tube or thermionic valve uses the phenomenon of thermionic emission of electrons from a heated cathode and is used for a number of fundamental electronic functions such as signal amplification and current rectification. Non-thermionic types, such as a vacuum phototube however, achieve electron emission through the photoelectric effect, are used for such as the detection of light levels. In both types, the electrons are accelerated from the cathode to the anode by the electric field in the tube; the simplest vacuum tube, the diode invented in 1904 by John Ambrose Fleming, contains only a heated electron-emitting cathode and an anode. Current can only flow in one direction through the device—from the cathode to the anode. Adding one or more control grids within the tube allows the current between the cathode and anode to be controlled by the voltage on the grid or grids.
These devices became a key component of electronic circuits for the first half of the twentieth century. They were crucial to the development of radio, radar, sound recording and reproduction, long distance telephone networks, analogue and early digital computers. Although some applications had used earlier technologies such as the spark gap transmitter for radio or mechanical computers for computing, it was the invention of the thermionic vacuum tube that made these technologies widespread and practical, created the discipline of electronics. In the 1940s the invention of semiconductor devices made it possible to produce solid-state devices, which are smaller, more efficient and durable, cheaper than thermionic tubes. From the mid-1960s, thermionic tubes were being replaced with the transistor. However, the cathode-ray tube remained the basis for television monitors and oscilloscopes until the early 21st century. Thermionic tubes still have some applications, such as the magnetron used in microwave ovens, certain high-frequency amplifiers, amplifiers that audio enthusiasts prefer for their tube sound.
Not all electronic circuit valves/electron tubes are vacuum tubes. Gas-filled tubes are similar devices, but containing a gas at low pressure, which exploit phenomena related to electric discharge in gases without a heater. One classification of thermionic vacuum tubes is by the number of active electrodes. A device with two active elements is a diode used for rectification. Devices with three elements are triodes used for switching. Additional electrodes create tetrodes, so forth, which have multiple additional functions made possible by the additional controllable electrodes. Other classifications are: by frequency range by power rating by cathode/filament type and Warm-up time by characteristic curves design by application specialized parameters specialized functions tubes used to display information Tubes have different functions, such as cathode ray tubes which create a beam of electrons for display purposes in addition to more specialized functions such as electron microscopy and electron beam lithography.
X-ray tubes are vacuum tubes. Phototubes and photomultipliers rely on electron flow through a vacuum, though in those cases electron emission from the cathode depends on energy from photons rather than thermionic emission. Since these sorts of "vacuum tubes" have functions other than electronic amplification and rectification they are described in their own articles. A vacuum tube consists of two or more electrodes in a vacuum inside an airtight envelope. Most tubes have glass envelopes with a glass-to-metal seal based on kovar sealable borosilicate glasses, though ceramic and metal envelopes have been used; the electrodes are attached to leads. Most vacuum tubes have a limited lifetime, due to the filament or heater burning out or other failure modes, so they are made as replaceable units. Tubes were a frequent cause of failure in electronic equipment, consumers were expected to be able to replace tubes themselves. In addition to the base terminals, some tubes had an electrode terminating at a top cap.
The principal reason for doing this was to avoid leakage resistance through the tube base for the high impedance grid input. The bases were made with phenolic insulation which performs poorly as an insulator in humid conditions. Other reasons for using a top cap include improving stability by reducing grid-to-anode capacitance, improved high-frequency performance, keeping a high plate voltage away from lower voltages, accommodating one more electrode than allowed by the base. There was an occasional design that had two top cap connections; the earliest vacuum tubes evolved from incandescent light bulbs, containing a filament sealed in an evacuated glass envelope. When hot, the filament releases electrons into the vacuum, a process called thermio
Swedish Americans
Swedish Americans are an American ethnic group of people who have ancestral roots from Sweden. They include the 1.2 million Swedish immigrants during 1885–1915 and their descendants. They formed tight-knit communities in the American Midwest, intermarried with other Swedish-Americans. Most were Lutheran Christians with origins in the state Church of Sweden who were affiliated with predecessor bodies of what are now the Evangelical Lutheran Church in America from the mergers of 1988 or the Lutheran Church–Missouri Synod, or the recent North American Lutheran Church of 2010. Today, Swedish Americans are found throughout the United States, with Minnesota and Illinois being the top three states with the highest number of Swedish Americans. Newly arrived Swedish immigrants settled in the Midwest, namely Minnesota, the Dakotas and Wisconsin, just as other Scandinavian Americans. Populations grew in the Pacific Northwest in the states of Oregon and Washington at the turn of the twentieth century; the first Swedish Americans were the settlers of New Sweden.
