1.
Consumer electronics
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Consumer electronics or home electronics are electronic or digital equipment intended for everyday use, typically in private homes. Consumer electronics include devices used for entertainment, communications, and home-office activities, in British English, they are often called brown goods by producers and sellers, to distinguish them from white goods such as washing machines and refrigerators. Radio broadcasting in the early 20th century brought the first major consumer product, later products included telephones, personal computers, MP3 players, audio equipment, televisions and calculators. In the 2010s, consumer electronics stores often sell GPS, automotive electronics, video game consoles, electronic instruments, karaoke machines, digital cameras. Stores also sell digital cameras, camcorders, cell phones, some consumer electronics stores, such as Best Buy have also begun selling office and baby furniture. Consumer electronics stores may be bricks and mortar retail stores, online stores. The CEA estimated the value of 2015 consumer electronics sales at US$220 billion, for its first fifty years the phonograph turntable did not use electronics, the needle and soundhorn were purely mechanical technologies. However, in the 1920s radio broadcasting became the basis of production of radio receivers. The vacuum tubes that had made radios practical were used with record players as well, television was soon invented, but remained insignificant in the consumer market until the 1950s. The transistor, invented in 1947 by Bell Laboratories, led to significant research in the field of semiconductors in the early 1950s. The transistors advantages revolutionized that industry along with other electronics, by 1959 Fairchild Semiconductor had introduced the first planar transistor from which come the origins of Moores Law. Integrated circuits followed when manufacturers built circuits on a substrate using electrical connections between circuits within the chip itself. One overriding characteristic of consumer products is the trend of ever-falling prices. This is driven by gains in manufacturing efficiency and automation, lower costs as manufacturing has moved to lower-wage countries. Semiconductor components benefit from Moores Law, a principle which states that, for a given price. While consumer electronics continues in its trend of convergence, combining elements of many products, there is an ever increasing need to keep product information updated and comparable, for the consumer to make an informed choice. Style, price, specification, and performance are all relevant, there is a gradual shift towards e-commerce web-storefronts. Many products include Internet connectivity using technologies such as Wi-Fi, Bluetooth, products not traditionally associated with computer use now provide options to connect to the Internet or to a computer using a home network to provide access to digital content
2.
Surface-mount technology
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Surface-mount technology is a method for producing electronic circuits in which the components are mounted or placed directly onto the surface of printed circuit boards. An electronic device so made is called a surface-mount device, in the industry it has largely replaced the through-hole technology construction method of fitting components with wire leads into holes in the circuit board. By employing SMT, the production process speeds up, but the risk of defects also increase due to the components miniaturization, in those conditions, the failures detection have become critical for any SMT manufacturing process. An SMT component is usually smaller than its through-hole counterpart because it has either smaller leads or no leads at all and it may have short pins or leads of various styles, flat contacts, a matrix of solder balls, or terminations on the body of the component. Surface mounting was originally called planar mounting, surface-mount technology was developed in the 1960s and became widely used in the late 1980s. Much of the work in this technology was by IBM. Components were mechanically redesigned to have metal tabs or end caps that could be directly soldered to the surface of the PCB. Components became much smaller and component placement on both sides of a board became far more common with surface mounting than through-hole mounting, allowing much higher circuit densities. Adhesive is sometimes used to hold SMT components on the side of a board if a wave soldering process is used to solder both SMT and through-hole components simultaneously. Surface mounting lends itself well to a degree of automation, reducing labor cost. SMDs can be one-quarter to one-tenth the size and weight, solder paste, a sticky mixture of flux and tiny solder particles, is first applied to all the solder pads with a stainless steel or nickel stencil using a screen printing process. It can also be applied by a mechanism, similar to an inkjet printer. After pasting, the boards then proceed to the pick-and-place machines, the components to be placed on the boards are usually delivered to the production line in either paper/plastic tapes wound on reels or plastic tubes. Some large integrated circuits are delivered in static-free trays, numerical control pick-and-place machines remove the parts from the tapes, tubes or trays and place them on the PCB. The boards are then conveyed into the reflow soldering oven and they first enter a pre-heat zone, where the temperature of the board and all the components is gradually, uniformly raised. The boards then enter a zone where the temperature is high enough to melt the solder particles in the solder paste, there are a number of techniques for reflowing solder. One is to use infrared lamps, this is called infrared reflow, another is to use a hot gas convection. Another technology which is becoming popular again is special fluorocarbon liquids with high boiling points which use a method called vapor phase reflow, due to environmental concerns, this method was falling out of favor until lead-free legislation was introduced which requires tighter controls on soldering
3.
