A pager is a wireless telecommunications device that receives and displays alphanumeric or voice messages. One-way pagers can only receive messages, while response pagers and two-way pagers can acknowledge, reply to, originate messages using an internal transmitter. Pagers operate as part of a paging system which includes one or more fixed transmitters, as well as a number of pagers carried by mobile users; these systems can range from a restaurant system with a single low-power transmitter, to a nationwide system with thousands of high-power base stations. Pagers were developed in the 1950s and 1960s, became used by the 1980s. In the 21st century, the widespread availability of cellphones and smartphones has diminished the pager industry. Pagers continue to be used by some emergency services and public safety personnel, because modern pager systems' coverage overlap, combined with use of satellite communications, can make paging systems more reliable than terrestrial-based cellular networks in some cases, including during natural and man-made disasters.
This resilience has led public safety agencies to adopt pagers over cellular and other commercial services for critical messaging. The UK National Health Service is thought to use over 10% of remaining pagers in 2017, with an annual cost of £6.6 million. Matt Hancock announced in February 2019; the first telephone pager system was patented in 1949 by Alfred J. Gross. One of the first practical paging services was launched in 1950 for physicians in the New York City area. Physicians paid $12 per month for the service and carried a 200-gram pager that would receive phone messages within 40 kilometres of a single transmitter tower; the system was operated by Telanswerphone. In 1960, John Francis Mitchell combined elements of Motorola's walkie-talkie and automobile radio technologies to create the first transistorized pager, from that time, paging technology continued to advance, pager adoption among emergency personnel is still popular, as of July 2016. In 1962 the Bell System—the U. S. telephone monopoly colloquially known as "Ma Bell"—presented its Bellboy radio paging system at the Seattle World's Fair.
Bellboy was the first commercial system for personal paging. It marked one of the first consumer applications of the transistor, for which three Bell Labs inventors received a Nobel Prize in Physics in 1956. Solid-state circuitry enabled the Bellboy pager, about the size of a small TV remote device, to fit into a customer's pocket or purse, quite a feat at that time; the Bellboy was a terminal. When the person received an audible signal on the pager, he found a telephone and called the service centre, which informed him of the caller's message. Bell System Bellboy radio pagers each used three reed receiver relays, each relay tuned to one of 33 different frequencies, selectively ringing a particular customer when all three relays were activated at the same time—a precursor of DTMF; the ReFLEX protocol was developed in the mid-1990s. While Motorola announced the end of its new pager manufacturing in 2001, pagers remained in use in large hospital complexes. Another is a facility handling classified information, where various radio transmitter or data storage devices are excluded to ensure security.
First responders in rural areas with inadequate cellular coverage are issued pagers. The 2005 London bombings resulted in overload of TETRA systems by the emergency services, showed that pagers, with their absence of necessity to transmit an acknowledgement before showing the message, the related capability to operate on low signal levels, are not outclassed by their successors. Volunteer firefighters, EMS paramedics, rescue squad members carry pagers to alert them of emergency call outs for their department; these pagers receive a special tone from a fire department radio frequency. Restaurant pagers were in wide use in the 2000s. Customers were given a portable receiver that vibrates, flashes, or beeps when a table becomes free or when their meal is ready. Pagers have been popular with birdwatchers in Britain and Ireland since 1991, with companies Rare Bird Alert and Birdnet Information offering news of rare birds sent to pagers that they sell; the U. S. paging industry generated $2.1 billion in revenue in 2008, down from $6.2 billion in 2003.