A colony established by Queen Christina of Sweden in 1638, it centered around the Delaware Valley including parts of the present-day states of Delaware, New Jersey, Pennsylvania. New Sweden was incorporated into New Netherland in 1655, ceased to be an official territory of the Realm of Sweden. However, many Swedish and Finnish colonists remained and were allowed some political and cultural autonomy. A victim of one of the earliest recorded murders in North America was an immigrant from Sweden. In 1665 in Brooklyn, New York, Barent Jansen Blom, progenitor of the Blom/Bloom family of Brooklyn and the lower Hudson Valley, was stabbed to death by Albert Cornelis Wantenaer. Present day reminders of the history of New Sweden are reflected in the presence of the American Swedish Historical Museum in Philadelphia, Fort Christina State Park in Wilmington, Governor Printz Park, The Printzhof in Essington, Pennsylvania. Swedish emigration to the United States had reached new heights in 1896, it was in this year that the Vasa Order of America, a Swedish American fraternal organization, was founded to help immigrants, who lacked an adequate network of social services.
Swedish Americans came through New York City and subsequently settled in the upper Midwest. Most were Lutheran and belonged to synods now associated with the Evangelical Lutheran Church in America, including the Augustana Evangelical Lutheran Church. Theologically, they were pietistic. In the year 1900, Chicago was the city with the second highest number of Swedes after Stockholm, the capital of Sweden. By Swedes in Chicago had founded the Evangelical Covenant Church and established such enduring institutions as Swedish Covenant Hospital and North Park University. Many others settled in Minnesota followed by Wisconsin. Like their Norwegian American and Danish American brethren, many Swedes sought out the agrarian lifestyle they had left behind in Sweden, as many immigrants settled on farms throughout the Midwest. There are towns scattered throughout the Midwest, such as Lindsborg, that to this day continue to celebrate their Swedish heritage. In the east, New England became a destination for many skilled industrial workers and Swedish centers developed in areas such as Jamestown, New York.
A small Swedish settlement was begun in New Sweden, Maine. 51 Swedish settlers came to the wooded area, led by W. W. Thomas, who called them "mina barn i skogen". Upon arrival, they knelt in thanksgiving to God; this area soon expanded and other settlements were named Stockholm and Westmanland, in honor of their Swedish heritage. The town of New Sweden, Maine celebrates St. Lucia and Founders Day, it is a Swedish-American community. Gustaf Adolph Lutheran Church was served by a native of Sweden as as 1979-1985, known to conduct special worship services in Swedish; the largest settlement in New England was Massachusetts. Here, Swedes were drawn to abrasive industries. By the early 20th century numerous churches, organizations and benevolent associations had been organized. Among them, the Swedish Cemetery Corporation, the Swedish Lutheran Old People's Home, Fairlawn Hospital, the Scandinavian Athletic Club; these institutions survive today. Numerous local lodges of national Swedish American organizations flourished and a few remain solvent as of 2008.
Within the city's largest historic "Swedish" neighborhood—Quinsigamond Village—street signs read like a map of Sweden: Stockholm Street, Halmstad Street, Malmo Street among others. Worcester's Swedes were staunch Republicans and this political loyalty is behind why Worcester remained a Republican stronghold in an otherwise Democratic state well into the 1950s. Many Swedes came to the Pacific Northwest during the turn of the twentieth century, along with Norwegians and Finns, settling in Washington and Oregon. According to research by the Oregon Historical Society, Swedish immigrants "felt a kinship with the natural surroundings and economic opportunities in the Pacific Northwest," and the region experienced a signi
General Electric Realty Plot
The General Electric Realty Plot referred to locally as the GE Realty Plot, GE Plots or just The Plot, is a residential neighborhood in Schenectady, New York, United States. It is an area of 90 acres just east of Union College. An undeveloped tract owned by the college, it was sold to General Electric at the end of the 19th century to help the college pay off a debt; the company's executives subdivided it, laid out streets according to a plan inspired by New York's Central Park and built houses on the land, with covenants requiring a minimum lot size and house value. Two of them were among the first electric houses in the U. S. used as models by GE. Settling in the neighborhood were some local businessmen and politicians, the research scientists who worked at the company's research laboratory a short distance away, they were collectively responsible for over 400 patents. Some of the key events in their research happened within the Plot, as many took things home to work on. By 1927 a hundred houses had been built, including one owned by chemist Irving Langmuir, a GE researcher, for his life.