Electron
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The electron is a subatomic particle, symbol e− or β−, with a negative elementary electric charge. Electrons belong to the first generation of the lepton particle family, the electron has a mass that is approximately 1/1836 that of the proton. Quantum mechanical properties of the include a intrinsic angular momentum of a half-integer value, expressed in units of the reduced Planck constant. As it is a fermion, no two electrons can occupy the same state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of particles and waves, they can collide with other particles and can be diffracted like light. Since an electron has charge, it has an electric field. Electromagnetic fields produced from other sources will affect the motion of an electron according to the Lorentz force law, electrons radiate or absorb energy in the form of photons when they are accelerated. Laboratory instruments are capable of trapping individual electrons as well as electron plasma by the use of electromagnetic fields, special telescopes can detect electron plasma in outer space. Electrons are involved in applications such as electronics, welding, cathode ray tubes, electron microscopes, radiation therapy, lasers, gaseous ionization detectors. Interactions involving electrons with other particles are of interest in fields such as chemistry. The Coulomb force interaction between the positive protons within atomic nuclei and the negative electrons without, allows the composition of the two known as atoms, ionization or differences in the proportions of negative electrons versus positive nuclei changes the binding energy of an atomic system. The exchange or sharing of the electrons between two or more atoms is the cause of chemical bonding. In 1838, British natural philosopher Richard Laming first hypothesized the concept of a quantity of electric charge to explain the chemical properties of atoms. Irish physicist George Johnstone Stoney named this charge electron in 1891, electrons can also participate in nuclear reactions, such as nucleosynthesis in stars, where they are known as beta particles. Electrons can be created through beta decay of isotopes and in high-energy collisions. The antiparticle of the electron is called the positron, it is identical to the electron except that it carries electrical, when an electron collides with a positron, both particles can be totally annihilated, producing gamma ray photons. The ancient Greeks noticed that amber attracted small objects when rubbed with fur, along with lightning, this phenomenon is one of humanitys earliest recorded experiences with electricity. In his 1600 treatise De Magnete, the English scientist William Gilbert coined the New Latin term electricus, both electric and electricity are derived from the Latin ēlectrum, which came from the Greek word for amber, ἤλεκτρον
4.
Vacuum
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Vacuum is space void of matter. The word stems from the Latin adjective vacuus for vacant or void, an approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. In engineering and applied physics on the hand, vacuum refers to any space in which the pressure is lower than atmospheric pressure. The Latin term in vacuo is used to describe an object that is surrounded by a vacuum, the quality of a partial vacuum refers to how closely it approaches a perfect vacuum. Other things equal, lower gas pressure means higher-quality vacuum, for example, a typical vacuum cleaner produces enough suction to reduce air pressure by around 20%. Ultra-high vacuum chambers, common in chemistry, physics, and engineering, operate below one trillionth of atmospheric pressure, outer space is an even higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average. In the electromagnetism in the 19th century, vacuum was thought to be filled with a medium called aether, in modern particle physics, the vacuum state is considered the ground state of matter. Vacuum has been a frequent topic of debate since ancient Greek times. Evangelista Torricelli produced the first laboratory vacuum in 1643, and other techniques were developed as a result of his theories of atmospheric pressure. A torricellian vacuum is created by filling a glass container closed at one end with mercury. Vacuum became an industrial tool in the 20th century with the introduction of incandescent light bulbs and vacuum tubes. The recent development of human spaceflight has raised interest