Hydroelectricity is electricity produced from hydropower. In 2015, hydropower generated 16.6% of the world's total electricity and 70% of all renewable electricity, was expected to increase about 3.1% each year for the next 25 years. Hydropower is produced in 150 countries, with the Asia-Pacific region generating 33 percent of global hydropower in 2013. China is the largest hydroelectricity producer, with 920 TWh of production in 2013, representing 16.9 percent of domestic electricity use. The cost of hydroelectricity is low, making it a competitive source of renewable electricity; the hydro station consumes no water, unlike gas plants. The average cost of electricity from a hydro station larger than 10 megawatts is 3 to 5 U. S. cents per kilowatt hour. With a dam and reservoir it is a flexible source of electricity since the amount produced by the station can be varied up or down rapidly to adapt to changing energy demands. Once a hydroelectric complex is constructed, the project produces no direct waste, in many cases, has a lower output level of greenhouse gases than fossil fuel powered energy plants.
Hydropower has been used since ancient times to perform other tasks. In the mid-1770s, French engineer Bernard Forest de Bélidor published Architecture Hydraulique which described vertical- and horizontal-axis hydraulic machines. By the late 19th century, the electrical generator was developed and could now be coupled with hydraulics; the growing demand for the Industrial Revolution would drive development as well. In 1878 the world's first hydroelectric power scheme was developed at Cragside in Northumberland, England by William Armstrong, it was used to power a single arc lamp in his art gallery. The old Schoelkopf Power Station No. 1 near Niagara Falls in the U. S. side began to produce electricity in 1881. The first Edison hydroelectric power station, the Vulcan Street Plant, began operating September 30, 1882, in Appleton, with an output of about 12.5 kilowatts. By 1886 there were 45 hydroelectric power stations in the U. S. and Canada. By 1889 there were 200 in the U. S. alone. At the beginning of the 20th century, many small hydroelectric power stations were being constructed by commercial companies in mountains near metropolitan areas.
Grenoble, France held the International Exhibition of Hydropower and Tourism with over one million visitors. By 1920 as 40% of the power produced in the United States was hydroelectric, the Federal Power Act was enacted into law; the Act created the Federal Power Commission to regulate hydroelectric power stations on federal land and water. As the power stations became larger, their associated dams developed additional purposes to include flood control and navigation. Federal funding became necessary for large-scale development and federally owned corporations, such as the Tennessee Valley Authority and the Bonneville Power Administration were created. Additionally, the Bureau of Reclamation which had begun a series of western U. S. irrigation projects in the early 20th century was now constructing large hydroelectric projects such as the 1928 Hoover Dam. The U. S. Army Corps of Engineers was involved in hydroelectric development, completing the Bonneville Dam in 1937 and being recognized by the Flood Control Act of 1936 as the premier federal flood control agency.
Hydroelectric power stations continued to become larger throughout the 20th century. Hydropower was referred to as white coal for its plenty. Hoover Dam's initial 1,345 MW power station was the world's largest hydroelectric power station in 1936; the Itaipu Dam opened in 1984 in South America as the largest, producing 14,000 MW but was surpassed in 2008 by the Three Gorges Dam in China at 22,500 MW. Hydroelectricity would supply some countries, including Norway, Democratic Republic of the Congo and Brazil, with over 85% of their electricity; the United States has over 2,000 hydroelectric power stations that supply 6.4% of its total electrical production output, 49% of its renewable electricity. The technical potential for hydropower development around the world is much greater than the actual production: the percent of potential hydropower capacity that has not been developed is 71% in Europe, 75% in North America, 79% in South America, 95% in Africa, 95% in the Middle East, 82% in Asia-Pacific.
The political realities of new reservoirs in western countries, economic limitations in the third world and the lack of a transmission system in undeveloped areas result in the possibility of developing 25% of the remaining technically exploitable potential before 2050, with the bulk of that being in the Asia-Pacific area. Some countries have developed their hydropower potential and have little room for growth: Switzerland produces 88% of its potential and Mexico 80%. Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator; the power extracted from the water depends on the volume and on the difference in height between the source and the water's outflow. This height difference is called the head. A large pipe delivers water from the reservoir to the turbine; this method produces electricity to supply high peak demands by moving water between reservoirs at different elevations. At times of low electrical demand, the excess generation capacity is used to pump water into the higher reservoir.
