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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]



The Niagara Falls are voluminous waterfalls on the Niagara River, straddling the international border between the Canadian province of Ontario and the U.S. state of New York. The falls are 17 miles north-northwest of Buffalo, New York and 75 miles south-southeast of Toronto, Ontario, between the twin cities of Niagara Falls, Ontario, and Niagara Falls, New York. Niagara Falls is composed of two major sections separated by Goat Island: Horseshoe Falls, the majority of which lies on the Canadian side of the border, and American Falls on the American side. The smaller Bridal Veil Falls are also located on the American side, separated from the main falls by Luna Island. Niagara Falls were formed when glaciers receded at the end of the Wisconsin glaciation (the last ice age), and water from the newly formed Great Lakes carved a path through the Niagara Escarpment en route to the Atlantic Ocean. Niagara Falls is divided into the Horseshoe Falls and the American Falls. The Horseshoe Falls drop about 173 feet, the height of the American Falls varies between 70–100 feet because of the presence of giant boulders at its base. The larger Horseshoe Falls are about 2,600 feet wide, while the American Falls are 1,060 feet wide. The volume of water approaching the falls during peak flow season may sometimes be as much as 202,000 cubic feet per second. Since the flow is a direct function of the Lake Erie water elevation, it typically peaks in late spring or early summer. During the summer months, 100,000 cubic feet per second of water actually traverses the Falls, some 90% of which goes over the Horseshoe Falls, while the balance is diverted to hydroelectric facilities. This is accomplished by employing a weir with movable gates upstream from the Horseshoe Falls. The Falls flow is further halved at night, and during the low tourist season in the winter, remains a flat 50,000 cubic feet per second. Water diversion is regulated by the 1950 Niagara Treaty and is administered by the International Niagara Board of Control (IJC). Viewpoints on the American shore generally are astride or behind the falls. The falls face directly toward the Canadian shore. It is the most powerful waterfall in North America. The Niagara Falls are renowned both for their beauty and as a valuable source of hydroelectric power. Managing the balance between recreational, commercial, and industrial uses has been a challenge for the stewards of the falls since the 1800s. In October 1829, Sam Patch, who called himself "the Yankee Leapster", jumped from a high tower into the gorge below the falls and survived; this began a long tradition of daredevils trying to go over the Falls. [READ THE REST OF THIS ARTICLE]



Castle Bravo was the code name given to the first U.S. test of a dry fuel thermonuclear hydrogen bomb device, detonated on March 1, 1954, at Bikini Atoll, Marshall Islands, by the United States, as the first test of Operation Castle (a longer series of tests of various devices). Fallout from the detonation—intended to be a secret test—poisoned the islanders who inhabited the test site, as well as the crew of Daigo Fukuryu Maru ("Lucky Dragon No. 5"), a Japanese fishing boat, and created international concern about atmospheric thermonuclear testing. The bomb used lithium deuteride fuel for the fusion stage, unlike the cryogenic liquid deuterium–tritium used as fuel for the fusion stage of the U.S. experimental Ivy Mike device, which, being the size of a small office building, was an impractical weapon for use at war. The bomb tested at Castle Bravo was the first practical deliverable fusion bomb in the U.S. arsenal. The Soviet Union had previously used lithium deuteride in a nuclear bomb, their Sloika design (known as the "Alarm Clock" in the U.S.), in 1953. It was not a "true" hydrogen bomb, deriving most of its yield from boosted fission reactions, and was limited to a yield of 400 kt. By comparison, the Teller-Ulam-based Ivy Mike device had a yield of 10.4 MT, and Castle Bravo had an even larger yield of 15 MT. In the Teller-Ulam design, the fission and fusion stages were kept physically separate in a reflective cavity. The radiation from the exploding fission primary compressed the fusion secondary to high densities, which could then undergo thermonuclear reactions after being heated by the exploding fission weapon. The Soviets, led by Andrei Sakharov, independently developed and tested their first Teller-Ulam device in 1955. Castle Bravo was the most powerful nuclear device ever detonated by the United States, with a yield of 15 Megatons. That yield, far exceeding the expected yield of 4 to 6 megatons, combined with other factors, led to the most significant accidental radiological contamination ever caused by the United States. [READ THE REST OF THIS ARTICLE]



The Boeing 747-8 is a wide-body commercial jet airliner being developed by Boeing Commercial Airplanes. Officially announced in 2005, the 747-8 is the fourth-generation Boeing 747 version, with lengthened fuselage, redesigned wings and improved efficiency. The 747-8 is the largest 747 version, the largest commercial aircraft built in the United States, and the longest passenger aircraft in the world. Boeing had considered larger-capacity versions of the 747 several times during the 1990s and 2000s. The 747-500X and -600X, proposed at the 1996 Farnborough Airshow, would have stretched the 747 and used a 777-derived wing, but did not attract enough interest to enter development. In 2000, Boeing offered the 747X and 747X Stretch derivatives as alternatives to the Airbus A3XX. This was a more modest proposal than the previous -500X and -600X. The 747-8 is offered in two main variants: the 747-8 Intercontinental (747-8I) for passengers and the 747-8 Freighter (747-8F) for cargo. The first 747-8F performed the model's maiden flight on February 8, 2010. Delivery of the first freighter aircraft has been postponed multiple times and is now expected in mid-2011; passenger model deliveries are to begin in late 2011 or early 2012. In December 2010, orders for the 747-8 totaled 107, including 74 of the freighter version, and 33 of the passenger version. The 747-8's first engine runs were completed in December 2009. Boeing announced the new model had successfully completed high-speed taxi tests on February 7, 2010. On February 8, 2010, after a 2.5-hour weather delay, the 747-8 Freighter made its maiden flight, taking off from Paine Field, Washington at 12:39 PST. The aircraft landed at 4:18 pm PST. Boeing estimates that more than 1,600 flight hours will be needed in order to certify the 747-8. The second test flight in late February, a ferry flight to Moses Lake, Washington, tested new navigation equipment. Further flight testing will take place in Moses Lake, conducting initial airworthiness and flutter tests, before moving to Palmdale, California for the majority of flight tests, so as to not interfere with 787 flight tests based out of Boeing Field in Seattle. Assembly of the first 747-8I was completed in February 2011. It was unveiled during a rollout ceremony in Everett, Washington on February 13, 2011. Deliveries are to begin in late 2011 or early 2012. On March 7, 2011, it was announced that Air China had agreed to purchase five 747-8Is, subject to government approval, at a price of $1.54 billion. Air China said it would use the aircraft to increase international service. The U.S. Air Force is seeking to upgrade Air Force One by replacing the Boeing VC-25 (two heavily modified 747-200Bs). Boeing is reported to be exploring a 747-8 proposal, along with a Boeing 787 variant. In 2010, South Korea government sources indicated that the country was considering purchasing the 747-8 to serve as the country's presidential aircraft. [READ THE REST OF THIS ARTICLE]



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