in the impact of vacuum on human health, the word vacuum comes from Latin an empty space, void, noun use of neuter of vacuus, meaning empty, related to vacare, meaning be empty. Vacuum is one of the few words in the English language that contains two consecutive letters u. Historically, there has been dispute over whether such a thing as a vacuum can exist. Ancient Greek philosophers debated the existence of a vacuum, or void, in the context of atomism, Aristotle believed that no void could occur naturally, because the denser surrounding material continuum would immediately fill any incipient rarity that might give rise to a void. Almost two thousand years after Plato, René Descartes also proposed a geometrically based alternative theory of atomism, without the problematic nothing–everything dichotomy of void, by the ancient definition however, directional information and magnitude were conceptually distinct. The explanation of a clepsydra or water clock was a topic in the Middle Ages. Although a simple wine skin sufficed to demonstrate a partial vacuum, in principle and he concluded that airs volume can expand to fill available space, and he suggested that the concept of perfect vacuum was incoherent. However, according to Nader El-Bizri, the physicist Ibn al-Haytham and the Mutazili theologians disagreed with Aristotle and Al-Farabi, using geometry, Ibn al-Haytham mathematically demonstrated that place is the imagined three-dimensional void between the inner surfaces of a containing body
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Matter
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All the everyday objects that we can bump into, touch or squeeze are ultimately composed of atoms. This ordinary atomic matter is in turn made up of interacting subatomic particles—usually a nucleus of protons and neutrons, typically, science considers these composite particles matter because they have both rest mass and volume. By contrast, massless particles, such as photons, are not considered matter, however, not all particles with rest mass have a classical volume, since fundamental particles such as quarks and leptons are considered point particles with no effective size or volume. Nevertheless, quarks and leptons together make up ordinary matter, Matter exists in states, the classical solid, liquid, and gas, as well as the more exotic plasma, Bose–Einstein condensates, fermionic condensates, and quark–gluon plasma. For much of the history of the natural sciences people have contemplated the nature of matter. The idea that matter was built of discrete building blocks, the so-called particulate theory of matter, was first put forward by the Greek philosophers Leucippus and Democritus, Matter should not be confused with mass, as the two are not the same in modern physics. Matter is itself a physical substance of which systems may be composed, while mass is not a substance, while there are different views on what should be considered matter, the mass of a substance or system is the same irrespective of any such definition of matter. Another difference is that matter has an opposite called antimatter, antimatter has the same mass property as its normal matter counterpart. Different fields of use the term matter in different, and sometimes incompatible. Some of these ways are based on loose historical meanings, from a time there was no reason to distinguish mass from simply a quantity of matter. As such, there is no universally agreed scientific meaning of the word matter. Scientifically, the mass is well-defined, but matter can be defined in several ways. Sometimes in the field of matter is simply equated with particles that exhibit rest mass, such as quarks. However, in physics and chemistry, matter exhibits both wave-like and particle-like properties, the so-called wave–particle duality. A definition of based on its physical and chemical structure is. Such atomic matter is sometimes termed ordinary matter. As an example, deoxyribonucleic acid molecules are matter under this definition because they are made of atoms and this definition can extend to include charged atoms and molecules, so as to include plasmas and electrolytes, which are not obviously included in the atoms definition. Alternatively, one can adopt the protons, neutrons, and electrons definition, at a microscopic level, the constituent particles of matter such as protons, neutrons, and electrons obey the laws of quantum mechanics and exhibit wave–particle duality
6.