When the demand becomes greater, water is released back into the lower reservoir through a turbine. Pumped-storage schemes provide the most commercially important means of large-scale grid energy storage and improve the daily capacity factor of the generation system. Pumped storag
The Czechs or the Czech people, are a West Slavic ethnic group and a nation native to the Czech Republic in Central Europe, who share a common ancestry, culture and Czech language. Ethnic Czechs were called Bohemians in English until the early 20th century, referring to the medieval land of Bohemia which in turn was adapted from late Iron Age tribe of Celtic Boii. During the Migration Period, West Slavic tribes of Bohemians settled in the area, "assimilated the remaining Celtic and Germanic populations", formed a principality in the 9th century, part of Great Moravia, in form of Duchy of Bohemia and Kingdom of Bohemia, the predecessors of the modern republic; the Czech diaspora is found in notable numbers in the United States, Israel, Germany, Switzerland, the United Kingdom, Russia and Brazil, among others. The Czech ethnic group is part of the West Slavic subgroup of the larger Slavic ethno-linguistical group; the West Slavs have their origin in early Slavic tribes which settled in Central Europe after East Germanic tribes had left this area during the migration period.
The West Slavic tribe of Bohemians settled in the area of Bohemia during the migration period, assimilated the remaining Celtic and Germanic populations. They formed a principality in the 9th century, the Duchy of Bohemia, under the Přemyslid dynasty, part of the Great Moravia under Svatopluk I. According to mythology, the founding father of the Czech people were Forefather Čech, who according to legend brought the tribe of Czechs into its land; the Czech are related to the neighbouring Slovaks. The Czech–Slovak languages form a dialect continuum rather than being two distinct languages. Czech cultural influence in Slovak culture is noted as having been much higher than the other way around. Czech people have a long history of coexistence with Germanic people. In the 17th century, German replaced Czech in local administration; the Czech National Revival took place in the 18th and 19th centuries aiming to revive Czech language and national identity. The Czech were the initiators of Pan-Slavism; the Czech ethnonym was the name of a Slavic tribe in central Bohemia that subdued the surrounding tribes in the late 9th century and created the Czech/Bohemian state.
The origin of the name of the tribe itself is unknown. According to legend, it comes from their leader Čech. Research regards Čech as a derivative of the root čel-; the Czech ethnonym was adopted by the Moravians in the 19th century. The population of the Czech lands has been influenced by different human migrations that wide-crossed Europe over time. In their Y-DNA haplogroups, which are inherited along the male line, Czechs have shown a mix of Eastern and Western European traits. According to a 2007 study, 34.2% of Czech males belong to R1a. Within the Czech Republic, the proportion of R1a seems to increase from west to east According to a 2000 study, 35.6% of Czech males have haplogroup R1b, common in Western Europe among Germanic and Celtic nations, but rare among Slavic nations. A mtDNA study of 179 individuals from Western Bohemia showed that 3% had East Eurasian lineages that entered the gene pool through admixture with Central Asian nomadic tribes in the early Middle Ages. A group of scientists suggested that the high frequency of a gene mutation causing cystic fibrosis in Central European and Celtic populations supports the theory of some Celtic ancestry among the Czech population.
The population of the Czech Republic descends from diverse peoples of Slavic and Germanic origin. Presence of West Slavs in the 6th century during the Migration Period has been documented on the Czech territory. Slavs settled in Bohemia and Austria sometime during the 6th or 7th centuries, "assimilated the remaining Celtic and Germanic populations". According to a popular myth, the Slavs came with Forefather Čech. During the 7th century, the Frankish merchant Samo, supporting the Slavs fighting against nearby settled Avars, became the ruler of the first known Slav state in Central Europe, the Samo's Empire; the principality Great Moravia, controlled by the Moymir dynasty, arose in the 8th century and reached its zenith in the 9th when it held off the influence of the Franks. Great Moravia was Christianized, the crucial role played Byzantine mission of Methodius; the Duchy of Bohemia emerged in the late 9th century. In 880, Prague Castle was constructed by Prince Bořivoj, founder of the Přemyslid dynasty and the city of Prague was established.
