1.
Baggage
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Baggage or luggage consists of bags, cases, and containers which hold a travellers articles during transit. The modern traveller can be expected to have packages containing clothing, toiletries, small possessions, trip necessities, for some people, luggage and the style thereof is representative of the owners wealth. Baggage, or baggage train, can refer to the train of people and goods. Historically the most common types of luggage were chests or trunks made of wood or other heavy materials and these would be shipped by professional movers. Since the Second World War smaller and more lightweight suitcases and bags that can be carried by an individual have become the form of luggage. According to the Oxford English Dictionary, the word comes from Old French bagage or from bagues. It may also be related to the word bag, trunk - A wooden box, generally much larger than other kinds of luggage. Trunks come in sizes as in the case of footlockers. These days trunks are more used for storage than transportation. Items large enough to require a trunk are now shipped in transport cases. Some of the better known trunk makers are Louis Vuitton, Goyard, Moynat, M. M. Secor, Suitcase - A general term that may refer to wheeled or non-wheeled luggage, as well as soft or hard side luggage. Garment bag - A style of luggage that folds over on itself to allow long garments such as suits or dresses to be packed flat to avoid creasing, carpet bag - travel luggage traditionally made from carpets. Since 2003 most locks integrated into luggage use the TSA Lock standard developed by Travel Sentry to allow opening by the US Transportation Security Administration, expandable Luggage - suitcases that can be unzipped to expand for more packing space. Rolling suitcases were invented in 1970, when Bernard D. Sadow applied for a patent that was granted in 1972 as United States patent 3,653,474 for Rolling Luggage. The patent application cited the increase in air travel, and baggage handling become perhaps the single biggest difficulty encountered by an air passenger, Sadow attributes the late invention of luggage on wheels to a macho thing where men would not accept suitcases with wheels. Others attribute the invention to the abundance of luggage porters with carts in the 1960s, the ease of curbside drop-offs at much smaller airports. Some vehicles have a specifically for luggage to be held. Items stored in the hold are known as hold luggage, a typical example would be a suitcase
2.
Cart
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A cart is a vehicle designed for transport, using two wheels and normally pulled by one or a pair of draught animals. A handcart is pulled or pushed by one or more people and it is different from a dray or wagon, which is a heavy transport vehicle with four wheels and typically two or more horses, or a carriage, which is used exclusively for transporting humans. Over time, the cart has come to mean nearly any small conveyance, from shopping carts to golf carts, without regard to number of wheels, load carried. The draught animals used for carts may be horses or ponies, mules, oxen, water buffalo or donkeys, carts have been mentioned in literature as far back as the second millennium B. C. The Indian sacred book Rigveda states that men and women are as equal as two wheels of a cart, hand-carts pushed by humans have been used around the world. In the 19th century, for instance, some Mormons travelling across the plains of the United States between 1856 and 1860 used handcarts, the history of the cart is closely tied to the history of the wheel. Larger carts may be drawn by animals, such as horses, mules and they have been in continuous use since the invention of the wheel, in the 4th millennium BC. Carts may be named for the animal that pulls them, such as horsecart or oxcart, in modern times, horsecarts are used in competition while draft horse showing. A dogcart, however, is usually a cart designed to carry hunting dogs, the term cart is also used for various kinds of lightweight, two-wheeled carriages, some of them sprung carts, especially those used as open pleasure or sporting vehicles. They could be drawn by a horse, pony or dog, the name survives today as a milkfloat. The seat is adjustable fore-and-aft to keep the vehicle balanced for two or four people, carts have many different shapes, but the basic idea of transporting material remains. Carts may have a pair of shafts, one each side of the draught animal that supports the forward-balanced load in the cart. The shafts are supported by a saddle on the horse, alternatively, the cart may have a single pole between a pair of animals. The draught traces attach to the axle of the vehicle or to the shafts, the traces are attached to a collar, to a yoke or to a harness on dogs or other light animals. Traces are made from a range of materials depending on the load, heavy draught traces are made from iron or steel chain. Lighter traces are often leather and sometimes rope, but plaited horse-hair. The dray is often associated with the transport of barrels, particularly of beer, of the cart types not animal-drawn, perhaps the most common example today is the shopping cart, which has also come to have a metaphorical meaning in relation to online purchases. Shopping carts first made their appearance in Oklahoma City in 1937, in golf, both manual push or pull and electric golf trolleys are designed to carry a golfers bag, clubs and other equipment
3.
Dead man's switch
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A dead mans switch is a switch that is automatically operated if the human operator becomes incapacitated, such as through death, loss of consciousness, or being bodily removed from control. Originally applied to switches on a vehicle or machine, it has come to be used to describe other intangible uses like in computer software. Dead mans switches are not always used to stop machines and prevent harm and these switches can also be used as a fail-deadly. A spring-operated switch can also be used to complete a circuit when it is no longer held down and this means that a dead mans switch may be used to activate a harmful device, such as a bomb or IED. The user holds down a switch of some sort in their hand which arms the device, the device will activate when the switch is released, so that if the user is knocked out or killed while holding the switch, the bomb will detonate. The Special Weapons Emergency Separation System is an application of concept in the field of nuclear weapons. A more extreme version is Russias Dead Hand program, which allows for automatic launch of nuclear missiles should a number of conditions be met and this layout has continued to be used on some modern trams around the world. In conventional steam railroad trains, there was always a person with the engineer, the fireman. For many decades this practice continued on electric and diesel locomotives, with modern urban and suburban railway systems, the driver is typically alone in an enclosed cab. Automatic devices were already beginning to be deployed on newer installations of the New York City Subway system in the early 20th century. According to a Manhattan borough historian, there have been at least three instances where the switch was used successfully – in 1927,1940, and 2010. The status and operation of both vigilance and dead-mans switch may be recorded on the event recorder. Most dead mans switches are mounted in the handle of a vehicle or machine. Handle switches are used on modern streetcars and subway trains. Pneumatically or electrically linked dead-mans controls involve relatively simple modifications of the controller handle, if pressure is not maintained on the controller, the trains emergency brakes are applied. Typically, the handle is a horizontal bar, rotated to apply the required power for the train. Attached to the bottom of the handle is a rod that when pushed down contacts a solenoid or switch inside the control housing, the handle springs up if pressure is removed, releasing the rods contact with the internal switch, instantly cutting power and applying the brakes. Though there are ways that this type of control could conceivably fail
4.
Lawn mower
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A lawn mower is a machine utilizing one or more revolving blades to cut a grass surface to an even height. The most common source for lawn mowers is a small internal combustion engine. Smaller mowers often lack any form of propulsion, requiring power to move over a surface, walk-behind mowers are self-propelled, requiring a human only to walk behind. Larger lawn mowers are usually either self-propelled walk-behind types, or more often, are ride-on mowers, equipped so the operator can ride on the mower, a robotic lawn mower is designed to operate either entirely on its own, or less commonly by an operator by remote control. Two main styles of blades are used in lawn mowers, there are several types of mowers, each suited to a particular scale and purpose. The smallest types, unpowered push mowers, are suitable for residential lawns and gardens. Electrical or piston engine-powered push-mowers are used for residential lawns. The largest multi-gang mowers are mounted on tractors and are designed for large expanses of grass such as courses and municipal parks. The first lawn mower was invented by Edwin Budding in 1830 in Thrupp, just outside Stroud, in Gloucestershire, England. Buddings mower was designed primarily to cut the grass on sports grounds and extensive gardens, as an alternative to the scythe. Buddings first machine was 19 inches wide with a made of wrought iron. The mower was pushed from behind, cast-iron gear wheels transmitted power from the rear roller to the cutting cylinder, allowing the rear roller to drive the knives on the cutting cylinder, the ratio was 16,1. Another roller placed between the cylinder and the main or land roller could be raised or lowered to alter the height of cut. The grass clippings were hurled forward into a tray-like box and it was soon realized, however, that an extra handle was needed in front to help pull the machine along. Overall, these machines were similar to modern mowers. Two of the earliest Budding machines sold went to Regents Park Zoological Gardens in London and his machine was the catalyst for the preparation of modern-style sporting ovals, playing fields, grass courts, etc. This led to the codification of rules for many sports, including for football, lawn bowls, lawn tennis. It took ten years and further innovations to create a machine that could be drawn by animals
5.
Snow blower
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A snow blower or snow thrower is a machine for removing snow from an area where it is not wanted, such as a driveway, sidewalk, roadway, railroad track, ice rink, or runway. The misnomer snow blower is used, as the snow is moved using an auger or impeller instead of blowing it. It can use electric power, or a gasoline or diesel engine to throw snow to another location or into a truck to be hauled away. This is in contrast with the action of snow plows, which push snow to the front or side, Snow blowers can generally be divided into two classes, single-stage and two-stage. On a single-stage snow blower, the auger pulls snow into the machine, the auger contacts the ground, making single-stage snow blowers unsuitable for use on unpaved surfaces. On a two-stage snow blower, the auger pulls snow into the machine and feeds it into a high-speed impeller, two-stage snow blowers can generally handle deeper snow depths than single-stage ones, and because their augers dont touch the ground, they can be used on unpaved surfaces. Robert Carr Harris of Dalhousie, New Brunswick patented a Railway Screw Snow Excavator in 1870, in 1923, Robert E. Cole patented a snowplow that operated by using cutters and a fan to blow snow from a surface. However, it is Arthur Sicard who is credited as the inventor of the first practical snow blower. In 1925 Sicard completed his first prototype, based on a concept he described in 1894 and he founded Sicard Industries in Sainte-Thérèse, Quebec and by 1927 his vehicles were in use removing snow from the roadways of the town of Outremont, now a borough of Montreal. His company is now a division of SMI-Snowblast, Inc. of Watertown, consumer Product Safety Commission estimates that each year there are approximately 5,740 snowblower related injuries in the United States which require medical attention. One problem with the design of the blower is that snow can build up in the auger, jamming it. This is complicated by the fact that the auger could deform before applying enough resistance to the motor to turn it off, the correct procedure is to turn off the engine, disengage the clutch and then clear the jam with a broom handle or other long object. In an effort to improve safety, many now include a plastic tool to be used to clear jams. Most modern machines mitigate this problem by including a safety system known as the Dead mans switch to prevent the mechanism from rotating when the operator is not at the controls and they are mandatory in some jurisdictions. Jet engines and other gas turbines are used for large scale propelling and melting of snow over rails, the jet engine both melts and blows the snow, clearing the tracks faster than other methods. While offering considerably greater power in a lightweight machine, this method is much more expensive than traditional snow removing methods. In Russia, the high cost is partially negated by utilizing retired military jet engines, such as the Klimov VK-1
6.