Vacuum tube
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In electronics, a vacuum tube, an electron tube, or just a tube, or valve, is a device that controls electric current between electrodes in an evacuated container. Vacuum tubes mostly rely on thermionic emission of electrons from a hot filament or a cathode heated by the filament and this type is called a thermionic tube or thermionic valve. A phototube, however, achieves electron emission through the photoelectric effect, the simplest vacuum tube, the diode, contains only a heater, a heated electron-emitting cathode, and a plate. Current can only flow in one direction through the device between the two electrodes, as electrons emitted by the travel through the tube and are collected by the anode. Adding one or more control grids within the tube allows the current between the cathode and anode to be controlled by the voltage on the grid or grids, Tubes with grids can be used for many purposes, including amplification, rectification, switching, oscillation, and display. In the 1940s the invention of devices made it possible to produce solid-state devices, which are smaller, more efficient, more reliable, more durable. Hence, from the mid-1950s solid-state devices such as transistors gradually replaced tubes, the cathode-ray tube remained the basis for televisions and video monitors until superseded in the 21st century. However, there are still a few applications for which tubes are preferred to semiconductors, for example, the used in microwave ovens. One classification of vacuum tubes is by the number of active electrodes, a device with two active elements is a diode, usually used for rectification. Devices with three elements are used for amplification and switching. Additional electrodes create tetrodes, pentodes, and so forth, which have additional functions made possible by the additional controllable electrodes. X-ray tubes are vacuum tubes. Phototubes and photomultipliers rely on electron flow through a vacuum, though in those cases electron emission from the cathode depends on energy from photons rather than thermionic emission, since these sorts of vacuum tubes have functions other than electronic amplification and rectification they are described in their own articles. A vacuum tube consists of two or more electrodes in a vacuum inside an airtight enclosure, most tubes have glass envelopes, though ceramic and metal envelopes have been used. The electrodes are attached to leads which pass through the envelope via an airtight seal, Tubes were a frequent cause of failure in electronic equipment, and consumers were expected to be able to replace tubes themselves. In addition to the terminals, some tubes had an electrode terminating at a top cap. The principal reason for doing this was to avoid leakage resistance through the tube base, the bases were commonly made with phenolic insulation which performs poorly as an insulator in humid conditions. There was even a design that had two top cap connections
7.
Amplifier
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An amplifier, electronic amplifier or amp is an electronic device that can increase the power of a signal. An amplifier functions by taking power from a supply and controlling the output to match the input signal shape. In this sense, an amplifier modulates the output of the power supply based upon the properties of the input signal, an amplifier is effectively the opposite of an attenuator, while an amplifier provides gain, an attenuator provides loss. An amplifier can either be a piece of equipment or an electrical circuit contained within another device. Amplification is fundamental to modern electronics, and amplifiers are used in almost all electronic equipment. Amplifiers can be categorized in different ways, another is which quantity, voltage or current is being amplified, amplifiers can be divided into voltage amplifiers, current amplifiers, transconductance amplifiers, and transresistance amplifiers. A further distinction is whether the output is a linear or nonlinear representation of the input, amplifiers can also be categorized by their physical placement in the signal chain. The first practical device that could amplify was the triode vacuum tube, invented in 1906 by Lee De Forest. Vacuum tubes were used in almost all amplifiers until the 1960s–1970s when the transistor, invented in 1947, today most amplifiers use transistors, but vacuum tubes continue to be used in some applications. Before the invention of electronic amplifiers, mechanically coupled carbon microphones were used as amplifiers in telephone repeaters. After the turn of the century it was found that negative resistance mercury lamps could amplify, the first practical electronic device that could amplify was the Audion vacuum tube, invented in 1906 by Lee De Forest, which led to the first amplifiers around 1912. The terms amplifier and amplification were first used for this new capability around 1915 when triodes became widespread, the amplifying vacuum tube revolutionized electrical technology, creating the new field of electronics, the technology of active electrical devices. It made possible long distance lines, public address systems, radio broadcasting, talking motion pictures, practical audio recording, radar, television. For 50 years virtually all electronic devices used vacuum tubes. Early tube amplifiers often had positive feedback, which could increase gain but also make the amplifier unstable, much of the mathematical theory of amplifiers was developed at Bell Telephone Laboratories during the 1920s to 1940s. Other advances in the theory of amplification were made by Harry Nyquist, the vacuum tube was the only amplifying device for 40 years, and dominated electronics until 1947, when the first transistor, the BJT, was invented. Today most amplifiers use transistors, but vacuum tubes are used in some high power applications such as radio transmitters. All amplifiers have gain, a factor that relates the magnitude of some property of the output signal to a property of the input signal
8.