It has subsequently been designated a National Historic Landmark in recognition of Langmuir's scientific accomplishments, including the Nobel Prize in Chemistry. In 1980 the entire neighborhood was recognized as a historic district and listed on the National Register of Historic Places; the Plot is no longer so dominated by GE employees, due to the company's reduced presence in the city. Houses in it remain valued, residents pay some of the Capital District region's highest property taxes. Rules enforced by both the neighborhood association and the city's Historic Commission preserve its historic character; the district is bounded by streets, making it quadrilateral in shape. At the west, Lenox Road divides it from the college campus. Nott Street forms the northern boundary, separating the Realty Plot from Ellis Hospital until West Alley, the rear property lines between houses on Lowell Road and Glenwood Boulevard, which marks the eastern boundary. After crossing Rugby Road, the line turns west to take in the houses on the south side of that street to its intersection with Wendell Avenue following Union Avenue back to Lenox.
A small stream known as College Creek or Grooteskill flows west through the district to the Mohawk River, making a ravine 30 feet deep and 100 feet wide through the center of the district. It is spanned by four bluestone arch bridges, all included as contributing properties. Many mature trees grow throughout the district; the houses within are on lots with a minimum size of 70 by 140 feet, set back 35–45 feet from the streets they front on. They are predominantly in the Colonial Revival or Georgian Revival architectural styles, with some in the Mission Revival, Queen Anne or Shingle modes; the Steinmetz Memorial on Wendell Avenue is the only significant open space in the district. It is named for Charles Proteus Steinmetz, the German-born electrical engineer whose research made alternating current possible. Steinmetz's house stood on the grounds of the Memorial, he did research in a backyard lab, both the lab and the house were demolished in 1944. Steinmetz chaired the city's school board. A separate public park in Schenectady, known as The Steinmetz Park, is located on Lenox Road but is outside the GE Realty Plot district.
The building at 1184 Rugby Road was developed as a primary school and remained in use for that purpose for many years. Only five buildings have been added to the district since its initial period of development in the first quarter of the 20th century, they include a modern church by Edward Durrell Stone from the late 1950s on Wendell Avenue. Most of them are otherwise unintrusive, but are not considered contributing due to their more recent construction. In 1899 Union College announced that, in order to pay off a $30,000 debt, it would sell two adjacent tracts of land. One, the College Meadow, was a 30-acre parcel to the west. Prominent citizens of the city called for the land to be developed into a park, a petition to that effect gained 2,200 signatures; the college proposed to lease the land to the city for that purpose, but at a high annual rate. Shortly afterward, several members of the board of General Electric, which had grown since Thomas Edison had moved his Edison Machine Works to Schenectady in 1886, announced that the company would buy the land from the college for $57,000, retiring the college's debt.
They formed Schenectady Realty Company, to purchase and develop the land. Their goal was to offer another incentive for the company's executives and researchers to stay rather than leave for a competitor or start their own companies, it took until 1903 to lay water and sewer lines and build the bridges. The plan for the neighborhood was developed by Parse and DeForrest, specialists in the new discipline of urban planning, they took their inspiration from New York City's Central Park, designed by Frederick Law Olmsted and Calvert Vaux. Instead of being planned around a central street, like the way Schenectady had developed along Union Street, thinking which had guided Glenwood and Parkwood boulevards elsewhere in the city, they imagined the area as two smaller lots divided by Wendell Avenue, a street named after a former landowner in the area, they laid out the streets in gentle curves, like the pathways in Central Park. They tried something new on residential city streets at the time: the "sod gutter"
Sweden
Sweden the Kingdom of Sweden, is a Scandinavian Nordic country in Northern Europe. It borders Norway to the west and north and Finland to the east, is connected to Denmark in the southwest by a bridge-tunnel across the Öresund, a strait at the Swedish-Danish border. At 450,295 square kilometres, Sweden is the largest country in Northern Europe, the third-largest country in the European Union and the fifth largest country in Europe by area. Sweden has a total population of 10.2 million. It has a low population density of 22 inhabitants per square kilometre; the highest concentration is in the southern half of the country. Germanic peoples have inhabited Sweden since prehistoric times, emerging into history as the Geats and Swedes and constituting the sea peoples known as the Norsemen. Southern Sweden is predominantly agricultural, while the north is forested. Sweden is part of the geographical area of Fennoscandia; the climate is in general mild for its northerly latitude due to significant maritime influence, that in spite of this still retains warm continental summers.