Vratislav II was the first Czech king in 1085 and the duchy was raised to a hereditary kingdom under Ottokar I in 1198. The second half of the 13th century was a period of advancing German immigration into the Czech lands; the number of Czechs who have at least German ancestry today runs into hundreds of thousands. The Habsburg Monarchy focused much of its power on religious wars against the Protestants. While these religious wars were taking place, the Czech estates revolted against Habsburg from 1546 to 1547 but were defeated. Defenestrations of Prague in 1618, signaled an open revolt by the Bohemian estates against the Habsburgs and started the Thirty Years' War. After the Battle
In the Roman Republic and the Roman Empire, the Latin word castrum was a building, or plot of land, used as a fortified military camp. Castrum was the term used for different sizes of camps including a large legionary fortress, smaller auxiliary forts, temporary encampments, "marching" forts; the diminutive form castellum was used for fortlets occupied by a detachment of a cohort or a century. In English, the terms Roman fortress, Roman fort, Roman camp are used for castrum. However, scholastic convention tends toward the use of the words camp, marching camp, fortress as a translation of castrum. For a list of known castra see List of castra. Castrum appears in Oscan and Umbrian, two other Italic languages, suggests an origin at least as old as Proto-Italic language. Julius Pokorny traces a probable derivation from * k̂es -, schneiden in * k̂es - tro-m; these Italic reflexes based on * kastrom include Umbrian castruo, kastruvuf. They have the same meaning, says Pokorny, as Latin fundus, an estate, or tract of land.
This is not any land, but is a prepared or cultivated tract, such as a farm enclosed by a fence or a wooden or stone wall of some kind. Cornelius Nepos uses Latin castrum in that sense: when Alcibiades deserts to the Persians, Pharnabazus gives him an estate worth 500 talents in tax revenues; this is a change of meaning from the reflexes in other languages, which still mean some sort of knife, axe, or spear. Pokorny explains it as ’Lager’ als ‘abgeschnittenes Stück Land’, “a lager, as a cut-off piece of land.” If this is the civilian interpretation, the military version must be “military reservation,” a piece of land cut off from the common land around it and modified for military use. All castra must be defended by works no more than a stockade, for which the soldiers carried stakes, a ditch; the castra could be prepared under attack behind a battle line. Considering that the earliest military shelters were tents made of hide or cloth, all but the most permanent bases housed the men in tents placed in quadrangles and separated by numbered streets, one castrum may well have acquired the connotation of tent.
The commonest Latin syntagmata for the term castra are: castra stativa Permanent camp/fortresses castra aestiva Summer camp/fortresses castra hiberna Winter camp/fortresses castra navalia or castra nautica Navy camp/fortressesIn Latin the term castrum is much more used as a proper name for geographical locations: e.g. Castrum Album, Castrum Inui, Castrum Novum, Castrum Truentinum, Castrum Vergium; the plural was used as a place name, as Castra Cornelia, from this come the Welsh place name prefix caer- and English suffixes -caster and -chester. Castrorum Filius, "son of the camps," was one of the names used by the emperor Caligula and also by other emperors. Castro derived from Castrum, is a common Spanish family name as well as toponym in Italy, the Balkans and Spain and other Hispanophone countries, either by itself or in various compounds such as the World Heritage Site of Gjirokastër; the terms stratopedon and phrourion were used by Greek language authors to translate castrum and castellum, respectively.
A castrum was designed to house and protect the soldiers, their equipment and supplies when they were not fighting or marching. This most detailed description that survives about Roman military camps is De Munitionibus Castrorum, a manuscript of 11 pages that dates most from the late 1st to early 2nd century AD. Regulations required a major unit in the field to retire to a properly constructed camp every day. "… as soon as they have marched into an enemy's land, they do not begin to fight until they have walled their camp about. To this end a marching column ported the equipment needed to build and stock the camp in a baggage train of wagons and on the backs of the soldiers. Camps were the responsibility of engineering units to which specialists of many types belonged, officered by architecti, "chief engineers", who requisitioned manual labor from the soldiers at large as required, they could throw up a camp under enemy attack in as little as a few hours. Judging from the names, they used a repertory of camp plans, selecting the one appropriate to the length of time a legion would spend in it: tertia castra, quarta castra, etc..