Rectifier
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A rectifier is an electrical device that converts alternating current, which periodically reverses direction, to direct current, which flows in only one direction. The process is known as rectification, historically, even synchronous electromechanical switches and motors have been used. Early radio receivers, called crystal radios, used a cats whisker of fine wire pressing on a crystal of galena to serve as a rectifier or crystal detector. Rectifiers have many uses, but are found serving as components of DC power supplies. Rectification may serve in other than to generate direct current for use as a source of power. As noted, detectors of radio signals serve as rectifiers, in gas heating systems flame rectification is used to detect presence of a flame. Because of the nature of the input AC sine wave. Many applications of rectifiers, such as supplies for radio, television and computer equipment. In these applications the output of the rectifier is smoothed by a filter to produce a steady current. More complex circuitry that performs the function, converting DC to AC, is called an inverter. Before the development of semiconductor rectifiers, vacuum tube thermionic diodes. With the introduction of electronics, vacuum tube rectifiers became obsolete. For power rectification from very low to high current, semiconductor diodes of various types are widely used. Other devices that have control electrodes as well as acting as unidirectional current valves are used more than simple rectification is required—e. g. Where variable output voltage is needed, high-power rectifiers, such as those used in high-voltage direct current power transmission, employ silicon semiconductor devices of various types. These are thyristors or other controlled switching solid-state switches, which function as diodes to pass current in only one direction. Rectifier circuits may be single-phase or multi-phase, most low power rectifiers for domestic equipment are single-phase, but three-phase rectification is very important for industrial applications and for the transmission of energy as DC. Because only one half of the input waveform reaches the output, half-wave rectification requires a single diode in a single-phase supply, or three in a three-phase supply
9.
Transistor
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A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit, a voltage or current applied to one pair of the transistors terminals controls the current through another pair of terminals. Because the controlled power can be higher than the controlling power, today, some transistors are packaged individually, but many more are found embedded in integrated circuits. The transistor is the building block of modern electronic devices. Julius Edgar Lilienfeld patented a field-effect transistor in 1926 but it was not possible to construct a working device at that time. The first practically implemented device was a point-contact transistor invented in 1947 by American physicists John Bardeen, Walter Brattain, the transistor revolutionized the field of electronics, and paved the way for smaller and cheaper radios, calculators, and computers, among other things. The transistor is on the list of IEEE milestones in electronics, and Bardeen, Brattain, the thermionic triode, a vacuum tube invented in 1907, enabled amplified radio technology and long-distance telephony. The triode, however, was a device that consumed a substantial amount of power. Physicist Julius Edgar Lilienfeld filed a patent for a transistor in Canada in 1925. Lilienfeld also filed patents in the United States in 1926 and 1928. However, Lilienfeld did not publish any research articles about his devices nor did his patents cite any examples of a working prototype. In 1934, German inventor Oskar Heil patented a device in Europe. Solid State Physics Group leader William Shockley saw the potential in this, the term transistor was coined by John R. Pierce as a contraction of the term transresistance. Instead, what Bardeen, Brattain, and Shockley invented in 1947 was the first point-contact transistor, Mataré had previous experience in developing crystal rectifiers from silicon and germanium in the German radar effort during World War II. Using this knowledge, he began researching the phenomenon of interference in 1947, realizing that Bell Labs scientists had already invented the transistor before them, the company rushed to get its transistron into production for amplified use in Frances telephone network. The first bipolar junction transistors were invented by Bell labs William Shockley, on April 12,1950, Bell Labs chemists Gordon Teal and Morgan Sparks had successfully produced a working bipolar NPN junction amplifying germanium transistor. Bell Labs had made this new sandwich transistor discovery announcement, in a release on July 4,1951. The first high-frequency transistor was the surface-barrier germanium transistor developed by Philco in 1953 and these were made by etching depressions into an N-type germanium base from both sides with jets of Indium sulfate until it was a few ten-thousandths of an inch thick
10.