Today, the sovereign state of Sweden is a constitutional monarchy and parliamentary democracy, with a monarch as head of state, like its neighbour Norway. The capital city is Stockholm, the most populous city in the country. Legislative power is vested in the 349-member unicameral Riksdag. Executive power is exercised by the government chaired by the prime minister. Sweden is a unitary state divided into 21 counties and 290 municipalities. An independent Swedish state emerged during the early 12th century. After the Black Death in the middle of the 14th century killed about a third of the Scandinavian population, the Hanseatic League threatened Scandinavia's culture and languages; this led to the forming of the Scandinavian Kalmar Union in 1397, which Sweden left in 1523. When Sweden became involved in the Thirty Years War on the Reformist side, an expansion of its territories began and the Swedish Empire was formed; this became one of the great powers of Europe until the early 18th century. Swedish territories outside the Scandinavian Peninsula were lost during the 18th and 19th centuries, ending with the annexation of present-day Finland by Russia in 1809.
The last war in which Sweden was directly involved was in 1814, when Norway was militarily forced into personal union. Since Sweden has been at peace, maintaining an official policy of neutrality in foreign affairs; the union with Norway was peacefully dissolved in 1905. Sweden was formally neutral through both world wars and the Cold War, albeit Sweden has since 2009 moved towards cooperation with NATO. After the end of the Cold War, Sweden joined the European Union on 1 January 1995, but declined NATO membership, as well as Eurozone membership following a referendum, it is a member of the United Nations, the Nordic Council, the Council of Europe, the World Trade Organization and the Organisation for Economic Co-operation and Development. Sweden maintains a Nordic social welfare system that provides universal health care and tertiary education for its citizens, it has the world's eleventh-highest per capita income and ranks in numerous metrics of national performance, including quality of life, education, protection of civil liberties, economic competitiveness, equality and human development.
The name Sweden was loaned from Dutch in the 17th century to refer to Sweden as an emerging great power. Before Sweden's imperial expansion, Early Modern English used Swedeland. Sweden is derived through back-formation from Old English Swēoþēod, which meant "people of the Swedes"; this word is derived from Sweon/Sweonas. The Swedish name Sverige means "realm of the Swedes", excluding the Geats in Götaland. Variations of the name Sweden are used in most languages, with the exception of Danish and Norwegian using Sverige, Faroese Svøríki, Icelandic Svíþjóð, the more notable exception of some Finnic languages where Ruotsi and Rootsi are used, names considered as referring to the people from the coastal areas of Roslagen, who were known as the Rus', through them etymologically related to the English name for Russia; the etymology of Swedes, thus Sweden, is not agreed upon but may derive from Proto-Germanic Swihoniz meaning "one's own", referring to one's own Germanic tribe. Sweden's prehistory begins in the Allerød oscillation, a warm period around 12,000 BC, with Late Palaeolithic reindeer-hunting camps of the Bromme culture at the edge of the ice in what is now the country's southernmost province, Scania.