More permanent camps were castra stativa. The least permanent of these were castra aestiva or aestivalia, "summer camps", in which the soldiers were housed sub pellibus or sub tentoriis, "under tents". Summer was the campaign season. For the winter the soldiers retired to castra hiberna containing barracks and other buildings of more solid materials, with timber construction being replaced by stone; the camp supplied army in the field. Neither the Celtic nor Germanic armies had this capability: they found it necessary to disperse after only a few days; the largest castra were legionary fortresses built as bases for one or more whole legions. From the time of Augustus more permanent castra with wooden or stone buildings and walls were introduced as the distant and hard-won boundaries of the expanding empire required permanent garrisons to control local and external threats
Holy Crown of Hungary
The Holy Crown of Hungary known as the Crown of Saint Stephen, was the coronation crown used by the Kingdom of Hungary for most of its existence. The Crown was bound to the Lands of the Hungarian Crown. No king of Hungary was regarded as having been legitimate without being crowned with it. In the history of Hungary, more than fifty kings were crowned with it, up to the last, Charles IV, in 1916; the enamels on the crown are or Byzantine work, presumed to have been made in Constantinople in the 1070s. The crown was presented by the Byzantine Emperor Michael VII Doukas to King Géza I of Hungary, it is one of the two known Byzantine crowns to survive, the other being the earlier Monomachus Crown, in Budapest, in the Hungarian National Museum. However, the Monomachus Crown may have had another function, the Holy Crown has been remodelled, uses elements of different origins; the date assigned to the present configuration of the Holy Crown varies, but is most put around the late 12th century. The Hungarian coronation insignia consists of the Holy Crown, the sceptre, the orb, the mantle.
The orb has the coat-of-arms of Charles I of Hungary. In popular tradition the Holy Crown was thought to be older, dating to the time of the first King Stephen I of Hungary, crowned in 1000/1001, it was first called the Holy Crown in 1256. During the 14th century, royal power came to be represented not by a crown, but by just one specific object: the Holy Crown; this meant that the Kingdom of Hungary was a special state: they were not looking for a crown to inaugurate a king, but rather, they were looking for a king for the crown. He depicts that "the Holy Crown is for the Hungarians what the Lost Ark is for the Jewish people". Since 2000, the Holy Crown has been on display in the central Domed Hall of the Hungarian Parliament Building; the Crown's shape is larger than a human head. During coronations, the king had to wear a leather'kapa' liner, made to fit, inside the crown; the weight of the Crown is 2056 g. The gold-silver alloys in the upper and the lower parts of the Crown differ in alloy ratio.
The lower part of the Crown is asymmetric. As is the case with all European Christian crowns, it symbolizes a halo and thus signifies that the wearer rules by Divine Right. According to popular tradition, St Stephen I held up the crown before his death to offer it to the Virgin Mary to seal a divine contract between her and the divine crown. After this, Mary was depicted not only as patrona for the Kingdom of Hungary but as regina; this contract was supposed to empower the crown with divine force to help the future kings of Hungary and did help reinforce the political system based on the so-called "Doctrine of the Holy Crown". Péter Révay, a Crown Guard, expounded this doctrine in his works Commentarius De Sacra Regni Hungariae Corona and De monarchia et Sacra Corona Regni Hungariae. At the core of this doctrine was the notion that the crown itself had personhood and as a legal entity is identical to the state of Hungary, it is superior to the ruling monarch, who rules "in the name of the crown".
According to the most accepted theory, represented in the publications of the Hungarian Academy of Sciences and of the Hungarian Catholic Episcopal Conference, the Holy Crown of Hungary consists of two main parts: "abroncs" the corona graeca, "keresztpántok" the corona latina. It was created during the reign of Béla III under Byzantine influence; the crowning of Stephen I, the first king of Hungary, canonized Saint Stephen, marks the beginning of Hungarian statehood. The date is variously given as Christmas 1000 or 1 January 1001. One version of the origin of the crown is written by bishop Hartvik, in which the "Pope" has sent King Stephen I "his blessings and a crown"; the basis for this belief is a biography by bishop Hartvik written around 1100-1110 at the request of King Könyves Kálmán. According to "Hartvik’s legend", St Stephen sent Archbishop Astrik of Esztergom to Rome to ask for or require a crown from the "Pope", but it does not tell the name of the Pope. No matter how much Astrik hurried, the Polish prince, Mieszko I's envoy was quicker, the crown was prepared for the future Polish king.