Railway air brake
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A railway air brake is a railway brake power braking system with compressed air as the operating medium. Modern trains rely upon a fail-safe air brake system that is based upon a design patented by George Westinghouse on March 5,1868, the Westinghouse Air Brake Company was subsequently organized to manufacture and sell Westinghouses invention. In various forms, it has nearly universally adopted. The Westinghouse system uses air pressure to charge air reservoirs on each car, full air pressure signals each car to release the brakes. A reduction or loss of air pressure signals each car to apply its brakes, in the air brakes simplest form, called the straight air system, compressed air pushes on a piston in a cylinder. The piston is connected through mechanical linkage to brake shoes that can rub on the train wheels, the mechanical linkage can become quite elaborate, as it evenly distributes force from one pressurized air cylinder to 8 or 12 wheels. The pressurized air comes from an air compressor in the locomotive and is sent from car to car by a line made up of pipes beneath each car. The principal problem with the air braking system is that any separation between hoses and pipes causes loss of air pressure and hence the loss of the force applying the brakes. This could easily cause a runaway train, straight air brakes are still used on locomotives, although as a dual circuit system, usually with each bogie having its own circuit. Unlike the straight air system, the Westinghouse system uses a reduction in air pressure in the line to apply the brakes. The triple valve is described as being so named as it performs three functions, Charging air into an air tank ready to be used, applying the brakes, in so doing, it supports certain other actions. When he soon improved the device by removing the poppet valve action, if the pressure in the train line is lower than that of the reservoir, the brake cylinder exhaust portal is closed and air from the cars reservoir is fed into the brake cylinder. Pressure increases in the cylinder, applying the brakes, while decreasing in the reservoir and this action continues until equilibrium between the brake pipe pressure and reservoir pressure is achieved. At that point, the airflow from the reservoir to the cylinder is lapped off. If the pressure in the line is higher than that of the reservoir. The triple valve also causes the cylinder to be exhausted to the atmosphere. As the pressure in the line and that of the reservoir equalize, the triple valve closes, causing the air in the reservoir to be sealed in. When the engine operator releases the brake, the brake valve portal to atmosphere is closed
7.
Train
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A train is a form of rail transport consisting of a series of vehicles that usually runs along a rail track to transport cargo or passengers. Motive power is provided by a locomotive or individual motors in self-propelled multiple units. Although historically steam propulsion dominated, the most common forms are diesel and electric locomotives. Other energy sources include horses, engine or water-driven rope or wire winch, gravity, pneumatics, batteries, the word train comes from the Old French trahiner, from the Latin trahere pull, draw. There are various types of trains that are designed for particular purposes, a train may consist of a combination of one or more locomotives and attached railroad cars, or a self-propelled multiple unit. The first trains were rope-hauled, gravity powered or pulled by horses, from the early 19th century almost all were powered by steam locomotives. A passenger train is one which includes passenger-carrying vehicles which can often be very long, one notable and growing long-distance train category is high-speed rail. In order to much faster operation over 500 km/h, innovative Maglev technology has been researched for years. In most countries, such as the United Kingdom, the distinction between a tramway and a railway is precise and defined in law, a freight train uses freight cars to transport goods or materials. Freight and passengers may be carried in the train in a mixed consist. Rail cars and machinery used for maintenance and repair of tracks, etc. are termed maintenance of way equipment, similarly, dedicated trains may be used to provide support services to stations along a train line, such as garbage or revenue collection. There are various types of trains that are designed for particular purposes, a train can consist of a combination of one or more locomotives and attached railroad cars, or a self-propelled multiple unit. Trains can also be hauled by horses, pulled by a cable, special kinds of trains running on corresponding special railways are atmospheric railways, monorails, high-speed railways, maglev, rubber-tired underground, funicular and cog railways. A passenger train may consist of one or several locomotives and coaches, alternatively, a train may consist entirely of passenger carrying coaches, some or all of which are powered as a multiple unit. In many parts of the world, particularly the Far East and Europe, freight trains are composed of wagons or trucks rather than carriages, though some parcel and mail trains are outwardly more like passenger trains. Trains can also be mixed, comprising both passenger accommodation and freight vehicles, special trains are also used for track maintenance, in some places, this is called maintenance of way. A train with a locomotive attached at each end is described as top and tailed, where a second locomotive is attached temporarily to assist a train up steep banks or grades it is referred to as banking in the UK, or helper service in North America. Recently, many loaded trains in the United States have been made up one or more locomotives in the middle or at the rear of the train
8.
Air brake (road vehicle)
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George Westinghouse first developed air brakes for use in railway service. He patented an air brake on March 5,1872. Westinghouse made numerous alterations to improve his air pressured brake invention, in the early 20th century, after its advantages were proven in railway use, it was adopted by manufacturers of trucks and heavy road vehicles. Air brake systems are used on heavy trucks and buses. The system consists of service brakes, parking brakes, a pedal. For the parking brake, theres a disc or drum brake arrangement which is designed to be held in the position by spring pressure. Air pressure must be produced to release these spring brake parking brakes, for the service brakes to be applied, the brake pedal is pushed, routing the air under pressure to the brake chamber, causing the brake to be engaged. Most types of air brakes are drum brakes, though there is an increasing trend towards the use of disc brakes in this application. The air compressor draws filtered air from the atmosphere and forces it into high-pressure reservoirs at around 120 psi, most heavy vehicles have a gauge within the drivers view, indicating the availability of air pressure for safe vehicle operation, often including warning tones or lights. A mechanical wig wag that automatically drops down into the field of vision when the pressure drops below a certain point is also common. Setting of the parking/emergency brake releases the air in the lines between the compressed air storage tank and the brakes, thus allowing the spring actuated parking brake to engage. A sudden loss of air pressure would result in full spring brake pressure immediately, a compressed air brake system is divided into a supply system and a control system. The supply system compresses, stores and supplies high-pressure air to the system as well as to additional air operated auxiliary truck systems. The air compressor is driven by the engine either by crankshaft pulley via a belt or directly from the timing gears. It is lubricated and cooled by the lubrication and cooling systems. As an alternative to the air dryer, the system can be equipped with an anti-freeze device. The system also includes various check, pressure limiting, drain, Air brake systems may include a wig wag device which deploys to warn the driver if the system air pressure drops too low. The control system is divided into two service brake circuits, the parking brake circuit and the trailer brake circuit
9.
Truck
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A truck is a motor vehicle designed to transport cargo. Trucks vary greatly in size, power, and configuration, smaller varieties may be similar to some automobiles. Commercial trucks can be large and powerful, and may be configured to mount specialized equipment, such as in the case of fire trucks and concrete mixers. Modern trucks are powered by diesel engines, although small to medium size trucks with gasoline engines exist in the US. In the European Union, vehicles with a gross mass of up to 3.5 t are known as light commercial vehicles. Trucks and cars have an ancestor, the steam-powered fardier Nicolas-Joseph Cugnot built in 1769. However, steam wagons were not common until the mid-1800s, the roads of the time, built for horse and carriages, limited these vehicles to very short hauls, usually from a factory to the nearest railway station. The first semi-trailer appeared in 1881, towed by a tractor manufactured by De Dion-Bouton. In 1895 Karl Benz designed and built the first truck in history using the internal combustion engine, later that year some of Benzs trucks were modified to become the first bus by the Netphener, the first motorbus company in history. A year later, in 1896, another internal combustion engine truck was built by Gottlieb Daimler, other companies, such as Peugeot, Renault and Büssing, also built their own versions. The first truck in the United States was built by Autocar in 1899 and was available with optional 5 or 8 horsepower motors, trucks of the era mostly used two-cylinder engines and had a carrying capacity of 3,300 to 4,400 lb. In 1904,700 heavy trucks were built in the United States,1000 in 1907,6000 in 1910, after World War I, several advances were made, pneumatic tires replaced the previously common full rubber versions. Electric starters, power brakes,4,6, and 8 cylinder engines, closed cabs, the first modern semi-trailer trucks also appeared. Touring car builders such as Ford and Renault entered the heavy truck market, although it had been invented in 1890, the diesel engine was not common in trucks in Europe until the 1930s. In the United States, it took longer for diesel engines to be accepted. The word truck might come from a back-formation of truckle, meaning small wheel or pulley, from Middle English trokell, another possible source is the Latin trochus, meaning iron hoop. In turn, both sources emanate from the Greek trokhos, meaning wheel, from trekhein, the first known usage of truck was in 1611, when it referred to the small strong wheels on ships cannon carriages. In its extended usage it came to refer to carts for carrying heavy loads and its expanded application to motor-powered load carrier has been in usage since 1930, shortened from motor truck, which dates back to 1901
10.
