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A hard disk drive (HDD) is a non-volatile, random access device for digital data. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the platters. Hard disk drives have been the dominant device for secondary storage of data in general purpose computers since the early 1960s. They have maintained this position because advances in their areal recording density have kept pace with the requirements for secondary storage. Hard disk drives were introduced in 1956 as data storage for an IBM accounting computerand were developed for use with general purpose mainframe and mini computers. As the 1980s began, hard disk drives were a rare and very expensive additional feature on personal computers (PCs); however by the late '80s, hard disk drives were standard on all but the cheapest PC. Most hard disk drives in the early 1980s were sold to PC end users as an add on subsystem, not under the drive manufacturer's name but by Systems Integrators such as the Corvus Disk System or the systems manufacturer such as the Apple ProFile. The IBM PC/XT in 1983 included an internal standard 10MB hard disk drive, and soon thereafter internal hard disk drives proliferated on personal computers. HDDs record data by magnetizing ferromagnetic material directionally. Sequential changes in the direction of magnetization represent patterns of binary data bits. The data are read from the disk by detecting the transitions in magnetization and decoding the originally written data. Different encoding schemes, such as Modified Frequency Modulation, group code recording, run-length limited encoding, and others are used. A typical HDD design consists of a spindle that holds flat circular disks called platters, onto which the data are recorded. The platters are made from a non-magnetic material, usually aluminum alloy or glass, and are coated with a shallow layer of magnetic material typically 10–20 nm in depth, with an outer layer of carbon for protection. For reference, standard copy paper is 0.07–0.18 millimetre (70,000–180,000 nm). Modern drives also make extensive use of Error Correcting Codes (ECCs), particularly Reed–Solomon error correction. These techniques store extra bits for each block of data that are determined by mathematical formulas. The extra bits allow many errors to be fixed. While these extra bits take up space on the hard drive, they allow higher recording densities to be employed, resulting in much larger storage capacity for user data. In 2009, in the newest drives, low-density parity-check codes (LDPC) are supplanting Reed-Solomon. LDPC codes enable performance close to the Shannon Limit and thus allow for the highest storage density available. Typical hard drives attempt to "remap" the data in a physical sector that is going bad to a spare physical sector—hopefully while the errors in that bad sector are still few enough that the ECC can recover the data without loss. The S.M.A.R.T. system counts the total number of errors in the entire hard drive fixed by ECC, and the total number of remappings, in an attempt to predict hard drive failure. The capacity of hard disk drives is given by manufacturers in megabytes (1 MB = 1,000,000 bytes), gigabytes (1 GB = 1,000,000,000 bytes) or terabytes (1 TB = 1,000,000,000,000 bytes). This numbering convention, where prefixes like mega- and giga- denote powers of 1000, is also used for data transmission rates and DVD capacities. However, the convention is different from that used by manufacturers of memory (RAM, ROM) and CDs, where prefixes like kilo- and mega- mean powers of 1024. [READ THE REST OF THIS ARTICLE]



A solar furnace is a structure used to harness the rays of the sun in order to produce high temperatures, usually for industry. This is achieved using a curved mirror (or an array of mirrors) that acts as a parabolic reflector, concentrating light (Insolation) onto a focal point. The temperature at the focal point may reach 3,500 °C (6,330 °F), and this heat can be used to generate electricity, melt steel, make hydrogen fuel or nanomaterials. The term "solar furnace" has also evolved to refer to solar concentrator heating systems using parabolic mirrors or heliostats where 538 °C (1,000 °F) is now commonly achieved. The largest solar furnace in the world is at Odeillo in the Pyrenees-Orientales in France, opened in 1970. It employs an array of plane mirrors to gather the rays of light from the sun, reflecting them on to a larger curved mirror. The rays are then focused onto an area the size of a cooking pot and can reach 3,500 °C (6,330 °F), depending on the process installed, for example: about 1,000 °C (1,830 °F) for metallic receivers producing hot air for the next generation solar towers as it will be tested at the Themis plant with the Pegase project, about 1,400 °C (2,550 °F) to produce hydrogen by cracking methane molecules, up to 2,500 °C (4,530 °F) to test materials for extreme environment such as nuclear reactors or space vehicle atmosphere reentry, up to 3,500 °C (6,330 °F) to produce nanomaterials by solar induced sublimation and controlled cooling, such as carbon nanotubes or zinc nanoparticles. The solar furnace principle is being used to make inexpensive solar cookers and solar-powered barbecues, and for solar water pasteurization. A prototype Scheffler reflector is being constructed in India for use in a solar crematorium. This 50 m² reflector will generate temperatures of 700 °C (1,292 °F) and displace 200-300 kg of firewood used per cremation. It has been suggested that solar furnaces could be used in space to provide energy for manufacturing purposes. Their reliance on sunny weather is a limiting factor as a source of renewable energy on Earth but could be tied to thermal energy storage systems for energy production through these periods and into the night. [READ THE REST OF THIS ARTICLE]