This period was characterised by small bands of hunter-gatherer-fishers using flint technology. Sweden is first described in a written source in Germania by Tacitus in 98 AD. In Germania 44 and 45 he mentions the Swedes as a powerful tribe with ships that had a prow at each end. Which kings ruled these Suiones is unknown, but Norse mythology presents a long line of legendary and semi-legendary kings going back to the last centuries BC; as for literacy in Sweden itself, the runic script was in use among the south Scandinavian elite by at least the 2nd century AD, but all that has come down to the present from the Roman Period is curt inscriptions on artefacts of male names, demonstrating th
Tuned radio frequency receiver
A tuned radio frequency receiver is a type of radio receiver, composed of one or more tuned radio frequency amplifier stages followed by a detector circuit to extract the audio signal and an audio frequency amplifier. This type of receiver was popular in the 1920s. Early examples could be tedious to operate because when tuning in a station each stage had to be individually adjusted to the station's frequency, but models had ganged tuning, the tuning mechanisms of all stages being linked together, operated by just one control knob. By the mid 1930s, it was replaced by the superheterodyne receiver patented by Edwin Armstrong; the TRF receiver was patented in 1916 by Ernst Alexanderson. His concept was that each stage would amplify the desired signal while reducing the interfering ones. Multiple stages of RF amplification would make the radio more sensitive to weak stations, the multiple tuned circuits would give it a narrower bandwidth and more selectivity than the single stage receivers common at that time.
All tuned stages of the radio tune to the desired reception frequency. This is in contrast to the modern superheterodyne receiver that must only tune the receiver's RF front end and local oscillator to the desired frequencies. Antique TRF receivers can be identified by their cabinets, they have a long, low appearance, with a flip-up lid for access to the vacuum tubes and tuned circuits. On their front panels there are two or three large dials, each controlling the tuning for one stage. Inside, along with several vacuum tubes, there will be a series of large coils; these will be with their axes at right angles to each other to reduce magnetic coupling between them. A problem with the TRF receiver built with triode vacuum tubes is the triode's interelectrode capacitance; the interelectrode capacitance allows energy in the output circuit to feedback into the input. That feedback can cause instability and oscillation that frustrate reception and produce squealing or howling noises in the speaker. In 1922, Louis Alan Hazeltine invented the technique of neutralization that uses additional circuitry to cancel the effect of the interelectrode capacitance.
Neutralization was used in the popular Neutrodyne series of TRF receivers. Under certain conditions, "the neutralization is independent of frequency over a wide frequency band." "Perfect neutralization cannot be maintained in practice over a wide band of frequencies because leakage inductances and stray capacities" are not canceled. The development of the tetrode and pentode vacuum tubes minimized the effect of interelectrode capacitances and could make neutralization unnecessary; the classic TRF receivers of the 1920s and 30s consisted of three sections: one or more tuned RF amplifier stages. These amplify the signal of the desired station to a level sufficient to drive the detector, while rejecting all other signals picked up by the antenna. A detector, which extracts the audio signal from the radio carrier signal by rectifying it. Optionally, but always included, one or more audio amplifier stages which increase the power of the audio signal; each tuned RF stage consists of an amplifying device, a triode vacuum tube, a tuned circuit which performs the filtering function.
The tuned circuit consisted of an air-core RF coupling transformer which served to couple the signal from the plate circuit of one tube to the input grid circuit of the next tube. One of the windings of the transformer had a variable capacitor connected across it to make a tuned circuit. A variable capacitor was used, with a knob on the front panel to tune the receiver; the RF stages had identical circuits to simplify design. Each RF stage had to be tuned to the same frequency, so the capacitors had to be tuned in tandem when bringing in a new station. In some sets the capacitors were "ganged", mounted on the same shaft or otherwise linked mechanically so that the radio could be tuned with a single knob, but in most sets the resonant frequencies of the tuned circuits could not be made to "track" well enough to allow this, each stage had its own tuning knob; the detector was a grid-leak detector. Some sets used a crystal detector instead. A regenerative detector was used, to increase selectivity.