Pressure
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Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the relative to the ambient pressure. Various units are used to express pressure, Pressure may also be expressed in terms of standard atmospheric pressure, the atmosphere is equal to this pressure and the torr is defined as 1⁄760 of this. Manometric units such as the centimetre of water, millimetre of mercury, Pressure is the amount of force acting per unit area. The symbol for it is p or P, the IUPAC recommendation for pressure is a lower-case p. However, upper-case P is widely used. The usage of P vs p depends upon the field in one is working, on the nearby presence of other symbols for quantities such as power and momentum. Mathematically, p = F A where, p is the pressure, F is the normal force and it relates the vector surface element with the normal force acting on it. It is incorrect to say the pressure is directed in such or such direction, the pressure, as a scalar, has no direction. The force given by the relationship to the quantity has a direction. If we change the orientation of the element, the direction of the normal force changes accordingly. Pressure is distributed to solid boundaries or across arbitrary sections of normal to these boundaries or sections at every point. It is a parameter in thermodynamics, and it is conjugate to volume. The SI unit for pressure is the pascal, equal to one newton per square metre and this name for the unit was added in 1971, before that, pressure in SI was expressed simply in newtons per square metre. Other units of pressure, such as pounds per square inch, the CGS unit of pressure is the barye, equal to 1 dyn·cm−2 or 0.1 Pa. Pressure is sometimes expressed in grams-force or kilograms-force per square centimetre, but using the names kilogram, gram, kilogram-force, or gram-force as units of force is expressly forbidden in SI. The technical atmosphere is 1 kgf/cm2, since a system under pressure has potential to perform work on its surroundings, pressure is a measure of potential energy stored per unit volume. It is therefore related to density and may be expressed in units such as joules per cubic metre. Similar pressures are given in kilopascals in most other fields, where the prefix is rarely used
11.
Wig wag (truck braking systems)
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A wig wag is a warning device for low air pressure found in air brake systems on large commercial trucks. This device drops a mechanical arm into view when the pressure in the system drops below the threshold of sufficient pressure to deploy the brakes. An automatic wig wag will rise out of view when the pressure in the rises above the threshold. The manual-reset type must be placed in the out-of-view position manually, neither will stay in place unless the pressure in the system is above the threshold. Most U. S. state commercial drivers license manuals, published by the states’ Departments of Motor Vehicles or equivalents, california Dept. of Motor Vehicle CDL Manual section on air brakes
12.
Safety valve
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A safety valve is a valve that acts as a fail-safe in a thermal-hydraulics plant. Pilot-operated relief valves are a type of pressure safety valve. Safety valves were first developed for use on steam boilers during the Industrial Revolution, early boilers operating without them were prone to explosion. Vacuum safety valves are used to prevent a tank from collapsing while it is being emptied, when sizing a vacuum safety valve, the calculation method is not defined in any norm, particularly in the hot CIP / cold water scenario, but some manufacturers have developed sizing simulations. The earliest and simplest safety valve was used on a 1679 steam digester and utilized a weight to retain the steam pressure, however, on the Stockton and Darlington Railway, the safety valve tended to go off when the engine hit a bump in the track. A valve less sensitive to sudden accelerations used a spring to contain the steam pressure and this dangerous practice was sometimes used to marginally increase the performance of a steam engine. In 1856, John Ramsbottom invented a tamper-proof spring safety valve that became universal on railways, Safety valves also evolved to protect equipment such as pressure vessels and heat exchangers. The term safety valve should be limited to compressible fluid applications, the two general types of protection encountered in industry are thermal protection and flow protection. Flow protection is characterized by safety valves that are larger than those mounted for thermal protection. They are generally sized for use in situations where significant quantities of gas or high volumes of liquid must be discharged in order to protect the integrity of the vessel or pipeline. This protection can alternatively be achieved by installing a high integrity pressure protection system, Relief valve, an automatic system that is actuated by the static pressure in a liquid-filled vessel. It specifically opens proportionally with increasing pressure, Safety valve, an automatic system that relieves the static pressure on a gas. It usually opens completely, accompanied by a popping sound, Safety relief valve, an automatic system that relieves by static pressure on both gas and liquid. Pilot-operated safety relief valve, a system that relieves on remote command from a pilot. Low pressure safety valve, a system that relieves static pressure on a gas. Used when the difference between the pressure and the ambient atmospheric pressure is small. Vacuum pressure safety valve, a system that relieves static pressure on a gas. Used when the difference between the vessel pressure and the ambient pressure is small, negative and near to atmospheric pressure
13.
Railway semaphore signal
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One of the earliest forms of fixed railway signal is the semaphore. These signals display their different indications to train drivers by changing the angle of inclination of a pivoted arm, Semaphore signals were patented in the early 1840s by Joseph James Stevens, and soon became the most widely used form of mechanical signal. Designs have altered over the years, and colour light signals have replaced semaphore signals in some countries. Note that this article doesnt represent a worldwide view of the semaphore, john Urpeth Rastrick claimed to have suggested the idea to Hutton Gregory. The semaphore was afterwards rapidly adopted as a fixed signal throughout Britain, such signals were widely adopted in the U. S. after 1908. Usually these were combined into a frame, though in some types. The arm projects horizontally in its most restrictive aspect, other angles indicate less restrictive aspects, the appropriately coloured lens, depending on the arms position, is illuminated from behind by either an oil lamp, a gas lamp, or an incandescent lamp run at a low voltage. Where a green light was required, a blue lens would usually be used, later signals using electric lamps used green lenses. Some signals converted to electric lamps from oil used a yellow-tinted bulb with the original blue lens to maintain the correct colour, materials that were commonly used to make signal posts for semaphore signals included timber, lattice steel, tubular steel and concrete. The Southern Railway in Great Britain frequently made use of old rail for signal posts, semaphores come in lower quadrant and upper quadrant forms. In a lower quadrant signal, the arm pivots downwards for the less restrictive indication, upper quadrant signals, as the name implies, pivot the arm upward for off. During the 1870s, all the British railway companies standardised on the use of semaphore signals, from the 1920s onwards, upper quadrant semaphores almost totally supplanted lower quadrant signals in Great Britain, except on former GWR lines. The advantage of the upper quadrant signal is that should the signal break, or the signal arm be weighed down by snow. In a lower quadrant signal, the opposite may happen, sending the signal to off when in fact it should be illustrating danger. Their spectacle cases, which are on the side of the spindle on which the signal arm is pivoted, are therefore required to be sufficiently heavy to prevent this happening. Current British practice mandates that semaphore signals, both upper and lower quadrant types, are inclined at an angle of 45 degrees from horizontal to display an off indication, the first railway semaphore signals had arms that could be worked to three positions, in the lower quadrant. Used in conjunction with the system, the arm horizontal meant danger, inclined downwards at 45 degrees meant caution. The vertical indication gradually came to be discontinued as the block system superseded time-interval working
14.
Isolation valve
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An isolation valve is a valve in a fluid handling system that stops the flow of process media to a given location, usually for maintenance or safety purposes. They can also be used to provide flow logic, and to external equipment to a system. A valve is classified as an isolation valve because of its function in a system. Therefore, many different types of valves can be classified as isolation valves, to easily understand the concept of an isolation valve, one can think of the valves under a kitchen or bathroom sink in a typical household. These valves are left open so that the user can control the flow of water with the spigot above the sink. However, if the needs to be replaced, the isolation valves are shut to stop the flow of water when the spigot is removed. In this system, the valves and the spigot may even be the same type of valve. However, due to their function they are classified as the valves and, in the case of the spigot. Insulation of valves done before installation With Factory Pre Insulated technology 1, extremely low thermal conductivity, higher thermal resistance & less water vapor diffusion rate. The product has shown prominent results in different atmospheric conditions against condensation & corrosion which also provide efficient life cycle performance of the product, firewater control Pipeline safety systems Residential plumbing systems Nuclear reactors Oil and gas wells
15.
Elevator
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An elevator or lift is a type of vertical transportation that moves people or goods between floors of a building, vessel, or other structure. Elevators are generally powered by electric motors that either drive traction cables or counterweight systems like a hoist, in agriculture and manufacturing, an elevator is any type of conveyor device used to lift materials in a continuous stream into bins or silos. Several types exist, such as the chain and bucket elevator, grain auger screw conveyor using the principle of Archimedes screw, or the chain, languages other than English may have loanwords based on either elevator or lift. Because of wheelchair access laws, elevators are often a requirement in new multistory buildings. The earliest known reference to an elevator is in the works of the Roman architect Vitruvius, some sources from later historical periods mention elevators as cabs on a hemp rope powered by hand or by animals. In 1000, the Book of Secrets by al-Muradi in Islamic Spain described the use of a lifting device. In the 17th century the prototypes of elevators were located in the buildings of England. Louis XV of France had a flying chair built for one of his mistresses at the Chateau de Versailles in 1743. Ancient and medieval elevators used drive systems based on hoists or winders, the invention of a system based on the screw drive was perhaps the most important step in elevator technology since ancient times, leading to the creation of modern passenger elevators. The first screw drive elevator was built by Ivan Kulibin and installed in Winter Palace in 1793, several years later another of Kulibins elevators was installed in Arkhangelskoye near Moscow. The development of elevators was led by the need for movement of raw materials including coal, the technology developed by these industries and the introduction of steel beam construction worked together to provide the passenger and freight elevators in use today. Starting in the mines, by the mid-19th century elevators were operated with steam power and were used for moving goods in bulk in mines and factories. It elevated paying customers to a height in the center of London. Early, crude steam-driven elevators were refined in the decade, – in 1835 an innovative elevator called the Teagle was developed by the company Frost. The elevator was belt-driven and used a counterweight for extra power, the hydraulic crane was invented by Sir William Armstrong in 1846, primarily for use at the Tyneside docks for loading cargo. These quickly supplanted the steam driven elevators, exploiting Pascals law. A water pump supplied a variable level of pressure to a plunger encased inside a vertical cylinder. Counterweights and balances were used to increase the lifting power of the apparatus
16.