In Canada, 161,500 Canadians paid $18.5 million for pager service in 2013. Telus, one of the three major mobile carriers, announced the end to its Canadian pager service as of March 31, 2015, but rivals Bell and PageNet intend to continue service. Many paging network operators now allow numeric and textual pages to be submitted to the paging networks via email; this is convenient for many users, due to the widespread adoption of email. This can result in pager messages being lost. Older forms of message submission using the Telelocator Alphanumeric Protocol involve modem connections directly to a paging network, are less subject to these delays. For this reason, older forms of message submission retain their usefulness for disseminating highly-important alerts to users such as emergency services personnel. Common paging protocols include TAP, FLEX, ReFLEX, POCSAG, GOLAY, ERMES and NTT. Past paging protocols include 5/6-tone. In the United States, pagers receive signals using the FLEX protocol in th
A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as rheostat; the measuring instrument called a potentiometer is a voltage divider used for measuring electric potential. Potentiometers are used to control electrical devices such as volume controls on audio equipment. Potentiometers operated by a mechanism can be used as position transducers, for example, in a joystick. Potentiometers are used to directly control significant power, since the power dissipated in the potentiometer would be comparable to the power in the controlled load. There are a number of terms in the electronics industry used to describe certain types of potentiometers: slide pot or slider pot: a potentiometer, adjusted by sliding the wiper left or right with a finger or thumb thumb pot or thumbwheel pot: a small rotating potentiometer meant to be adjusted infrequently by means of a small thumbwheel trimpot or trimmer pot: a trimmer potentiometer meant to be adjusted once or infrequently for "fine-tuning" an electrical signal Potentiometers consist of a resistive element, a sliding contact that moves along the element, making good electrical contact with one part of it, electrical terminals at each end of the element, a mechanism that moves the wiper from one end to the other, a housing containing the element and wiper.
See drawing. Many inexpensive potentiometers are constructed with a resistive element formed into an arc of a circle a little less than a full turn and a wiper sliding on this element when rotated, making electrical contact; the resistive element can be angled. Each end of the resistive element is connected to a terminal on the case; the wiper is connected to a third terminal between the other two. On panel potentiometers, the wiper is the center terminal of three. For single-turn potentiometers, this wiper travels just under one revolution around the contact; the only point of ingress for contamination is the narrow space between the shaft and the housing it rotates in. Another type is the linear slider potentiometer, which has a wiper which slides along a linear element instead of rotating. Contamination can enter anywhere along the slot the slider moves in, making effective sealing more difficult and compromising long-term reliability. An advantage of the slider potentiometer is that the slider position gives a visual indication of its setting.
While the setting of a rotary potentiometer can be seen by the position of a marking on the knob, an array of sliders can give a visual impression of, for example, the effect of a multi-band equalizer. The resistive element of inexpensive potentiometers is made of graphite. Other materials used include resistance wire, carbon particles in plastic, a ceramic/metal mixture called cermet. Conductive track potentiometers use conductive polymer resistor pastes that contain hard-wearing resins and polymers and lubricant, in addition to the carbon that provides the conductive properties. Multiturn potentiometers are operated by rotating a shaft, but by several turns rather than less than a full turn; some multiturn potentiometers have a linear resistive element with a sliding contact moved by a lead screw. Multiturn potentiometers, both user-accessible and preset, allow finer adjustments. A string potentiometer is a multi-turn potentiometer operated by an attached reel of wire turning against a spring, enabling it to convert linear position to a variable resistance.
User-accessible rotary potentiometers can be fitted with a switch which operates at the anti-clockwise extreme of rotation. Before digital electronics became the norm such a component was used to allow radio and television receivers and other equipment to be switched on at minimum volume with an audible click the volume increased, by turning a knob. Multiple resistance elements can be ganged together with their sliding contacts on the same shaft, for example, in stereo audio amplifiers for volume control. In other applications, such as domestic light dimmers, the normal usage pattern is best satisfied if the potentiometer remains set at its current position, so the switch is operated by a push action, alternately on and off, by axial presses of the knob. Others are enclosed within the equipment and are intended to be adjusted to calibrate equipment during manufacture or repair, not otherwise touched, they are physically much smaller than user-accessible potentiometers, may need to be operated by a screwdriver rather than having a knob.
They are called "preset potentiometers" or "trim pots". Some presets are accessible by a small screwdriver poked through a hole in the case to allow servicing without dismantling; the relationship between slider position and resistance, known as the "taper" or "law", is controlled by the manufacturer. In principle any relationship is possible, but for most purposes linear or logarithmic potentiometers are sufficient. A letter code may be used to identify which taper is used, but the letter code definitions are not standardized. Potentiometers made in Asia and the USA are marked with an "A" for logarithmic taper or a "B" for linear taper
History of the Internet
The history of the Internet begins with the development of electronic computers in the 1950s. Initial concepts of wide area networking originated in several computer science laboratories in the United States, United Kingdom, France; the U. S. Department of Defense awarded contracts as early as the 1960s, including for the development of the ARPANET project, directed by Robert Taylor and managed by Lawrence Roberts; the first message was sent over the ARPANET in 1969 from computer science Professor Leonard Kleinrock's laboratory at University of California, Los Angeles to the second network node at Stanford Research Institute. Packet switching networks such as the NPL network, ARPANET, Merit Network, CYCLADES, Telenet, were developed in the late 1960s and early 1970s using a variety of communications protocols. Donald Davies first demonstrated packet switching in 1967 at the National Physics Laboratory in the UK, which became a testbed for UK research for two decades; the ARPANET project led to the development of protocols for internetworking, in which multiple separate networks could be joined into a network of networks.