Some TRF sets that were listened to with earphones didn't need an audio amplifier, but most sets had one to three transformer-coupled or RC-coupled audio amplifier stages to provide enough power to drive a loudspeaker. The schematic diagram shows a typical TRF receiver; this particular example uses six triodes. It has two radio frequency amplifier stages, one grid-leak detector/amplifier and three class ‘A’ audio amplifier stages. There are 3 tuned circuits T1-C1, T2-C2, T3-C3; the second and third tuning capacitors, C2 and C3, are ganged together and controlled by a single knob, to simplify tuning. Two or three RF amplifiers were required to filter and amplify the received signal enough for good reception. Terman characterizes the TRF's disadvantages as "poor selectivity and low sensitivity in proportion to the number of tubes employed, they are accordingly obsolete." Selectivity requires narrow bandwidth, but the bandwidth of a filter with a given Q factor increases with frequency. So to achieve a narrow bandwidth at a high radio frequency required high-Q filters o
RCA
The RCA Corporation was a major American electronics company, founded as the Radio Corporation of America in 1919. It was a wholly owned subsidiary of General Electric. An innovative and progressive company, RCA was the dominant electronics and communications firm in the United States for over five decades. RCA was at the forefront of the mushrooming radio industry in the early 1920s, as a major manufacturer of radio receivers, the exclusive manufacturer of the first superheterodyne models. RCA created the first American radio network, the National Broadcasting Company; the company was a pioneer in the introduction and development of television, both black-and-white and color. During this period, RCA was identified with the leadership of David Sarnoff, he was general manager at the company's founding, became president in 1930, remained active, as chairman of the board, until the end of 1969. RCA's impregnable stature began to weaken in the mid-1970s, as it attempted to diversify and expand into a multifaceted conglomerate.
The company suffered enormous financial losses in the mainframe computer industry and other failed projects such as the CED videodisc. In 1986, RCA was reacquired by General Electric, which over the next few years liquidated most of the corporation's assets. Today, RCA exists as a brand name only. RCA originated as a reorganization of the Marconi Wireless Telegraph Company of America. In 1897, the Wireless Telegraph and Signal Company, was founded in London to promote the radio inventions of Guglielmo Marconi; as part of worldwide expansion, in 1899 American Marconi was organized as a subsidiary company, holding the rights to use the Marconi patents in the United States and Cuba. In 1912 it took over the assets of the bankrupt United Wireless Telegraph Company, from that point forward it had been the dominant radio communications company in the United States. With the entry of the United States into World War One in April 1917, the government took over most civilian radio stations, to use them for the war effort.
Although the overall U. S. government plan was to restore civilian ownership of the seized radio stations once the war ended, many Navy officials hoped to retain a monopoly on radio communication after the war. Defying instructions to the contrary, the Navy began purchasing large numbers of stations outright. With the conclusion of the conflict, Congress turned down the Navy's efforts to have peacetime control of the radio industry, instructed the Navy to make plans to return the commercial stations it controlled, including the ones it had improperly purchased, to the original owners. Due to national security considerations, the Navy was concerned about returning the high-powered international stations to American Marconi, since a majority of its stock was in foreign hands, the British largely controlled the international undersea cables; this concern was increased by the announcement in late 1918 of the formation of the Pan-American Wireless Telegraph and Telephone Company, a joint venture between American Marconi and the Federal Telegraph Company, with plans to set up service between the United States and South America.
The Navy had installed a high-powered Alexanderson alternator, built by General Electric, at the American Marconi transmitter site in New Brunswick, New Jersey. It proved to be superior for transatlantic transmissions to the spark transmitters, traditionally used by the Marconi companies. Marconi officials were so impressed by the capabilities of the Alexanderson alternators that they began making preparations to adopt them as their standard transmitters for international communication. A tentative plan made with General Electric proposed that over a two-year period the Marconi companies would purchase most of GE's alternator production. However, this proposal was met with disapproval, on national security grounds, by the U. S. Navy, concerned that this would guarantee British domination of international radio communication; the Navy, claiming it was acting with the support of President Wilson, looked for an alternative that would result in an "all-American" company taking over the American Marconi assets.
In April 1919 two naval officers, Admiral H. G. Bullard and Commander S. C. Hooper, met with GE's president, Owen D. Young, asking that he suspend the pending alternator sales to the Marconi companies; this move would leave General Electric without a buyer for its transmitters, so the officers proposed that GE purchase American Marconi, use the assets to form its own radio communications subsidiary. Young consented to this proposal, effective November 20, 1919, transformed American Marconi into the Radio Corporation of America; the new company was promoted as being a patriotic gesture. RCA's incorporation papers required that its officers needed to be U. S. citizens, with a majority of its stock held by Americans. RCA retained most of the American Marconi staff, although Owen Young became the new company's head as the chairman of the board. Former American Marconi vice president and general manager E. J. Nally become RCA's first president. Nally's term ended on December 31, 1922, he was succeeded the next day by Major General James G. Harbord.