Short circuit
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A short circuit is an electrical circuit that allows a current to travel along an unintended path with no or a very low electrical impedance. The electrical opposite of a circuit is an open circuit. It is common to misuse short circuit to describe any electrical malfunction, a short circuit is an abnormal connection between two nodes of an electric circuit intended to be at different voltages. This results in an electric current limited only by the Thévenin equivalent resistance of the rest of the network and potentially causes circuit damage, overheating. Although usually the result of a fault, there are cases where short circuits are caused intentionally, for example, in circuit analysis, a short circuit is a connection between two nodes that forces them to be at the same voltage. In an ideal circuit, this means there is no resistance. In real circuits, the result is a connection with almost no resistance, in such a case, the current is limited by the rest of the circuit. A common type of short circuit occurs when the positive and negative terminals of a battery are connected with a low-resistance conductor, with low resistance in the connection, a high current exists, causing the cell to deliver a large amount of energy in a short time. Overloaded wires can also overheat, sometimes causing damage to the wires insulation, in mains circuits, short circuits may occur between two phases, between a phase and neutral or between a phase and earth. Such short circuits are likely to result in a high current. However, it is possible for circuits to arise between neutral and earth conductors, and between two conductors of the same phase. Such short circuits can be dangerous, particularly as they may not immediately result in a current and are therefore less likely to be detected. Possible effects include unexpected energisation of a circuit presumed to be isolated, to help reduce the negative effects of short circuits, power distribution transformers are deliberately designed to have a certain amount of leakage reactance. The leakage reactance helps limit both the magnitude and rate of rise of the fault current, a short circuit may lead to formation of an electric arc. The arc, a channel of hot ionized plasma, is highly conductive, surface erosion is a typical sign of electric arc damage. Even short arcs can remove significant amount of materials from the electrodes, a short circuit fault current can, within milliseconds, be thousands of times larger than the normal operating current of the system. Damage from short circuits can be reduced or prevented by employing fuses, circuit breakers, or other overload protection, overload protection must be chosen according to the current rating of the circuit. Circuits for large home appliances require protective devices set or rated for higher currents than lighting circuits, wire gauges specified in building and electrical codes are chosen to ensure safe operation in conjunction with the overload protection
17.
Fuse (electrical)
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In electronics and electrical engineering, a fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit including the source of power and the load. Its essential component is a wire or strip that melts when too much current flows through it. It is a device and once a fuse has operated it is an open circuit. The time and current operating characteristics of fuses are chosen to provide adequate protection without needless interruption, wiring regulations usually define a maximum fuse current rating for particular circuits. Short circuits, overloading, mismatched loads, or device failure are the reasons for fuse operation. A fuse is a means of removing power from a faulty system. Circuit breakers can be used as a design solution to fuses. Breguet recommended the use of conductors to protect telegraph stations from lightning strikes, by melting. A variety of wire or foil fusible elements were in use to protect telegraph cables, a fuse was patented by Thomas Edison in 1890 as part of his electric distribution system. The fuse is arranged in series to carry all the current passing through the protected circuit, the resistance of the element generates heat due to the current flow. The size and construction of the element is determined so that the heat produced for a current does not cause the element to attain a high temperature. If too high a current flows, the element rises to a temperature and either directly melts, or else melts a soldered joint within the fuse. The fuse element is made of zinc, copper, silver, aluminum, the fuse ideally would carry its rated current indefinitely, and melt quickly on a small excess. The element must not be damaged by minor harmless surges of current, the fuse elements may be shaped to increase heating effect. In large fuses, current may be divided between multiple strips of metal, fuse elements may be supported by steel or nichrome wires, so that no strain is placed on the element, but a spring may be included to increase the speed of parting of the element fragments. The fuse element may be surrounded by air, or by materials intended to speed the quenching of the arc, silica sand or non-conducting liquids may be used. A maximum current that the fuse can continuously conduct without interrupting the circuit, the speed at which a fuse blows depends on how much current flows through it and the material of which the fuse is made. The operating time is not a fixed interval, but decreases as the current increases, Fuses have different characteristics of operating time compared to current, characterized as, according to time required to respond to an overcurrent condition
18.
Circuit breaker
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A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current, typically resulting from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected, unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset to resume normal operation. The generic function of a breaker, RCD or a fuse. An early form of circuit breaker was described by Thomas Edison in an 1879 patent application and its purpose was to protect lighting circuit wiring from accidental short circuits and overloads. A modern miniature circuit breaker similar to the now in use was patented by Brown. Hugo Stotz, an engineer who had sold his company to BBC, was credited as the inventor on DRP458392, stotzs invention was the forerunner of the modern thermal-magnetic breaker commonly used in household load centers to this day. All circuit breaker systems have features in their operation, but details vary substantially depending on the voltage class, current rating. The circuit breaker must firstly detect a fault condition, in small mains and low voltage circuit breakers, this is usually done within the device itself. Typically, the heating and/or magnetic effects of current are employed. Circuit breakers for large currents or high voltages are usually arranged with protective relay pilot devices to sense a fault condition, Circuit breakers may also use the higher current caused by the fault to separate the contacts, such as thermal expansion or a magnetic field. The circuit breaker contacts must carry the current without excessive heating. Contacts are made of copper or copper alloys, silver alloys, service life of the contacts is limited by the erosion of contact material due to arcing while interrupting the current. Miniature and molded-case circuit breakers are usually discarded when the contacts have worn, when a high current or voltage is interrupted, an arc is generated. The length of the arc is generally proportional to the voltage while the intensity is proportional to the current and this arc must be contained, cooled and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium the arc forms in, finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit. Low-voltage miniature circuit breaker uses air alone to extinguish the arc and these circuit breakers contain so-called arc chutes, a stack of mutually insulated parallel metal plates which divide and cool the arc. By splitting the arc into smaller arcs the arc is cooled down while the arc voltage is increased, the number of plates in the arc chute is dependent on the short-circuit rating and nominal voltage of the circuit breaker. In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc
19.
Current limiting
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An inrush current limiter is a device or group of devices used to limit inrush current. Negative temperature coefficient thermistors and resistors are two of the simplest options, with time and power dissipation being their main drawbacks. More complex solutions can be used when design constraints make simpler options infeasible, a fuse generally is too slow in operation and the time it takes to blow may well be enough to destroy the devices. A typical short-circuit/overload protection scheme is shown in the image, the schematic is representative of a simple protection mechanism employed in regulated DC supplies and class-AB power amplifiers. Q1 is the pass or output transistor, Rsens is the load current sensing device. Q2 is the transistor which turns on as soon as the voltage across Rsens becomes about 0.65 V. This voltage is determined by the value of Rsens and the current through it. When Q2 turns on, it removes base current from Q1 thereby reducing the current of Q1. Neglecting the base currents of Q1 and Q2, the current of Q1 is also the load current. Thus, Rsens fixes the maximum current to a given by 0. 65/Rsens, for any given output voltage. For example, if Rsens =0.33 Ω, the current is limited to about 2 A even if Rload becomes a short, with the absence of Q2, Q1 would attempt to drive a very large current and the result would be greater power dissipation in Q1. If Rload is zero the dissipation will be much greater, with Q2 in place, the current is limited and the maximum power dissipation in Q1 is also limited to a safe value. Further, this power dissipation will remain as long as the overload exists, for example, the pass-transistor in a regulated DC power supply system rated for 25 V at 1.5 A will normally dissipate about 7.5 W for a Vcc of 30 V. With current limiting, the dissipation will increase to about 60 W if the output is shorted‡‡. Without current limiting the dissipation would be greater than 300 W‡‡ - so limiting does have a benefit, in short, an 80–100 W device will be needed where a 10–20 W device would have been sufficient. In this technique, beyond the current limit the voltage will decrease to a value depending on the current limit. An issue with the circuit is that Q1 will not be saturated unless its base is biased about 0.5 volts above Vcc. The circuits at right and left operate more efficiently from a single supply, in both circuits, R1 allows Q1 to turn on and pass voltage and current to the load
20.
Avionics
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Avionics are the electronic systems used on aircraft, artificial satellites, and spacecraft. Avionic systems include communications, navigation, the display and management of systems. These can be as simple as a searchlight for a helicopter or as complicated as the tactical system for an airborne early warning platform. The term avionics is a portmanteau of the aviation and electronics. The term avionics was coined by the journalist Philip J. Klass as a portmanteau of aviation electronics, many modern avionics have their origins in World War II wartime developments. For example, autopilot systems that are today were started to help bomber planes fly steadily enough to hit precision targets from high altitudes. Famously, radar was developed in the UK, Germany, modern avionics is a substantial portion of military aircraft spending. Aircraft like the F‑15E and the now retired F‑14 have roughly 20 percent of their budget spent on avionics, most modern helicopters now have budget splits of 60/40 in favour of avionics. The civilian market has seen a growth in cost of avionics. Flight control systems and new navigation needs brought on by tighter airspaces, have pushed up development costs, the major change has been the recent boom in consumer flying. As more people begin to use planes as their method of transportation. The majority of power their avionics using 14- or 28‑volt DC electrical systems, however, larger. One source of international standards for equipment are prepared by the Airlines Electronic Engineering Committee. Communications connect the deck to the ground and the flight deck to the passengers. On‑board communications are provided by systems and aircraft intercoms. The VHF aviation communication system works on the airband of 118.000 MHz to 136.975 MHz, each channel is spaced from the adjacent ones by 8.33 kHz in Europe,25 kHz elsewhere. VHF is also used for line of communication such as aircraft-to-aircraft. Amplitude modulation is used, and the conversation is performed in simplex mode, aircraft communication can also take place using HF or satellite communication
21.
Redundancy (engineering)
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In many safety-critical systems, such as fly-by-wire and hydraulic systems in aircraft, some parts of the control system may be triplicated, which is formally termed triple modular redundancy. An error in one component may then be out-voted by the other two, in a triply redundant system, the system has three sub components, all three of which must fail before the system fails. Redundancy may also be known by the majority voting systems or voting logic. Applied in the case of software, code written independently and distinctly different, structures are usually designed with redundant parts as well, ensuring that if one part fails, the entire structure will not collapse. A structure without redundancy is called fracture-critical, meaning that a broken component can cause the collapse of the entire structure. Bridges that failed due to lack of redundancy include the Silver Bridge, parallel and combined systems demonstrate different level of redundancy. The models are subject of studies in reliability and safety engineering, the two functions of redundancy are passive redundancy and active redundancy. Both functions prevent performance decline from exceeding specification limits without human intervention using extra capacity, passive redundancy uses excess capacity to reduce the impact of component failures. One common form of redundancy is the extra strength of cabling. This extra strength allows some structural components to fail without bridge collapse, the extra strength used in the design is called the margin of safety. Eyes and ears provide working examples of passive redundancy, vision loss in one eye does not cause blindness but depth perception is impaired. Hearing loss in one ear does not cause deafness but directionality is impaired, performance decline is commonly associated with passive redundancy when a limited number of failures occur. Active redundancy eliminates performance declines by monitoring the performance of devices. The voting logic is linked to switching that automatically reconfigures the components, error detection and correction and the Global Positioning System are two examples of active redundancy. Electrical power distribution provides an example of active redundancy, several power lines connect each generation facility with customers. Each power line includes monitors that detect overload, each power line also includes circuit breakers. The combination of power lines provides excess capacity, circuit breakers disconnect a power line when monitors detect an overload. Power is redistributed across the remaining lines, charles Perrow, author of Normal Accidents, has said that sometimes redundancies backfire and produce less, not more reliability
22.