The Internet protocol suite was developed by Robert E. Kahn and Vint Cerf in the 1970s and became the standard networking protocol on the ARPANET, incorporating concepts from the French CYCLADES project directed by Louis Pouzin. In the early 1980s the NSF funded the establishment for national supercomputing centers at several universities, provided interconnectivity in 1986 with the NSFNET project, which created network access to the supercomputer sites in the United States from research and education organizations. Commercial Internet service providers began to emerge in the late 1980s; the ARPANET was decommissioned in 1990. Limited private connections to parts of the Internet by commercial entities emerged in several American cities by late 1989 and 1990, the NSFNET was decommissioned in 1995, removing the last restrictions on the use of the Internet to carry commercial traffic. In the 1980s, research at CERN in Switzerland by British computer scientist Tim Berners-Lee resulted in the World Wide Web, linking hypertext documents into an information system, accessible from any node on the network.
Since the mid-1990s, the Internet has had a revolutionary impact on culture and technology, including the rise of near-instant communication by electronic mail, instant messaging, voice over Internet Protocol telephone calls, two-way interactive video calls, the World Wide Web with its discussion forums, social networking, online shopping sites. The research and education community continues to develop and use advanced networks such as JANET in the United Kingdom and Internet2 in the United States. Increasing amounts of data are transmitted at higher and higher speeds over fiber optic networks operating at 1 Gbit/s, 10 Gbit/s, or more; the Internet's takeover of the global communication landscape was instant in historical terms: it only communicated 1% of the information flowing through two-way telecommunications networks in the year 1993 51% by 2000, more than 97% of the telecommunicated information by 2007. Today the Internet continues to grow, driven by greater amounts of online information, commerce and social networking.
However, the future of the global internet may be shaped by regional differences in the world. The concept of data communication – transmitting data between two different places through an electromagnetic medium such as radio or an electric wire – pre-dates the introduction of the first computers; such communication systems were limited to point to point communication between two end devices. Semaphore lines, telegraph systems and telex machines can be considered early precursors of this kind of communication; the Telegraph in the late 19th century was the first digital communication system. Fundamental theoretical work in data transmission and information theory was developed by Claude Shannon, Harry Nyquist, Ralph Hartley in the early 20th century. Early computers had remote terminals; as the technology evolved, new systems were devised to allow communication over longer distances or with higher speed that were necessary for the mainframe computer model. These technologies made it possible to exchange data between remote computers.
However, the point-to-point communication model was limited, as it did not allow for direct communication between any two arbitrary systems. The technology was considered unsafe for strategic and military use because there were no alternative paths for the communication in case of an enemy attack. With limited exceptions, the earliest computers were connected directly to terminals used by individual users in the same building or site; such networks became known as local area networks. Networking beyond this scope, known as wide area networks, emerged during the 1950s and became established during the 1960s. J. C. R. Licklider, Vice President at Bolt Beranek and Newman, Inc. proposed a global network in his January 1960 paper Man-Computer Symbiosis: A network of such centers, connected to one another by wide-band communication lines the functions of present-day libraries together with anticipated advances in information storage and retrieval and symbiotic functions suggested earlier in this paper In August 1962, Licklider and Welden Clark published the paper "On-Line Man-Computer Communication", one of the first descriptions of a networked future.
In October 1962, Licklider was hired by Jack Ruina as director of the newly established Information Processing Techniques Office w
Smartphones are a class of mobile phones and of multi-purpose mobile computing devices. They are distinguished from feature phones by their stronger hardware capabilities and extensive mobile operating systems, which facilitate wider software and multimedia functionality, alongside core phone functions such as voice calls and text messaging. Smartphones include various sensors that can be leveraged by their software, such as a magnetometer, proximity sensors, barometer and accelerometer, support wireless communications protocols such as Bluetooth, Wi-Fi, satellite navigation. Early smartphones were marketed towards the enterprise market, attempting to bridge the functionality of standalone personal digital assistant devices with support for cellular telephony, but were limited by their battery life, bulky form, the immaturity of wireless data services. In the 2000s, BlackBerry, Nokia's Symbian platform, Windows Mobile began to gain market traction, with models featuring QWERTY keyboards or resistive touchscreen input, emphasizing access to push email and wireless internet.