Harbord
World Heritage Site
A World Heritage Site is a landmark or area, selected by the United Nations Educational and Cultural Organization as having cultural, scientific or other form of significance, is protected by international treaties. The sites are judged important to the collective interests of humanity. To be selected, a World Heritage Site must be an classified landmark, unique in some respect as a geographically and identifiable place having special cultural or physical significance, it may signify a remarkable accomplishment of humanity, serve as evidence of our intellectual history on the planet. The sites are intended for practical conservation for posterity, which otherwise would be subject to risk from human or animal trespassing, unmonitored/uncontrolled/unrestricted access, or threat from local administrative negligence. Sites are demarcated by UNESCO as protected zones; the list is maintained by the international World Heritage Program administered by the UNESCO World Heritage Committee, composed of 21 "states parties" that are elected by their General Assembly.
The programme catalogues and conserves sites of outstanding cultural or natural importance to the common culture and heritage of humanity. Under certain conditions, listed sites can obtain funds from the World Heritage Fund; the program began with the Convention Concerning the Protection of the World's Cultural and Natural Heritage, adopted by the General Conference of UNESCO on 16 November 1972. Since 193 state parties have ratified the convention, making it one of the most recognized international agreements and the world's most popular cultural program; as of July 2018, a total of 1,092 World Heritage Sites exist across 167 countries. Italy, with 54 sites, has the most of any country, followed by China, France, Germany and Mexico. In 1954, the government of Egypt decided to build the new Aswan High Dam, whose resulting future reservoir would inundate a large stretch of the Nile valley containing cultural treasures of ancient Egypt and ancient Nubia. In 1959, the governments of Egypt and Sudan requested UNESCO to assist their countries to protect and rescue the endangered monuments and sites.
In 1960, the Director-General of UNESCO launched an appeal to the member states for an International Campaign to Save the Monuments of Nubia. This appeal resulted in the excavation and recording of hundreds of sites, the recovery of thousands of objects, as well as the salvage and relocation to higher ground of a number of important temples, the most famous of which are the temple complexes of Abu Simbel and Philae; the campaign, which ended in 1980, was considered a success. As tokens of its gratitude to countries which contributed to the campaign's success, Egypt donated four temples: the Temple of Dendur was moved to the Metropolitan Museum of Art in New York City, the Temple of Debod was moved to the Parque del Oeste in Madrid, the Temple of Taffeh was moved to the Rijksmuseum van Oudheden in the Netherlands, the Temple of Ellesyia to Museo Egizio in Turin; the project cost $80 million, about $40 million of, collected from 50 countries. The project's success led to other safeguarding campaigns: saving Venice and its lagoon in Italy, the ruins of Mohenjo-daro in Pakistan, the Borobodur Temple Compounds in Indonesia.
UNESCO initiated, with the International Council on Monuments and Sites, a draft convention to protect the common cultural heritage of humanity. The United States initiated the idea of cultural conservation with nature conservation; the White House conference in 1965 called for a "World Heritage Trust" to preserve "the world's superb natural and scenic areas and historic sites for the present and the future of the entire world citizenry". The International Union for Conservation of Nature developed similar proposals in 1968, they were presented in 1972 to the United Nations Conference on the Human Environment in Stockholm. Under the World Heritage Committee, signatory countries are required to produce and submit periodic data reporting providing the World Heritage Committee with an overview of each participating nation's implementation of the World Heritage Convention and a "snapshot" of current conditions at World Heritage properties. A single text was agreed on by all parties, the "Convention Concerning the Protection of the World Cultural and Natural Heritage" was adopted by the General Conference of UNESCO on 16 November 1972.
The Convention came into force on 17 December 1975. As of May 2017, it has been ratified by 193 states parties, including 189 UN member states plus the Cook Islands, the Holy See and the State of Palestine. Only four UN member states have not ratified the Convention: Liechtenstein, Nauru and Tuvalu. A country must first list its significant natural sites. A country may not nominate sites. Next, it can place sites selected from that list into a Nomination File; the Nomination File is evaluated by the International Council on Monuments and Sites and the World Conservation Union. These bodies make their recommendations to the World Heritage Committee; the Committee meets once per year to determine whether or not to inscribe each nominated property on the World Heritage List and sometimes defers or refers the decision to request more information from the country which nominated the site. There are ten selection criteria – a site must meet at least one of them to be included on the list