Triple modular redundancy
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If any one of the three systems fails, the other two systems can correct and mask the fault. The TMR concept can be applied to forms of redundancy, such as software redundancy in the form of N-version programming. Some ECC memory uses triple modular redundancy hardware, because triple modular redundancy hardware is faster than Hamming error correction software, space satellite systems often use TMR, although satellite RAM usually uses Hamming error correction. Some communication systems use N-modular redundancy as a form of forward error correction. For example, 5-modular redundancy communication systems use the majority of 5 samples – if any 2 of the 5 results are erroneous, the other 3 results can correct and mask the fault. Modular redundancy is a concept, dating to antiquity, while the first use of TMR in a computer was the Czechoslovak computer SAPO. Three chronometers provided triple modular redundancy, allowing error correction if one of the three was wrong, so the pilot would take the average of the two with closer reading. There is an old adage to this effect, stating, Never go to sea with two chronometers, take one or three, mainly this means that if two chronometers contradict, how do you know which one is correct. At one time this observation or rule was a one as the cost of three sufficiently accurate chronometers was more than the cost of many types of smaller merchant vessels. Some vessels carried more than three chronometers – for example, the HMS Beagle carried 22 chronometers, in the modern era, ships at sea use GNSS navigation receivers -- mostly running with WAAS or ENGOS support so as to provide accurate time. In TMR, three identical logic circuits are used to compute the same set of specified Boolean function, if there are no circuit failures, the outputs of the three circuits are identical. But due to failures, the outputs of the three circuits may be different. A Majority Logic Gate is used to decide which of the outputs is the correct output. The majority gate output is 1 if two or more of the inputs of the majority gate are 1, output is 0 if two or more of the majority gate’s inputs are 0. e, the majority gate is a voting machine. If one circuit fails and produces an output of 0, while the two are working correctly and produce an output of 1, the majority gate output is 1, i. e. it still has the correct value. And similarly for the case when the Boolean function computed by the three identical circuits has value 0, thus, the majority gate output is guaranteed to be correct as long as no more than one of the three identical logic circuits has failed. TMR systems should use data scrubbing—rewrite flip-flops periodically—in order to avoid accumulation of errors, the majority gate itself could fail. Is there a way to mask that failure, in other words, who guards the guardians
23.
Fly-by-wire
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Fly-by-wire is a system that replaces the conventional manual flight controls of an aircraft with an electronic interface. Both systems often require redundant backup to deal with failures, which increases weight, both have limited ability to compensate for changing aerodynamic conditions. The term fly-by-wire implies a purely electrically signaled control system and it is used in the general sense of computer-configured controls, where a computer system is interposed between the operator and the final control actuators or surfaces. This modifies the manual inputs of the pilot in accordance with control parameters, side-sticks, centre sticks, or conventional flight control yokes can be used to fly FBW aircraft. Fly-by wire systems are complex, but their operation can be explained in simple terms. When a pilot moves the control column, a signal is sent to a computer the signal is sent through wires to ensure that the signal reaches the computer. A Triplex is when there are three channels being used, in an Analog system, the computer receives the signals, performs a calculation and adds another channel. These four Quadruplex signals are sent to the control surface actuator. Potentiometers in the actuator send a signal back to the computer reporting the position of the actuator, when the actuator reaches the desired position, the two signals cancel each other out and the actuator stops moving. The computer then commands the flight control surfaces to adopt a configuration that will achieve the flight path. Fly-by-wire control systems allow aircraft computers to perform tasks without pilot input, automatic stability systems operate in this way. Gyroscopes fitted with sensors are mounted in an aircraft to sense movement changes in the pitch, roll, any movement results in signals to the computer, which automatically moves control actuators to stabilize the aircraft. Aircraft systems may be quadruplexed to prevent loss of signals in the case of failure of one or even two channels, pre-flight safety checks of a fly-by-wire system are often performed using built-in test equipment. On programming the system, either by the pilot or groundcrew, any failure will be indicated to the crews. A FBW aircraft can be lighter than a design with conventional controls. These include the vertical and horizontal stabilizers that are at the rear of the fuselage, if these structures can be reduced in size, airframe weight is reduced. The advantages of FBW controls were first exploited by the military, the Airbus series of airliners used full-authority FBW controls beginning with their A320 series, see A320 flight control. Boeing followed with their 777 and later designs, electronic systems require less maintenance, whereas mechanical and hydraulic systems require lubrication, tension adjustments, leak checks, fluid changes, etc
24.
Traffic light
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The worlds first, manually operated gas-lit traffic signal was short lived. Installed in London in December 1868, it exploded less than a month later, Traffic control started to seem necessary in the late 1890s and Earnest Sirrine from Chicago patented the first automated traffic control system in 1910. It used the words STOP and PROCEED, although neither word lit up, Traffic lights alternate the right of way accorded to users by displaying lights of a standard colour following a universal colour code. In the typical sequence of phases, The green light allows traffic to proceed in the direction denoted, if it is safe to do so. The amber light warns that the signal is about to change to red, in a number of countries – among them the United Kingdom – a phase during which red and yellow are displayed together indicates that the signal is about to change to green. A flashing amber indication is a warning signal, in the United Kingdom, a flashing amber light is used only at pelican crossings, in place of the combined red–amber signal, and indicates that drivers may pass if no pedestrians are on the crossing. The red signal prohibits any traffic from proceeding, a flashing red indication is treated as a stop sign. In some countries traffic signals will go into a flashing mode if the Conflict Monitor detects a problem, the signal may display flashing yellow to the main road and flashing red to the side road, or flashing red in all directions. Flashing operation can also be used during times of day when traffic is light, before traffic lights traffic police controlled the flow of traffic, a well-documented example being that on London Bridge in 1722. Three men were given the task of directing traffic coming in, each officer would help direct traffic coming out of Southwark into London and he made sure all traffic stayed on the west end of the bridge. A second officer would direct traffic on the east end of the bridge to control the flow of people leaving London and going into Southwark. The main reason for the light was that there was an overflow of horse-drawn traffic over Westminster Bridge which forced thousands of pedestrians to walk next to the Houses of Parliament. The design combined three semaphore arms with red and green gas lamps for use, on a pillar. The gas lantern was manually turned by a police officer. The signal was 22 feet high, at night a red light would command Stop and a green light would mean use Caution. Although it was said to be successful at controlling traffic, its life was brief. It exploded on 2 January 1869, as a result of a leak in one of the gas lines underneath the pavement, injuring or killing the policeman who was operating it. In the first two decades of the 20th century semaphore traffic signals, like the one in London, were in use all over the United States with each state having its own design of the device, One good example was from Toledo, Ohio in 1908
25.
Green
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Green is the color between blue and yellow on the spectrum of visible light. It is evoked by light with a predominant wavelength of roughly 495–570 nm, the modern English word green comes from the Middle English and Anglo-Saxon word grene, from the same Germanic root as the words grass and grow. It is the color of living grass and leaves and as a result is the color most associated with springtime, growth, by far the largest contributor to green in nature is chlorophyll, the chemical by which plants photosynthesize and convert sunlight into chemical energy. Many creatures have adapted to their environments by taking on a green hue themselves as camouflage. Several minerals have a color, including the emerald, which is colored green by its chromium content. In surveys made in Europe and the United States, green is the color most commonly associated with nature, life, health, youth, spring, hope and envy. In Europe and the U. S. green is associated with death, sickness, or the devil. In the Middle Ages and Renaissance, when the color of clothing showed the social status, green was worn by merchants, bankers. The Mona Lisa by Leonardo da Vinci wears green, showing she is not from a noble family, Green is also the traditional color of safety and permission, a green light means go ahead, a green card permits permanent residence in the United States. It is the most important color in Islam and it was the color of the banner of Muhammad, and is found in the flags of nearly all Islamic countries, and represents the lush vegetation of Paradise. It is also associated with the culture of Gaelic Ireland. Because of its association with nature, it is the color of the environmental movement, political groups advocating environmental protection and social justice describe themselves as part of the Green movement, some naming themselves Green parties. This has led to campaigns in advertising, as companies have sold green, or environmentally friendly. The word green comes from the Middle English and Old English word grene, which, like the German word grün, has the same root as the words grass and grow. It is from a Common Germanic *gronja-, which is reflected in Old Norse grænn, Old High German gruoni, ultimately from a PIE root *ghre- to grow. The first recorded use of the word as a term in Old English dates to ca. Latin with viridis also has a genuine and widely used term for green, related to virere to grow and ver spring, it gave rise to words in several Romance languages, French vert, Italian verde. Likewise the Slavic languages with zelenъ, Ancient Greek also had a term for yellowish, pale green – χλωρός, chloros, cognate with χλοερός verdant and χλόη the green of new growth
26.