Integrated circuit
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An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material, normally silicon. The ICs mass production capability, reliability and building-block approach to circuit design ensured the rapid adoption of standardized ICs in place of using discrete transistors. ICs are now used in all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other home appliances are now inextricable parts of the structure of modern societies, made possible by the small size. These advances, roughly following Moores law, allow a computer chip of 2016 to have millions of times the capacity, ICs have two main advantages over discrete circuits, cost and performance. Cost is low because the chips, with all their components, are printed as a unit by photolithography rather than being constructed one transistor at a time, furthermore, packaged ICs use much less material than discrete circuits. Performance is high because the ICs components switch quickly and consume little power because of their small size, the main disadvantage of ICs is the high cost to design them and fabricate the required photomasks. This high initial cost means ICs are only practical when high production volumes are anticipated, Circuits meeting this definition can be constructed using many different technologies, including thin-film transistor, thick film technology, or hybrid integrated circuit. However, in general usage integrated circuit has come to refer to the single-piece circuit construction originally known as a integrated circuit. Jacobi disclosed small and cheap hearing aids as typical industrial applications of his patent, an immediate commercial use of his patent has not been reported. The idea of the circuit was conceived by Geoffrey Dummer. Dummer presented the idea to the public at the Symposium on Progress in Quality Electronic Components in Washington and he gave many symposia publicly to propagate his ideas, and unsuccessfully attempted to build such a circuit in 1956. A precursor idea to the IC was to create small ceramic squares, Components could then be integrated and wired into a bidimensional or tridimensional compact grid. This idea, which seemed very promising in 1957, was proposed to the US Army by Jack Kilby, however, as the project was gaining momentum, Kilby came up with a new, revolutionary design, the IC. In his patent application of 6 February 1959, Kilby described his new device as a body of semiconductor material … wherein all the components of the circuit are completely integrated. The first customer for the new invention was the US Air Force, Kilby won the 2000 Nobel Prize in Physics for his part in the invention of the integrated circuit. His work was named an IEEE Milestone in 2009, half a year after Kilby, Robert Noyce at Fairchild Semiconductor developed his own idea of an integrated circuit that solved many practical problems Kilbys had not. Noyces design was made of silicon, whereas Kilbys chip was made of germanium, Noyce credited Kurt Lehovec of Sprague Electric for the principle of p–n junction isolation, a key concept behind the IC
11.
Telecommunication
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Telecommunication is the transmission of signs, signals, messages, writings, images and sounds or intelligence of any nature by wire, radio, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology and it is transmitted either electrically over physical media, such as cables, or via electromagnetic radiation. Such transmission paths are divided into communication channels which afford the advantages of multiplexing. The term is used in its plural form, telecommunications. Early means of communicating over a distance included visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, other examples of pre-modern long-distance communication included audio messages such as coded drumbeats, lung-blown horns, and loud whistles. Zworykin, John Logie Baird and Philo Farnsworth, the word telecommunication is a compound of the Greek prefix tele, meaning distant, far off, or afar, and the Latin communicare, meaning to share. Its modern use is adapted from the French, because its use was recorded in 1904 by the French engineer. Communication was first used as an English word in the late 14th century, in the Middle Ages, chains of beacons were commonly used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could pass a single bit of information. One notable instance of their use was during the Spanish Armada, in 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system between Lille and Paris. However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres, as a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880. Homing pigeons have occasionally used throughout history by different cultures. Pigeon post is thought to have Persians roots and was used by the Romans to aid their military, frontinus said that Julius Caesar used pigeons as messengers in his conquest of Gaul. The Greeks also conveyed the names of the victors at the Olympic Games to various cities using homing pigeons, in the early 19th century, the Dutch government used the system in Java and Sumatra. And in 1849, Paul Julius Reuter started a service to fly stock prices between Aachen and Brussels, a service that operated for a year until the gap in the telegraph link was closed. Sir Charles Wheatstone and Sir William Fothergill Cooke invented the telegraph in 1837. Also, the first commercial electrical telegraph is purported to have constructed by Wheatstone and Cooke. Both inventors viewed their device as an improvement to the electromagnetic telegraph not as a new device, samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837
12.
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