Since the unveiling of the iPhone in 2007, the majority of smartphones have featured thin, slate-like form factors, with large, capacitive screens with support for multi-touch gestures rather than physical keyboards, offer the ability for users to download or purchase additional applications from a centralized store, use cloud storage and synchronization, virtual assistants, as well as mobile payment services. Improved hardware and faster wireless communication have bolstered the growth of the smartphone industry. In the third quarter of 2012, one billion smartphones were in use worldwide. Global smartphone sales surpassed the sales figures for feature phones in early 2013; the first commercially available device that could be properly referred to as a "smartphone" began as a prototype called "Angler" developed by Frank Canova in 1992 while at IBM and demonstrated in November of that year at the COMDEX computer industry trade show. A refined version was marketed to consumers in 1994 by BellSouth under the name Simon Personal Communicator.
In addition to placing and receiving cellular calls, the touchscreen-equipped Simon could send and receive faxes and emails. It included an address book, appointment scheduler, world time clock, notepad, as well as other visionary mobile applications such as maps, stock reports and news; the term "smart phone" or "smartphone" was not coined until a year after the introduction of the Simon, appearing in print as early as 1995, describing AT&T's PhoneWriter Communicator. Beginning in the mid-late 1990s, many people who had mobile phones carried a separate dedicated PDA device, running early versions of operating systems such as Palm OS, Newton OS, Symbian or Windows CE/Pocket PC; these operating systems would evolve into early mobile operating systems. Most of the "smartphones" in this era were hybrid devices that combined these existing familiar PDA OSes with basic phone hardware; the results were devices that were bulkier than either dedicated mobile phones or PDAs, but allowed a limited amount of cellular Internet access.
The trend at the time, that manufacturers competed on in both mobile phones and PDAs was to make devices smaller and slimmer. The bulk of these smartphones combined with their high cost and expensive data plans, plus other drawbacks such as expansion limitations and decreased battery life compared to separate standalone devices limited their popularity to "early adopters" and business users who needed portable connectivity. In March 1996, Hewlett-Packard released the OmniGo 700LX, a modified HP 200LX palmtop PC with a Nokia 2110 mobile phone piggybacked onto it and ROM-based software to support it, it had a 640×200 resolution CGA compatible four-shade gray-scale LCD screen and could be used to place and receive calls, to create and receive text messages and faxes. It was 100% DOS 5.0 compatible, allowing it to run thousands of existing software titles, including early versions of Windows. In August 1996, Nokia released the Nokia 9000 Communicator, a digital cellular PDA based on the Nokia 2110 with an integrated system based on the PEN/GEOS 3.0 operating system from Geoworks.
The two components were attached by a hinge in what became known as a clamshell design, with the display above and a physical QWERTY keyboard below. The PDA provided e-mail; when closed, the device could be used as a digital cellular telephone. In June 1999 Qualcomm released the "pdQ Smartphone", a CDMA digital PCS smartphone with an integrated Palm PDA and Internet connectivity. Subsequent landmark devices included: The Ericsson R380 by Ericsson Mobile Communications; the first device marketed as a "smartphone", it was the first Symbian-based phone, with PDA functionality and limited Web browsing on a resistive touchscreen utilizing a stylus. Users could not install their own software on the device, however; the Kyocera 6035, a dual-nature device with a separate Palm OS PDA operating system and CDMA mobile phone firmware. It supported limited Web browsing with the PDA software treating the phone hardware as an attached modem. Handspring's Treo 180, the first smartphone that integrated the Palm OS on a GSM mobile phone having telephony, SMS messaging and Internet access built in to the OS.