Computer hardware
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Computer hardware is the collection of physical components that constitute a computer system. By contrast, software is instructions that can be stored and run by hardware, hardware is directed by the software to execute any command or instruction. A combination of hardware and software forms a usable computing system, the template for all modern computers is the Von Neumann architecture, detailed in a 1945 paper by Hungarian mathematician John von Neumann. The meaning of the term has evolved to mean a stored-program computer in which an instruction fetch and this is referred to as the Von Neumann bottleneck and often limits the performance of the system. For the third year, U. S. business-to-business channel sales increased. The impressive growth was the fastest sales increase since the end of the recession, sales growth accelerated in the second half of the year peaking in fourth quarter with a 6.9 percent increase over the fourth quarter of 2012. There are a number of different types of system in use today. The personal computer, also known as the PC, is one of the most common types of computer due to its versatility, laptops are generally very similar, although they may use lower-power or reduced size components, thus lower performance. The computer case is a plastic or metal enclosure that houses most of the components, a case can be either big or small, but the form factor of motherboard for which it is designed matters more. A power supply unit converts alternating current electric power to low-voltage DC power for the components of the computer. Laptops are capable of running from a battery, normally for a period of hours. The motherboard is the component of a computer. It is usually cooled by a heatsink and fan, or water-cooling system, most newer CPUs include an on-die Graphics Processing Unit. The clock speed of CPUs governs how fast it executes instructions, many modern computers have the option to overclock the CPU which enhances performance at the expense of greater thermal output and thus a need for improved cooling. The chipset, which includes the bridge, mediates communication between the CPU and the other components of the system, including main memory. Random-Access Memory, which stores the code and data that are being accessed by the CPU. For example, when a web browser is opened on the computer it takes up memory, RAM usually comes on DIMMs in the sizes 2GB, 4GB, and 8GB, but can be much larger. Read-Only Memory, which stores the BIOS that runs when the computer is powered on or otherwise begins execution, the BIOS includes boot firmware and power management firmware
27.
Software
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Computer software, or simply software, is that part of a computer system that consists of data or computer instructions, in contrast to the physical hardware from which the system is built. In computer science and software engineering, computer software is all information processed by computer systems, programs, computer software includes computer programs, libraries and related non-executable data, such as online documentation or digital media. Computer hardware and software require each other and neither can be used on its own. At the lowest level, executable code consists of machine language instructions specific to an individual processor—typically a central processing unit, a machine language consists of groups of binary values signifying processor instructions that change the state of the computer from its preceding state. For example, an instruction may change the value stored in a storage location in the computer—an effect that is not directly observable to the user. An instruction may also cause something to appear on a display of the computer system—a state change which should be visible to the user. The processor carries out the instructions in the order they are provided, unless it is instructed to jump to a different instruction, the majority of software is written in high-level programming languages that are easier and more efficient for programmers, meaning closer to a natural language. High-level languages are translated into machine language using a compiler or an interpreter or a combination of the two, an outline for what would have been the first piece of software was written by Ada Lovelace in the 19th century, for the planned Analytical Engine. However, neither the Analytical Engine nor any software for it were ever created, the first theory about software—prior to creation of computers as we know them today—was proposed by Alan Turing in his 1935 essay Computable numbers with an application to the Entscheidungsproblem. This eventually led to the creation of the academic fields of computer science and software engineering. Computer science is more theoretical, whereas software engineering focuses on practical concerns. However, prior to 1946, software as we now understand it—programs stored in the memory of stored-program digital computers—did not yet exist, the first electronic computing devices were instead rewired in order to reprogram them. On virtually all platforms, software can be grouped into a few broad categories. There are many different types of software, because the range of tasks that can be performed with a modern computer is so large—see list of software. System software includes, Operating systems, which are collections of software that manage resources and provides common services for other software that runs on top of them. Supervisory programs, boot loaders, shells and window systems are parts of operating systems. In practice, an operating system bundled with additional software so that a user can potentially do some work with a computer that only has an operating system. Device drivers, which operate or control a particular type of device that is attached to a computer, utilities, which are computer programs designed to assist users in the maintenance and care of their computers
28.
Computer
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A computer is a device that can be instructed to carry out an arbitrary set of arithmetic or logical operations automatically. The ability of computers to follow a sequence of operations, called a program, such computers are used as control systems for a very wide variety of industrial and consumer devices. The Internet is run on computers and it millions of other computers. Since ancient times, simple manual devices like the abacus aided people in doing calculations, early in the Industrial Revolution, some mechanical devices were built to automate long tedious tasks, such as guiding patterns for looms. More sophisticated electrical machines did specialized analog calculations in the early 20th century, the first digital electronic calculating machines were developed during World War II. The speed, power, and versatility of computers has increased continuously and dramatically since then, conventionally, a modern computer consists of at least one processing element, typically a central processing unit, and some form of memory. The processing element carries out arithmetic and logical operations, and a sequencing, peripheral devices include input devices, output devices, and input/output devices that perform both functions. Peripheral devices allow information to be retrieved from an external source and this usage of the term referred to a person who carried out calculations or computations. The word continued with the same meaning until the middle of the 20th century, from the end of the 19th century the word began to take on its more familiar meaning, a machine that carries out computations. The Online Etymology Dictionary gives the first attested use of computer in the 1640s, one who calculates, the Online Etymology Dictionary states that the use of the term to mean calculating machine is from 1897. The Online Etymology Dictionary indicates that the use of the term. 1945 under this name, theoretical from 1937, as Turing machine, devices have been used to aid computation for thousands of years, mostly using one-to-one correspondence with fingers. The earliest counting device was probably a form of tally stick, later record keeping aids throughout the Fertile Crescent included calculi which represented counts of items, probably livestock or grains, sealed in hollow unbaked clay containers. The use of counting rods is one example, the abacus was initially used for arithmetic tasks. The Roman abacus was developed from used in Babylonia as early as 2400 BC. Since then, many forms of reckoning boards or tables have been invented. In a medieval European counting house, a checkered cloth would be placed on a table, the Antikythera mechanism is believed to be the earliest mechanical analog computer, according to Derek J. de Solla Price. It was designed to calculate astronomical positions and it was discovered in 1901 in the Antikythera wreck off the Greek island of Antikythera, between Kythera and Crete, and has been dated to circa 100 BC
29.
Watchdog timer
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A watchdog timer is an electronic timer that is used to detect and recover from computer malfunctions. During normal operation, the computer regularly resets the timer to prevent it from elapsing. If, due to a fault or program error, the computer fails to reset the watchdog. The timeout signal is used to initiate corrective action or actions, the corrective actions typically include placing the computer system in a safe state and restoring normal system operation. In such systems, the computer cannot depend on a human to reboot it if it hangs, a watchdog timer is usually employed in cases like these. Watchdog timers may also be used when running untrusted code in a sandbox, to limit the CPU time available to the code and thus prevent some types of denial-of-service attacks. The act of restarting a watchdog timer is referred to as kicking the dog or other similar terms. Alternatively, in microcontrollers that have a watchdog timer, the watchdog is sometimes kicked by executing a special machine language instruction. An example of this is the CLRWDT instruction found in the set of some PIC microcontrollers. In computers that are running operating systems, watchdog resets are usually invoked through a device driver, for example, in the Linux operating system, a user space program will kick the watchdog by interacting with the watchdog device driver, typically by writing a zero character to /dev/watchdog. The device driver, which serves to abstract the watchdog hardware from user programs, is also used to configure the time-out period and start. Watchdog timers come in many configurations, and many allow their configurations to be altered, microcontrollers often include an integrated, on-chip watchdog. In other computers the watchdog may reside in a chip that connects directly to the CPU. The watchdog and CPU may share a clock signal, as shown in the block diagram below. Two or more timers are sometimes cascaded to form a multistage watchdog timer, for example, the block diagram below shows a three-stage watchdog. In a multistage watchdog, only the first stage is kicked by the processor, upon first stage timeout, a corrective action is initiated and the next stage in the cascade is started. As each subsequent stage times out, it triggers a corrective action, upon final stage timeout, a corrective action is initiated, but no other stage is started because the end of the cascade has been reached. Watchdog timers may have fixed or programmable time intervals
30.
Fault-tolerant computer system
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Fault-tolerant computer systems are systems designed around the concepts of fault tolerance. In essence, they must be able to continue working to a level of satisfaction in the presence of faults, Fault tolerance is not just a property of individual machines, it may also characterise the rules by which they interact. Recovery from errors in fault-tolerant systems can be characterised as either roll-forward or roll-back, when the system detects that it has made an error, roll-forward recovery takes the system state at that time and corrects it, to be able to move forward. Roll-back recovery reverts the system back to some earlier, correct version, for example using checkpointing. Roll-back recovery requires that the operations between the checkpoint and the detected erroneous state can be made idempotent, some systems make use of both roll-forward and roll-back recovery for different errors or different parts of one error. Hardware fault-tolerance is the most common application of systems, designed to prevent failures due to hardware components. Most basically, this is provided by redundancy, particularly dual modular redundancy, typically, components have multiple backups and are separated into smaller segments that act to contain a fault, and extra redundancy is built into all physical connectors, power supplies, fans, etc. Software fault-tolerance is based more around nullifying programming errors using real-time redundancy, there are many ways to conduct such fault-regulation, depending on the application and the available hardware. The first known fault-tolerant computer was SAPO, built in 1951 in Czechoslovakia by Antonin Svoboda and its basic design was magnetic drums connected via relays, with a voting method of memory error detection. Several other machines were developed along this line, mostly for military use, most of the development in the so-called LLNM computing was done by NASA during the 1960s, in preparation for Project Apollo and other research aspects. NASAs first machine went into an observatory, and their second attempt. This computer had a backup of memory arrays to use memory recovery methods and it could detect its own errors and fix them or bring up redundant modules as needed. The computer is working today. Hyper-dependable computers were pioneered mostly by aircraft manufacturers, nuclear companies. Again, IBM developed the first computer of this kind for NASA for guidance of Saturn V rockets, but later on BNSF, Unisys, the 1970 F14 CADC had built-in self-test and redundancy. In general, the efforts at fault-tolerant designs were focused mainly on internal diagnosis, where a fault would indicate something was failing. SAPO, for instance, had a method by which faulty memory drums would emit a noise before failure. Later efforts showed that, to be effective, the system had to be self-repairing and diagnosing – isolating a fault
31.