The Pope had seen a dream during the night, seeing the angel of the Lord telling him there will be another envoy from another nation, asking for a crown for their own king. The angel told the Pope: "There will be another envoy from an unknown folk, who will ask for a crown, please give the crown to them, as they deserve it"; the next day Astrik approached the Pope. "Hartvik’s legend" appeared in the liturgical books and breviaries in Hungary around 1200, recalling the then-existing Pope, Pope Sylvester II. The story of how the crown had been sent by Pope Sylvester II spread throughout the
The Danube is Europe's second longest river, after the Volga. It is located in Eastern Europe; the Danube was once a long-standing frontier of the Roman Empire, today flows through 10 countries, more than any other river in the world. Originating in Germany, the Danube flows southeast for 2,850 km, passing through or bordering Austria, Hungary, Serbia, Bulgaria and Ukraine before draining into the Black Sea, its drainage basin extends into nine more countries. The Danube river basin is home to fish species such as pike, huchen, Wels catfish and tench, it is home to a large diversity of carp and sturgeon, as well as salmon and trout. A few species of euryhaline fish, such as European seabass and eel, inhabit the Danube Delta and the lower portion of the river. Since ancient times, the Danube has become a traditional trade route in Europe, nowadays 2,415 km of its total length being navigable; the river is an important source of energy and drinking water. Danube is an Old European river name derived from a Proto-Indo-European *dānu.
Other river names from the same root include the Dunaj, Dzvina/Daugava, Donets, Dniestr, Dysna and Tuoni. In Rigvedic Sanskrit, dānu means "fluid, drop", in Avestan, the same word means "river". In the Rigveda, Dānu once appears as the mother of Vrtra, "a dragon blocking the course of the rivers"; the Finnish word for Danube is Tonava, most derived from the word for the river in Swedish and German, Donau. Its Sámi name Deatnu means "Great River", it is possible that dānu in Scythian as in Avestan was a generic word for "river": Dnieper and Dniestr, from Danapris and Danastius, are presumed to continue Scythian *dānu apara "far river" and *dānu nazdya- "near river", respectively. The river was known to the ancient Greeks as the Istros a borrowing from a Daco-Thracian name meaning "strong, swift", from a root also encountered in the ancient name of the Dniester and akin to Iranic turos “swift” and Sanskrit iṣiras "swift", from the PIE *isro-, *sreu “to flow”. In the Middle Ages, the Greek Tiras was borrowed into Italian as Tyrlo and into Turkic languages as Tyrla, the latter further borrowed into Romanian as a regionalism.
The Thraco-Phrygian name was Matoas, "the bringer of luck". In Latin, the Danube was variously known as Ister; the Latin name is masculine, except Slovenian. The German Donau is feminine, as it has been re-interpreted as containing the suffix -ouwe "wetland". Romanian differs from other surrounding languages in designating the river with a feminine term, Dunărea; this form was not inherited from Latin. To explain the loss of the Latin name, scholars who suppose that Romanian developed near the large river propose that the Romanian name descends from a hypotetical Thracian *Donaris that shares the same PIE root with the Iranic don-/dan-, with the suffix -aris encountered in the ancient name of the Ialomița River, in the unidentified Miliare river mentioned by Jordanes in his Getica. Gábor Vékony says that this hypothesis is not plausible, because the Greeks borrowed the Istros form from the native Thracians, he proposes. The modern languages spoken in the Danube basin all use names related to Dānuvius: German: Donau.
Dunav. Dunai. Classified as an international waterway, it originates in the town of Donaueschingen, in the Black Forest of Germany, at the confluence of the rivers Brigach and Breg; the Danube flows southeast for about 2,730 km, passing through four capital cities before emptying into the Black Sea via the Danube Delta in Romania and Ukraine. Once a long-standing frontier of the Roman Empire, the river passes through or touches the borders of 10 countries: Romania, Serbia, Germany, Slovakia, Croatia and Moldova, its drainage basin extends into nine more. In addition to the bordering countries, the drainage basin includes parts of nine more countries: Bosnia and Herzegovina, the Czech Republic, Montenegro, Italy, North Macedonia and Albania, its total drainage basin is 801,463 km2. The highest point of the drainage basin is the summit of Piz Bernina at the Italy–Switzerland border, at 4,049 metres; the land drained by the Danube extends into many other countries. Many Danubian tributaries are important rivers in their own right, navigable by barges and other shallow-draught boats.