The 180 model had a thumb-type keyboard and the 180g version had a Graffiti handwriting recognition area, instead. In 1999, Japanese wireless provider NTT DoCoMo launched i-mode, a new
Chicago the City of Chicago, is the most populous city in Illinois, as well as the third most populous city in the United States. With an estimated population of 2,716,450, it is the most populous city in the Midwest. Chicago is the principal city of the Chicago metropolitan area referred to as Chicagoland, the county seat of Cook County, the second most populous county in the United States; the metropolitan area, at nearly 10 million people, is the third-largest in the United States, the fourth largest in North America and the third largest metropolitan area in the world by land area. Located on the shores of freshwater Lake Michigan, Chicago was incorporated as a city in 1837 near a portage between the Great Lakes and the Mississippi River watershed and grew in the mid-nineteenth century. After the Great Chicago Fire of 1871, which destroyed several square miles and left more than 100,000 homeless, the city made a concerted effort to rebuild; the construction boom accelerated population growth throughout the following decades, by 1900 Chicago was the fifth largest city in the world.
Chicago made noted contributions to urban planning and zoning standards, including new construction styles, the development of the City Beautiful Movement, the steel-framed skyscraper. Chicago is an international hub for finance, commerce, technology, telecommunications, transportation, it is the site of the creation of the first standardized futures contracts at the Chicago Board of Trade, which today is the largest and most diverse derivatives market gobally, generating 20% of all volume in commodities and financial futures. O'Hare International Airport is the one of the busiest airports in the world, the region has the largest number of U. S. highways and greatest amount of railroad freight. In 2012, Chicago was listed as an alpha global city by the Globalization and World Cities Research Network, it ranked seventh in the entire world in the 2017 Global Cities Index; the Chicago area has one of the highest gross domestic products in the world, generating $680 billion in 2017. In addition, the city has one of the world's most diversified and balanced economies, not being dependent on any one industry, with no single industry employing more than 14% of the workforce.
Chicago's 58 million domestic and international visitors in 2018, made it the second most visited city in the nation, behind New York City's approximate 65 million visitors. The city ranked first place in the 2018 Time Out City Life Index, a global quality of life survey of 15,000 people in 32 cities. Landmarks in the city include Millennium Park, Navy Pier, the Magnificent Mile, the Art Institute of Chicago, Museum Campus, the Willis Tower, Grant Park, the Museum of Science and Industry, Lincoln Park Zoo. Chicago's culture includes the visual arts, film, comedy and music jazz, soul, hip-hop and electronic dance music including house music. Of the area's many colleges and universities, the University of Chicago, Northwestern University, the University of Illinois at Chicago are classified as "highest research" doctoral universities. Chicago has professional sports teams in each of the major professional leagues, including two Major League Baseball teams; the name "Chicago" is derived from a French rendering of the indigenous Miami-Illinois word shikaakwa for a wild relative of the onion, known to botanists as Allium tricoccum and known more as ramps.
The first known reference to the site of the current city of Chicago as "Checagou" was by Robert de LaSalle around 1679 in a memoir. Henri Joutel, in his journal of 1688, noted that the eponymous wild "garlic" grew abundantly in the area. According to his diary of late September 1687:...when we arrived at the said place called "Chicagou" which, according to what we were able to learn of it, has taken this name because of the quantity of garlic which grows in the forests in this region. The city has had several nicknames throughout its history such as the Windy City, Chi-Town, Second City, the City of the Big Shoulders, which refers to the city's numerous skyscrapers and high-rises. In the mid-18th century, the area was inhabited by a Native American tribe known as the Potawatomi, who had taken the place of the Miami and Sauk and Fox peoples; the first known non-indigenous permanent settler in Chicago was Jean Baptiste Point du Sable. Du Sable arrived in the 1780s, he is known as the "Founder of Chicago".
In 1795, following the Northwest Indian War, an area, to be part of Chicago was turned over to the United States for a military post by native tribes in accordance with the Treaty of Greenville. In 1803, the United States Army built Fort Dearborn, destroyed in 1812 in the Battle of Fort Dearborn and rebuilt; the Ottawa and Potawatomi tribes had ceded additional land to the United States in the 1816 Treaty of St. Louis; the Potawatomi were forcibly removed from their land after the Treaty of Chicago in 1833. On August 12, 1833, the Town of Chicago was organized with a population of about 200. Within seven years it grew to more than 4,000 people. On June 15, 1835, the first public land sales began with Edmund Dick Taylor as U. S. Receiver of Public Monies; the City of Chicago was incorporated on Saturday, March 4, 1837, for several decades was the world's fastest-growing city. As the site of the Chicago Portage, the city became an important transportation hub between the eastern and western United States.