Catastrophic failure
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A catastrophic failure is a sudden and total failure from which recovery is impossible. Catastrophic failures often lead to cascading systems failure, the term is most commonly used for structural failures, but has often been extended to many other disciplines in which total and irrecoverable loss occurs. Such failures are investigated using the methods of engineering, which aims to isolate the cause or causes of failure. In firearms, catastrophic failure usually refers to a rupture or disintegration of the barrel or receiver of the gun when firing it. A failure of this type, known colloquially as a kaboom, or kB failure, can pose a threat not only to the user, in chemical engineering, thermal runaway can cause catastrophic failure. The bridge was designed and its replacement was built as a separate structure upstream of the old. The failure of the South Fork Dam in 1889 released 4.8 billion US gallons of water, the collapse of the St. Francis Dam in 1928 released 12.4 billion US gallons of water, resulting in a death toll of nearly 600 people. The collapse of the first Tacoma Narrows Bridge of 1940, where the deck of the road bridge was totally destroyed by dynamic oscillations in a 40 miles per hour wind. The De Havilland Comet disasters of 1954, later determined to be failures due to metal fatigue that had not been anticipated at the corners of square windows used by the Comet 1. The 62 Banqiao Dams failure event in China in 1975, due to Typhoon Nina, approximately 86,000 people died from flooding and another 145,000 died from subsequent diseases, total of 231,000 deaths. The Hyatt Regency walkway collapse of 1981, where a walkway in a hotel lobby in Kansas City, Missouri, collapsed completely, killing over 100 people on. The Space Shuttle Challenger disaster of 1986, in which an O-ring of a rocket booster failed, the nuclear reactor at the Chernobyl power plant, which exploded in 1986 causing the release of a substantial amount of radioactive materials. The collapse of the Warsaw radio mast of 1991, which had up to that point held the title of worlds tallest structure, the Sampoong Department Store collapse of 1995, which happened due to structural weaknesses, killed 502 people and injured 937. The terrorist attacks and subsequent fire at the World Trade Center on September 11,2001, the Space Shuttle Columbia disaster of 2003, where damage to a wing during launch resulted in total loss upon re-entry. The collapse of the multi-span I-35W Mississippi River bridge on August 1,2007, what Do You Care What Other People Think. Lewis, Peter R. Beautiful Railway Bridge of the Silvery Tay, Reinvestigating the Tay Bridge Disaster of 1879
32.
Electronic circuit
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In an integrated circuit or IC, the components and interconnections are formed on the same substrate, typically a semiconductor such as silicon or gallium arsenide. An electronic circuit can usually be categorized as an analog circuit, breadboards, perfboards, and stripboards are common for testing new designs. They allow the designer to make changes to the circuit during development. Analog electronic circuits are those in current or voltage may vary continuously with time to correspond to the information being represented. Analog circuitry is constructed from two building blocks, series and parallel circuits. In a series circuit, the current passes through a series of components. A string of Christmas lights is an example of a series circuit, if one goes out. In a parallel circuit, all the components are connected to the voltage. The basic components of analog circuits are wires, resistors, capacitors, inductors, diodes, analog circuits are very commonly represented in schematic diagrams, in which wires are shown as lines, and each component has a unique symbol. Analog circuit analysis employs Kirchhoffs circuit laws, all the currents at a node, wires are usually treated as ideal zero-voltage interconnections, any resistance or reactance is captured by explicitly adding a parasitic element, such as a discrete resistor or inductor. When the circuit size is comparable to a wavelength of the relevant signal frequency, wires are treated as transmission lines, with constant characteristic impedance, and the impedances at the start and end determine transmitted and reflected waves on the line. These values represent the information that is being processed, in the vast majority of cases, binary encoding is used, one voltage represents a binary 1 and another voltage represents a binary 0. Digital circuits make extensive use of transistors, interconnected to create gates that provide the functions of Boolean logic, AND, NAND, OR, NOR, XOR. Digital circuits therefore can provide both logic and memory, enabling them to perform arbitrary computational functions, the design process for digital circuits is fundamentally different from the process for analog circuits. Each logic gate regenerates the binary signal, so the designer need not account for distortion, gain control, offset voltages, as a consequence, extremely complex digital circuits, with billions of logic elements integrated on a single silicon chip, can be fabricated at low cost. Such digital integrated circuits are ubiquitous in electronic devices, such as calculators, mobile phone handsets. Digital circuitry is used to general purpose computing chips, such as microprocessors. Field-programmable gate arrays, chips with logic circuitry whose configuration can be modified after fabrication, are widely used in prototyping
33.
Track circuit
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A track circuit is a simple electrical device used to detect the absence of a train on rail tracks, used to inform signallers and control relevant signals. The basic principle behind the circuit lies in the connection of the two rails by the wheels and axle of locomotives and rolling stock to short out an electrical circuit. This circuit is monitored by electrical equipment to detect the absence of the trains, since this is a safety appliance, fail-safe operation is crucial, therefore the circuit is designed to indicate the presence of a train when failures occur. On the other hand, false occupancy readings are disruptive to operations and are to be minimized. Track circuits allow railway signalling systems to operate semi-automatically, by displaying signals for trains to slow down or stop in the presence of occupied track ahead of them. They help prevent dispatchers and operators from causing accidents, both by informing them of track occupancy and by preventing signals from displaying unsafe indications, a track circuit typically has power applied to each rail and a relay coil wired across them. When no train is present, the relay is energised by the current flowing from the source through the rails. When a train is present, its axles short the rails together, the current to the relay coil drops. Circuits through the relay contacts therefore report whether or not the track is occupied, each circuit detects a defined section of track, such as a block. These sections are separated by insulated joints, usually in both rails, to prevent one circuit from falsely powering another in the event of insulation failure, the electrical polarity is usually reversed from section to section. Circuits are powered at low voltages to protect against line power failures, the relays and the power supply are attached to opposite ends of the section to prevent broken rails from electrically isolating part of the track from the circuit. A series resistor limits the current when the circuit is short-circuited. In some railway electrification schemes, one or both of the rails are used to carry the return current. This prevents use of the basic DC track circuit because the substantial traction currents overwhelm the small track circuit currents. To accommodate this, AC track circuits use alternating current signals instead of current but typically. The relays are arranged to detect the frequency and to ignore DC. Again, failsafe principles dictate that the relay interprets the presence of the signal as unoccupied track, the AC signal can be coded and locomotives equipped with inductive pickups to create a cab signalling system. There are two approaches to provide a continuous path for traction current that spans multiple track circuit blocks
34.
Railway signalling
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Railway signalling is a system used to direct railway traffic and keep trains clear of each other at all times. Trains move on fixed rails, making them susceptible to collision. This susceptibility is exacerbated by the weight and inertia of a train. Most forms of control involve movement authority being passed from those responsible for each section of a rail network to the train crew. The set of rules and the equipment used to accomplish this determine what is known as the method of working. Not all these methods require the use of signals. The earliest rail cars were first hauled by horses or mules, a mounted flagman on a horse preceded some early trains. Hand and arm signals were used to direct the “train drivers”, foggy and poor-visibility conditions gave rise to flags and lanterns. Wayside signalling dates back as far as 1832, and used elevated flags or balls that could be seen from afar, the simplest form of operation, at least in terms of equipment, is to run the system according to a timetable. Every train crew understands and adheres to a fixed schedule, trains may only run on each track section at a scheduled time, during which they have possession and no other train may use the same section. When trains run in opposite directions on a railroad, meeting points are scheduled. Neither train is permitted to move before the other has arrived, in the US the display of two green flags is an indication that another train is following the first and the waiting train must wait for the next train to pass. In addition, the carrying the flags gives eight blasts on the whistle as it approaches. The waiting train must return eight blasts before the flag carrying train may proceed, the timetable system has several disadvantages. First, there is no confirmation that the track ahead is clear. A second problem is the systems inflexibility, trains cannot be added, delayed, or rescheduled without advance notice. A third problem is a corollary of the second, the system is inefficient, to provide flexibility, the timetable must give trains a broad allocation of time to allow for delays, so the line is not in the possession of each train for longer than is otherwise necessary. Nonetheless, this system permits operation on a vast scale, with no requirements for any kind of communication that travels faster than a train, timetable operation was the normal mode of operation in North America in the early days of the railroad
35.
Bathyscaphe
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The float is filled with gasoline because it is readily available, buoyant, and, for all practical purposes, incompressible. By contrast, the cabin must withstand a huge pressure differential and is massively built. Buoyancy at the surface can be trimmed easily by replacing gasoline with water, Auguste Piccard, inventor of the first bathyscaphe, composed the name bathyscaphe using the Ancient Greek words βαθύς bathys and σκάφος skaphos. For example, the pressure at the bottom of the Challenger Deep is more than seven times that in a standard H-type compressed gas cylinder, instead, ballast in the form of iron shot is released to ascend, the shot being lost to the ocean floor. The iron shot containers are in the form of one or more hoppers which are open at the bottom throughout the dive, the iron shot being held in place by an electromagnet at the neck. This is a device as it requires no power to ascend, in fact, in the event of a power failure, shot runs out by gravity. The first bathyscaphe was dubbed FNRS-2, named after the Fonds National de la Recherche Scientifique, propulsion was provided by battery-driven electric motors. The float held 37,850 litres of aviation gasoline, there was no access tunnel, the sphere had to be loaded and unloaded while on deck. The first journeys were detailed in the Jacques Cousteau book The Silent World, as described in the book, the vessel had serenely endured the pressure of the depths, but had been destroyed in a minor squall. FNRS-3 was a new submersible, using the crew sphere from the damaged FNRS-2, Piccards second bathyscaphe was actually a third vessel Trieste, which was purchased by the United States Navy from Italy in 1957. It had two water ballast tanks and eleven buoyancy tanks holding 120,000 litres of gasoline, in 1960 Trieste, carrying Piccards son Jacques Piccard and Lt. Don Walsh, reached the deepest known point on the Earths surface, the Challenger Deep, in the Mariana Trench. The onboard systems indicated a depth of 37,800 ft but this was corrected to 35,813 ft by taking into account variations arising from salinity. Later and more accurate measurements made in 1995 have found the Challenger Deep to be slightly shallower at 35,798 ft. The crew of the Trieste noted that the floor consisted of diatomaceous ooze and reported observing some type of flatfish, resembling a sole, in 1995, the Japanese sent an unmanned submersible to this depth, Kaikō, but it was later lost at sea. In 2009, a team from the Woods Hole Oceanographic Institution sent a submarine named Nereus to the bottom of the trench. On 25 March 2012, James Cameron reached the Challenger Deep in the Deepsea Challenger submersible, siphonophores and the Deep Scattering Layer. The US Navy account of the dive, with photographs Brewington, depth of the Deepest Dive in a Bathyscaphe. History of the Bathyscape Trieste FNRS-213,000 Feet Under the Sea in the French Bathyscaphe Popular Mechanics, May 1954, Deepsea Challenger – Mariana Trench Dive
36.