From its source to its outlet into the Black Sea, its main tribu
World War I
World War I known as the First World War or the Great War, was a global war originating in Europe that lasted from 28 July 1914 to 11 November 1918. Contemporaneously described as "the war to end all wars", it led to the mobilisation of more than 70 million military personnel, including 60 million Europeans, making it one of the largest wars in history, it is one of the deadliest conflicts in history, with an estimated nine million combatants and seven million civilian deaths as a direct result of the war, while resulting genocides and the 1918 influenza pandemic caused another 50 to 100 million deaths worldwide. On 28 June 1914, Gavrilo Princip, a Bosnian Serb Yugoslav nationalist, assassinated the Austro-Hungarian heir Archduke Franz Ferdinand in Sarajevo, leading to the July Crisis. In response, on 23 July Austria-Hungary issued an ultimatum to Serbia. Serbia's reply failed to satisfy the Austrians, the two moved to a war footing. A network of interlocking alliances enlarged the crisis from a bilateral issue in the Balkans to one involving most of Europe.
By July 1914, the great powers of Europe were divided into two coalitions: the Triple Entente—consisting of France and Britain—and the Triple Alliance of Germany, Austria-Hungary and Italy. Russia felt it necessary to back Serbia and, after Austria-Hungary shelled the Serbian capital of Belgrade on the 28th, partial mobilisation was approved. General Russian mobilisation was announced on the evening of 30 July; when Russia failed to comply, Germany declared war on 1 August in support of Austria-Hungary, with Austria-Hungary following suit on 6th. German strategy for a war on two fronts against France and Russia was to concentrate the bulk of its army in the West to defeat France within four weeks shift forces to the East before Russia could mobilise. On 2 August, Germany demanded free passage through Belgium, an essential element in achieving a quick victory over France; when this was refused, German forces invaded Belgium on 3 August and declared war on France the same day. On 12 August and France declared war on Austria-Hungary.
In November 1914, the Ottoman Empire entered the war on the side of the Alliance, opening fronts in the Caucasus and the Sinai Peninsula. The war was fought in and drew upon each power's colonial empire as well, spreading the conflict to Africa and across the globe; the Entente and its allies would become known as the Allied Powers, while the grouping of Austria-Hungary and their allies would become known as the Central Powers. The German advance into France was halted at the Battle of the Marne and by the end of 1914, the Western Front settled into a battle of attrition, marked by a long series of trench lines that changed little until 1917. In 1915, Italy opened a front in the Alps. Bulgaria joined the Central Powers in 1915 and Greece joined the Allies in 1917, expanding the war in the Balkans; the United States remained neutral, although by doing nothing to prevent the Allies from procuring American supplies whilst the Allied blockade prevented the Germans from doing the same the U. S. became an important supplier of war material to the Allies.
After the sinking of American merchant ships by German submarines, the revelation that the Germans were trying to incite Mexico to make war on the United States, the U. S. declared war on Germany on 6 April 1917. Trained American forces would not begin arriving at the front in large numbers until mid-1918, but the American Expeditionary Force would reach some two million troops. Though Serbia was defeated in 1915, Romania joined the Allied Powers in 1916 only to be defeated in 1917, none of the great powers were knocked out of the war until 1918; the 1917 February Revolution in Russia replaced the Tsarist autocracy with the Provisional Government, but continuing discontent at the cost of the war led to the October Revolution, the creation of the Soviet Socialist Republic, the signing of the Treaty of Brest-Litovsk by the new government in March 1918, ending Russia's involvement in the war. This allowed the transfer of large numbers of German troops from the East to the Western Front, resulting in the German March 1918 Offensive.