Chicago's first railway and Chicago Union Railroad, the Illi
A computer network is a digital telecommunications network which allows nodes to share resources. In computer networks, computing devices exchange data with each other using connections between nodes; these data links are established over cable media such as wires or optic cables, or wireless media such as Wi-Fi. Network computer devices that originate and terminate the data are called network nodes. Nodes are identified by network addresses, can include hosts such as personal computers and servers, as well as networking hardware such as routers and switches. Two such devices can be said to be networked together when one device is able to exchange information with the other device, whether or not they have a direct connection to each other. In most cases, application-specific communications protocols are layered over other more general communications protocols; this formidable collection of information technology requires skilled network management to keep it all running reliably. Computer networks support an enormous number of applications and services such as access to the World Wide Web, digital video, digital audio, shared use of application and storage servers and fax machines, use of email and instant messaging applications as well as many others.
Computer networks differ in the transmission medium used to carry their signals, communications protocols to organize network traffic, the network's size, traffic control mechanism and organizational intent. The best-known computer network is the Internet; the chronology of significant computer-network developments includes: In the late 1950s, early networks of computers included the U. S. military radar system Semi-Automatic Ground Environment. In 1959, Anatolii Ivanovich Kitov proposed to the Central Committee of the Communist Party of the Soviet Union a detailed plan for the re-organisation of the control of the Soviet armed forces and of the Soviet economy on the basis of a network of computing centres, the OGAS. In 1960, the commercial airline reservation system semi-automatic business research environment went online with two connected mainframes. In 1963, J. C. R. Licklider sent a memorandum to office colleagues discussing the concept of the "Intergalactic Computer Network", a computer network intended to allow general communications among computer users.
In 1964, researchers at Dartmouth College developed the Dartmouth Time Sharing System for distributed users of large computer systems. The same year, at Massachusetts Institute of Technology, a research group supported by General Electric and Bell Labs used a computer to route and manage telephone connections. Throughout the 1960s, Paul Baran and Donald Davies independently developed the concept of packet switching to transfer information between computers over a network. Davies pioneered the implementation of the concept with the NPL network, a local area network at the National Physical Laboratory using a line speed of 768 kbit/s. In 1965, Western Electric introduced the first used telephone switch that implemented true computer control. In 1966, Thomas Marill and Lawrence G. Roberts published a paper on an experimental wide area network for computer time sharing. In 1969, the first four nodes of the ARPANET were connected using 50 kbit/s circuits between the University of California at Los Angeles, the Stanford Research Institute, the University of California at Santa Barbara, the University of Utah.
Leonard Kleinrock carried out theoretical work to model the performance of packet-switched networks, which underpinned the development of the ARPANET. His theoretical work on hierarchical routing in the late 1970s with student Farouk Kamoun remains critical to the operation of the Internet today. In 1972, commercial services using X.25 were deployed, used as an underlying infrastructure for expanding TCP/IP networks. In 1973, the French CYCLADES network was the first to make the hosts responsible for the reliable delivery of data, rather than this being a centralized service of the network itself. In 1973, Robert Metcalfe wrote a formal memo at Xerox PARC describing Ethernet, a networking system, based on the Aloha network, developed in the 1960s by Norman Abramson and colleagues at the University of Hawaii. In July 1976, Robert Metcalfe and David Boggs published their paper "Ethernet: Distributed Packet Switching for Local Computer Networks" and collaborated on several patents received in 1977 and 1978.
In 1979, Robert Metcalfe pursued making Ethernet an open standard. In 1976, John Murphy of Datapoint Corporation created ARCNET, a token-passing network first used to share storage devices. In 1995, the transmission speed capacity for Ethernet increased from 10 Mbit/s to 100 Mbit/s. By 1998, Ethernet supported transmission speeds of a Gigabit. Subsequently, higher speeds of up to 400 Gbit/s were added; the ability of Ethernet to scale is a contributing factor to its continued use. Computer networking may be considered a branch of electrical engineering, electronics engineering, telecommunications, computer science, information technology or computer engineering, since it relies upon the theoretical and practical application of the related disciplines. A computer network facilitates interpersonal communications allowing users to communicate efficiently and via various means: email, instant messaging, online chat, video telephone calls, video conferencing. A network allows sharing of computing resources.