Electromagnet
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An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. The magnetic field disappears when the current is turned off, electromagnets usually consist of insulated wire wound into a coil. A current through the wire creates a field which is concentrated in the hole in the center of the coil. The main advantage of an electromagnet over a permanent magnet is that the field can be quickly changed by controlling the amount of electric current in the winding. However, unlike a permanent magnet that needs no power, an electromagnet requires a supply of current to maintain the magnetic field. Electromagnets are also employed in industry for picking up and moving heavy objects such as scrap iron. Danish scientist Hans Christian Ørsted discovered in 1820 that electric currents create magnetic fields, british scientist William Sturgeon invented the electromagnet in 1824. His first electromagnet was a piece of iron that was wrapped with about 18 turns of bare copper wire. The iron was varnished to insulate it from the windings, when a current was passed through the coil, the iron became magnetized and attracted other pieces of iron, when the current was stopped, it lost magnetization. Sturgeon displayed its power by showing that although it only weighed seven ounces, however, Sturgeons magnets were weak because the uninsulated wire he used could only be wrapped in a single spaced out layer around the core, limiting the number of turns. Beginning in 1830, US scientist Joseph Henry systematically improved and popularized the electromagnet, the first major use for electromagnets was in telegraph sounders. A portative electromagnet is one designed to just hold material in place, a tractive electromagnet applies a force and moves something. The solenoid is a coil of wire, and the plunger is made of a such as soft iron. Applying a current to the solenoid applies a force to the plunger, the plunger stops moving when the forces upon it are balanced. For example, the forces are balanced when the plunger is centered in the solenoid, the maximum uniform pull happens when one end of the plunger is at the middle of the solenoid. For units using inches, pounds force, and amperes with long, slender, solenoids, for example, a 12-inch long coil with a long plunger of 1-square inch cross section and 11,200 ampere-turns had a maximum pull of 8.75 pounds. The maximum pull is increased when a stop is inserted into the solenoid. The stop becomes a magnet that will attract the plunger, it adds little to the pull when the plunger is far away
37.
Nuclear reactor
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This article is a subarticle of Nuclear power. A nuclear reactor, formerly known as a pile, is a device used to initiate. Nuclear reactors are used at power plants for electricity generation. Heat from nuclear fission is passed to a fluid, which runs through steam turbines. These either drive a ships propellers or turn electrical generators, Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, some are run only for research. As of April 2014, the IAEA reports there are 435 nuclear power reactors in operation, when a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more nuclei, releasing kinetic energy, gamma radiation, and free neutrons. A portion of neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons. This is known as a chain reaction. To control such a chain reaction, neutron poisons and neutron moderators can change the portion of neutrons that will go on to cause more fission. Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if monitoring detects unsafe conditions, commonly-used moderators include regular water, solid graphite and heavy water. Some experimental types of reactor have used beryllium, and hydrocarbons have been suggested as another possibility, the reactor core generates heat in a number of ways, The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms. The reactor absorbs some of the rays produced during fission. Heat is produced by the decay of fission products and materials that have been activated by neutron absorption. This decay heat-source will remain for some even after the reactor is shut down. A kilogram of uranium-235 converted via nuclear processes releases approximately three times more energy than a kilogram of coal burned conventionally. A nuclear reactor coolant — usually water but sometimes a gas or a metal or molten salt — is circulated past the reactor core to absorb the heat that it generates
38.
Automation
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Some processes have been completely automated. The biggest benefit of automation is that it saves labor, however, it is used to save energy and materials and to improve quality, accuracy. The term automation, inspired by the word automatic, was not widely used before 1947. It was during this time industry was rapidly adopting feedback controllers. Automation has been achieved by means including mechanical, hydraulic, pneumatic, electrical, electronic devices and computers. Complicated systems, such as factories, airplanes and ships typically use all these combined techniques. Fundamentally, there are two types of control loop, open loop control, and closed loop control, in open loop control, the control action from the controller is independent of the process output. A good example of this is a central heating boiler controlled only by a timer, so heat is applied for a constant time. In closed loop control, the action from the controller is dependent on the process output. A closed loop controller therefore has a loop which ensures the controller exerts a control action to give a process output the same as the Reference input or set point. For this reason, closed loop controllers are also called feedback controllers, the theoretical basis of closed loop automation is control theory. The control action is the form of the output action. One of the simplest types of control is on-off control, an example is the thermostat used on household appliances which either opens or closes an electrical contact. Sequence control, in which a sequence of discrete operations is performed. An elevator control system is an example of sequence control, a proportional–integral–derivative controller is a control loop feedback mechanism widely used in industrial control systems. Sequential control may be either to a sequence or to a logical one that will perform different actions depending on various system states. An example of an adjustable but otherwise fixed sequence is a timer on a lawn sprinkler, States refer to the various conditions that can occur in a use or sequence scenario of the system. An example is an elevator, which uses logic based on the state to perform certain actions in response to its state
39.
Switch
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In electrical engineering, a switch is an electrical component that can make or break an electrical circuit, interrupting the current or diverting it from one conductor to another. The mechanism of a switch removes or restores the conducting path in a circuit when it is operated, the most familiar form of switch is a manually operated electromechanical device with one or more sets of electrical contacts, which are connected to external circuits. The mechanism actuating the transition between two states can be either a toggle or momentary type. A switch may be manipulated by a human as a control signal to a system, such as a computer keyboard button, or to control power flow in a circuit. Switches may be operated by process variables such as pressure, temperature, flow, current, voltage, for example, a thermostat is a temperature-operated switch used to control a heating process. A switch that is operated by electrical circuit is called a relay. Large switches may be operated by a motor drive mechanism. An ideal switch would have no voltage drop when closed, and it would have zero rise time and fall time during state changes, and would change state without bouncing between on and off positions. Practical switches fall short of ideal, they have resistance, limits on the current and voltage they can handle, finite switching time. The ideal switch is used in circuit analysis as it greatly simplifies the system of equations to be solved. Theoretical treatment of the effects of non-ideal properties is required in the design of networks of switches. In the simplest case, a switch has two pieces, often metal, called contacts, connected to an external circuit, that touch to complete the circuit. The contact material is chosen for its resistance to corrosion, because most metals form insulating oxides that would prevent the switch from working, contact materials are also chosen on the basis of electrical conductivity, hardness, mechanical strength, low cost and low toxicity. Sometimes the contacts are plated with noble metals and they may be designed to wipe against each other to clean off any contamination. Nonmetallic conductors, such as plastic, are sometimes used. To prevent the formation of insulating oxides, a minimum wetting current may be specified for a given switch design, in electronics, switches are classified according to the arrangement of their contacts. A pair of contacts is said to be closed when current can flow from one to the other, when the contacts are separated by an insulating air gap, they are said to be open, and no current can flow between them at normal voltages. The terms make for closure of contacts and break for opening of contacts are also widely used, the terms pole and throw are also used to describe switch contact variations
40.
Current loop
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In electrical signalling an analog current loop is used where a device must be monitored or controlled remotely over a pair of conductors. Only one current level can be present at any time and they are also used to transmit controller outputs to the modulating field devices such as control valves. These loops have the advantages of simplicity and noise immunity, and have an international user. Various Automation Protocols may replace analog current loops, but 4-20 mA is still an industrial standard. In industrial process control, analog 4–20 mA current loops are used for electronic signalling. These loops are used both for carrying information from field instrumentation, and carrying control signals to the process modulating devices. The key advantages of the current loop are, The loop can often power the device, with power supplied by the controller. Many instrumentation manufacturers produce 4-20 mA sensors which are loop powered, the live or elevated zero of 4 mA allows powering of the device even with no process signal output from the field transmitter. The accuracy of the signal is not affected by voltage drop in the interconnecting wiring and it has high noise immunity as it is low impedance circuit usually through twisted pair conductors. It is self-monitoring, currents less than 3.8 mA or more than 20.5 mA are taken to indicate a fault and it can be carried over long cables up to the limit of the resistance for the voltage used. In line displays can be inserted and powered by the loop, easy conversion to voltage using a resistor. Loop powered I to P converters can convert the 4-20 mA signal to a 3-15 psi pneumatic output for control valves, field instrumentation measurements are such as pressure, temperature, level, flow, pH or other process variables. A current loop can also be used to control a valve positioner or other output actuator, since input terminals of instruments may have one side of the current loop input tied to the chassis ground, analog isolators may be required when connecting several instruments in series. The relationship between current value and process variable measurement is set by calibration, which assigns different ranges of engineering units to the span between 4 and 20 mA. The mapping between engineering units and current can be inverted, so that 4 mA represents the maximum and 20 mA the minimum, depending on the source of current for the loop, devices may be classified as active or passive. For example, a chart recorder may provide power to a pressure transmitter. The pressure transmitter modulates the current on the loop to send the signal to the strip chart recorder, another loop may contain two passive chart recorders, a passive pressure transmitter, and a 24 V battery. Note that a 4-wire instrument has a power supply separate from the current loop