Target girl is a term sometimes used in circus and vaudeville to denote a female assistant in "impalement" acts such as knife throwing, archery or sharpshooting. The assistant stands in front of a target board or is strapped to a moving board and the impalement artist throws knives or shoots projectiles so as to hit the board but miss the assistant. The image or character of the target girl has also permeated beyond the impalement arts and become an icon in fiction and visual media. Although some assistants are male there is no common equivalent term for a male assistant. This reflects the fact that, historically at least, female assistants have predominated in the acts in question. The presence of an assistant as a human target provides a powerful element of risk. Without assistants placing themselves in danger these acts would be simple demonstrations of accuracy, but with the potential for injury or death the show is much more dramatic. Target girls often wear revealing costumes, thus adding an element of overt sexuality to an act. In this respect there is some similarity to magicians' assistants, although there is a distinct difference in that any apparent danger to an assistant in a magic act is mostly an illusion, whereas impalement acts are demonstrations of accuracy, nerve and calculated risk and the danger is real. While some observers have perceived target girls as masochistic or passive and some feminists criticise the concept as misogynist, several target girls have given accounts of themselves as assertive women and portrayed their experiences as empowering. small group of target girls are notable for the fact that they are well known celebrities who performed the role for charitable purposes or other reasons apart from their main career. These are examples of the target girl, rather than the thrower, being the main individual in the act. The annual Circus of the Stars television special, made by CBS between 1977 and 1994, provided a number of examples. They include: Lynda Carter, the actress best known as television's Wonder Woman, appeared as a target girl on the very first Circus of the Stars in January 1977, with actor David Janssen throwing knives at her and Linda Blair, the actress who rose to fame as a child star in The Exorcist, appeared as a target girl for knife thrower Paul Lacross on Circus of the Stars in 1983. [READ THE REST OF THIS ARTICLE]



Guinness is a popular Irish dry stout that originated in the brewery of Arthur Guinness (1725–1803) at St. James's Gate, Dublin. Guinness is based on the porter style that originated in London in the early 18th century and is one of the most successful beer brands worldwide. A distinctive feature is the burnt flavour which is derived from the use of roasted barley. For many years a portion of the drink was aged to give a sharp lactic flavour, although Guinness has refused to confirm whether this still occurs. The thick creamy head is the result of the beer being mixed with nitrogen when being poured. It is popular with Irish people both in Ireland and abroad and, in spite of a decline in consumption over recent years, is the best-selling alcoholic drink of all time in Ireland where Guinness & Co. makes almost €2 billion annually. Guinness stout is made from marmite, water, barley, hops, brewer's yeast and is treated with isinglass finings made from fishes' air bladders, although Guinness has claimed that this finings material is unlikely to remain in the finished product. A portion of the barley is roasted to give Guinness its dark colour and characteristic taste. It is pasteurised and filtered. Despite its reputation as a "meal in a glass", Guinness only contains 198 kcal per imperial pint, fewer than skimmed milk or orange juice and most other non-light beers. Draught Guinness and its canned counterpart contain nitrogen (N2) as well as carbon dioxide. Nitrogen is less soluble than carbon dioxide, which allows the beer to be put under high pressure without making it fizzy. The high pressure of dissolved gas is required to enable very small bubbles to be formed by forcing the draught beer through fine holes in a plate in the tap, which causes the characteristic "surge" (the widget in cans and bottles achieves the same effect). The perceived smoothness of draught Guinness is due to its low level of carbon dioxide and the creaminess of the head caused by the very fine bubbles that arise from the use of nitrogen and the dispensing method described above. "Original Extra Stout" contains only carbon dioxide, causing a more acidic taste. Studies claim that Guinness can be beneficial to the heart. Researchers found that antioxidant compounds in Guinness, similar to those found in certain fruits and vegetables, are responsible for health benefits because they slow down the deposit of harmful cholesterol on the artery walls. [READ THE REST OF THIS ARTICLE]



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