Users may access and use resources provided by devices on the network, such as printing a document on a shared network printer or use of a shared storage device. A network allows sharing of files, and
John Logie Baird
John Logie Baird FRSE was a Scottish engineer, one of the inventors of the mechanical television, demonstrating the first working television system on 26 January 1926, inventor of both the first publicly demonstrated colour television system, the first purely electronic colour television picture tube. In 1928 the Baird Television Development Company achieved the first transatlantic television transmission. Baird's early technological successes and his role in the practical introduction of broadcast television for home entertainment have earned him a prominent place in television's history. Baird was ranked number 44 in the BBC's list of the 100 Greatest Britons following a UK-wide vote in 2002. In 2006, Baird was named as one of the 10 greatest Scottish scientists in history, having been listed in the National Library of Scotland's'Scottish Science Hall of Fame'. In 2015 he was inducted into the Scottish Engineering Hall of Fame. Baird was born on 13 August 1888 in Helensburgh and was the youngest of four children of the Reverend John Baird, the Church of Scotland's minister for the local St Bride's Church and Jessie Morrison Inglis, the orphaned niece of a wealthy family of shipbuilders from Glasgow.
He was educated at Larchfield Academy in Helensburgh. While at college Baird undertook a series of engineering apprentice jobs as part of his course; the conditions in industrial Glasgow at the time helped form his socialist convictions but contributed to his ill health. He became an agnostic, his degree course was interrupted by the First World War and he never returned to graduate. At the beginning of 1915 he volunteered for service in the British Army but was classified as unfit for active duty. Unable to go to the Front, he took a job with the Clyde Valley Electrical Power Company, engaged in munitions work; the development of television was the result of work by many inventors. Among them, Baird made major advances in the field. Many historians credit Baird with being the first to produce a live, greyscale television image from reflected light. Baird achieved this, where other inventors had failed, by obtaining a better photoelectric cell and improving the signal conditioning from the photocell and the video amplifier.
Between 1902 and 1907, Arthur Korn invented and built the first successful signal-conditioning circuits for image transmission. The circuits overcame the image-destroying lag effect, part of selenium photocells. Korn's compensation circuit allowed him to send still fax pictures by telephone or wireless between countries and over oceans, while his circuit operated without benefit of electronic amplification. Korn's success at transmitting halftone still images suggested that such compensation circuits might work in television. Baird was the direct beneficiary of Korn's success. In his first attempts to develop a working television system, Baird experimented with the Nipkow disk. Paul Gottlieb Nipkow had invented this scanning disc system in 1884. Television historian Albert Abramson calls Nipkow's patent "the master television patent". Nipkow's work is important because Baird and many others chose to develop it into a broadcast medium. In early 1923, in poor health, Baird moved to 21 Linton Crescent, Hastings, on the south coast of England.
He rented a workshop in the Queen's Arcade in the town. Baird built what was to become the world's first working television set using items including an old hatbox and a pair of scissors, some darning needles, a few bicycle light lenses, a used tea chest, sealing wax and glue that he purchased. In February 1924, he demonstrated to the Radio Times that a semi-mechanical analogue television system was possible by transmitting moving silhouette images. In July of the same year, he received a 1000-volt electric shock, but survived with only a burnt hand, as a result his landlord, Mr Tree, asked him to vacate the premises. Baird gave the first public demonstration of moving silhouette images by television at Selfridges department store in London in a three-week series of demonstrations beginning on 25 March 1925. In his laboratory on 2 October 1925, Baird transmitted the first television picture with a greyscale image: the head of a ventriloquist's dummy nicknamed "Stooky Bill" in a 30-line vertically scanned image, at five pictures per second.
Baird went downstairs and fetched an office worker, 20-year-old William Edward Taynton, to see what a human face would look like, Taynton became the first person to be televised in a full tonal range. Looking for publicity, Baird visited the Daily Express newspaper to promote his invention; the news editor was terrified and he was quoted by one of his staff as saying: "For God's sake, go down to reception and get rid of a lunatic who's down there. He says he's got a machine for seeing by wireless! Watch him — he may have a razor on him." On 26 January 1926, Baird repeated the transmission for members of the Royal Institution and a reporter from The Times in his laboratory at 22 Frith Street in the Soho district of London, where Bar Italia is now located. By this time, he had improved the scan rate to 12.5 pictures per second. It was the first demonstration of a television system that could broadcast live moving images with tone graduation, he demonstrated the world's first colour transmission on 3 July 1928, using scanning discs at the transmitting and receiving ends with three spirals of apertures, each spiral with a filter of a different primary colour.