This offensive was successful, but the Allies rallied and drove the Germans back in their Hundred Days Offensive. Bulgaria was the first Central Power to sign an armistice—the Armistice of Salonica on 29 September 1918. On 30 October, the Ottoman Empire capitulated. On 4 November, the Austro-Hungarian empire agreed to the Armistice of Villa Giusti after being decisively defeated by Italy in the Battle of Vittorio Veneto. With its allies defeated, revolution at home, the military no longer willing to fight, Kaiser Wilhelm abdicated on 9 November and Germany signed an armistice on 11 November 1918. World War I was a significant turning point in the political, cultural and social climate of the world; the war and its immediate aftermath sparked numerous uprisings. The Big Four (Britain, the United States, It
Chromium is a chemical element with symbol Cr and atomic number 24. It is the first element in group 6, it is a steely-grey, lustrous and brittle transition metal. Chromium boasts a high usage rate as a metal, able to be polished while resisting tarnishing. Chromium is the main additive in stainless steel, a popular steel alloy due to its uncommonly high specular reflection. Simple polished chromium reflects 70% of the visible spectrum, with 90% of infrared light being reflected; the name of the element is derived from the Greek word χρῶμα, chrōma, meaning color, because many chromium compounds are intensely colored. Ferrochromium alloy is commercially produced from chromite by silicothermic or aluminothermic reactions and chromium metal by roasting and leaching processes followed by reduction with carbon and aluminium. Chromium metal is of high value for hardness. A major development in steel production was the discovery that steel could be made resistant to corrosion and discoloration by adding metallic chromium to form stainless steel.
Stainless steel and chrome plating together comprise 85% of the commercial use. In the United States, trivalent chromium ion is considered an essential nutrient in humans for insulin and lipid metabolism. However, in 2014, the European Food Safety Authority, acting for the European Union, concluded that there was not sufficient evidence for chromium to be recognized as essential. While chromium metal and Cr ions are not considered toxic, hexavalent chromium is both toxic and carcinogenic. Abandoned chromium production sites require environmental cleanup. Chromium is the fourth transition metal found on the periodic table, has an electron configuration of 3d5 4s1, it is the first element in the periodic table whose ground-state electron configuration violates the Aufbau principle. This occurs again in the periodic table with other elements and their electron configurations, such as copper and molybdenum; this occurs. In the previous elements, the energetic cost of promoting an electron to the next higher energy level is too great to compensate for that released by lessening inter-electronic repulsion.
However, in the 3d transition metals, the energy gap between the 3d and the next-higher 4s subshell is small, because the 3d subshell is more compact than the 4s subshell, inter-electron repulsion is smaller between 4s electrons than between 3d electrons. This lowers the energetic cost of promotion and increases the energy released by it, so that the promotion becomes energetically feasible and one or two electrons are always promoted to the 4s subshell. Chromium is the first element in the 3d series where the 3d electrons start to sink into the inert core. Chromium is a strong oxidising agent in contrast to the tungsten oxides. Chromium is hard, is the third hardest element behind carbon and boron, its Mohs hardness is 8.5, which means that it can scratch samples of quartz and topaz, but can be scratched by corundum. Chromium is resistant to tarnishing, which makes it useful as a metal that preserves its outermost layer from corroding, unlike other metals such as copper and aluminium. Chromium has a melting point of 1907 °C, low compared to the majority of transition metals.
However, it still has the second highest melting point out of all the Period 4 elements, being topped by vanadium by 3 °C at 1910 °C. The boiling point of 2671 °C, however, is comparatively lower, having the third lowest boiling point out of the Period 4 transition metals alone behind manganese and zinc. Chromium has an unusually high specular reflection in comparison to that of other transition metals. At 425 μm, chromium was found to have a relative maximum reflection of about 72% reflectance, before entering a depression in reflectivity, reaching a minimum of 62% reflectance at 750 μm before rising again to reflecting 90% of 4000 μm of infrared waves.. When chromium is formed into a stainless steel alloy and polished, the specular reflection decreases with the inclusion of additional metals, yet is still rather high in comparison with other alloys. Between 40% and 60% of the visible spectrum is reflected from polished stainless steel; the explanation on why chromium displays such a high turnout of reflected photon waves in general the 90% of infrared waves that were reflected, can be attributed to chromium's magnetic properties.
Chromium has unique magnetic properties in the sense that chromium is the only elemental solid which shows antiferromagnetic ordering at room temperature. Above 38 °C, its magnetic ordering changes to paramagnetic.. The antiferromagnetic properties, which cause the chromium atoms to temporarily ionize and bond with themselves, are present because the body-centric cubic's magnetic properties are disproportionate to the lattice periodicity; this is due to the fact that the magnetic moments at the cube's corners and the cube centers are not equal, but are still antiparallel. From here, the frequency-dependent relative permittivity of chromium, deriving from Maxwell's equations in conjunction with chromium's antiferromagnetivity, leaves chromium with a high infrared and visible light reflectance. Chromium metal left standing in air is passivated by oxidation, forming a th