An electric switchboard is a device that directs electricity from one or more sources of supply to several smaller regions of usage. It is an assembly of one or more panels, each of which contains switches that allow electricity to be redirected; the U. S. National Electrical Code defines a switchboard as "a large single panel, frame, or assembly of panels on which are mounted, on the face, back, or both, over-current and other protective devices and instruments"; the role of a switchboard is to allow the division of the current supplied to the switchboard into smaller currents for further distribution and to provide switching, current protection and metering for those various currents. In general, switchboards may distribute power to transformers, control equipment, to individual system loads. Inside a switchboard there will be one or more busbars; these are flat strips of aluminum, to which the switchgear is connected. Busbars carry large currents through the switchboard, are supported by insulators.
Bare busbars are common, but many types are now manufactured with an insulating cover on the bars, leaving only connection points exposed. The operator is protected from electrocution by safety fuses. There may be controls for the supply of electricity to the switchboard, coming from a generator or bank of electrical generators frequency control of AC power and load sharing controls, plus gauges showing frequency and a synchroscope; the amount of power going into a switchboard must always equal the power going out to the loads. Modern industrial switchboards are metal of "dead front" construction. Open switchboards were made with switches and other devices were mounted on panels made of slate, granite, or ebony asbestos board; the metal enclosure of the switchboard is bonded to earth ground for protection of personnel. Large switchboards may be free-standing floor-mounted enclosures with provision for incoming connections at either the top or bottom of the enclosure. A switchboard may have incoming bus bars or bus duct for the source connection, for large circuits fed from the board.
A switchboard may include a metering or control compartment separated from the power distribution conductors
An electric field surrounds an electric charge, exerts force on other charges in the field, attracting or repelling them. Electric field is sometimes abbreviated as E-field. Mathematically the electric field is a vector field that associates to each point in space the force per unit of charge exerted on an infinitesimal positive test charge at rest at that point; the SI unit for electric field strength is volt per meter. Newtons per coulomb is used as a unit of electric field strengh. Electric fields are created by time-varying magnetic fields. Electric fields are important in many areas of physics, are exploited electrical technology. On an atomic scale, the electric field is responsible for the attractive force between the atomic nucleus and electrons that holds atoms together, the forces between atoms that cause chemical bonding. Electric fields and magnetic fields are both manifestations of the electromagnetic force, one of the four fundamental forces of nature. From Coulomb's law a particle with electric charge q 1 at position x 1 exerts a force on a particle with charge q 0 at position x 0 of F = 1 4 π ε 0 q 1 q 0 2 r ^ 1, 0 where r 1, 0 is the unit vector in the direction from point x 1 to point x 0, ε0 is the electric constant in C2 m−2 N−1When the charges q 0 and q 1 have the same sign this force is positive, directed away from the other charge, indicating the particles repel each other.
When the charges have unlike signs the force is negative, indicating the particles attract. To make it easy to calculate the Coulomb force on any charge at position x 0 this expression can be divided by q 0, leaving an expression that only depends on the other charge E = F q 0 = 1 4 π ε 0 q 1 2 r ^ 1, 0 This is the electric field at point x 0 due to the point charge q 1. Since this formula gives the electric field magnitude and direction at any point x 0 in space it defines a vector field. From the above formula it can be seen that the electric field due to a point charge is everywhere directed away from the charge if it is positive, toward the charge if it is negative, its magnitude decreases with the inverse square of the distance from the charge. If there are multiple charges, the resultant Coulomb force on a charge can be found by summing the vectors of the forces due to each charge; this shows the electric field obeys the superposition principle: the total electric field at a point due to a collection of charges is just equal to the vector sum of the electric fields at that point due to the individual charges.
E = E 1 + E 2 + E 3 + ⋯ = 1 4 π ε 0 q 1 2 r ^ 1 + 1 4 π ε 0 q 2 ( x 2 −
In electrical wiring, a light switch is a switch most used to operate electric lights, permanently connected equipment, or electrical outlets. Portable lamps such as table lamps may have a light switch mounted on the socket, base, or in-line with the cord. Manually operated on/off switches may be substituted by dimmer switches that allow controlling the brightness of lamps as well as turning them on or off, time-controlled switches, occupancy-sensing switches, remotely controlled switches and dimmers. Light switches are found in flashlights and other devices. Switches for lighting may be in hand-held devices, moving vehicles, buildings. Residential and commercial buildings have wall-mounted light switches to control lighting within a room. Mounting height and other design factors vary from country to country; the switch mounting boxes, or enclosures are recessed within a finished wall. Surface mounting of enclosures is fairly common though is seen more in commercial industrial and outbuilding settings than in residential structures.
These light switch boxes are designed to house and mount the switch, protect the wiring and contain any heat or fire. Each kind uses some form of a plastic, ceramic, or metal cover to prevent accidental contact with live terminals of the switch. Wall plates are available in different styles and colours to blend in with the style of a room available in weatherproof varieties for outdoors; these covers are quite easy to mount. The first light switch employing "quick-break technology" was invented by John Henry Holmes in 1864 in the Shieldfield district of Newcastle upon Tyne; the "quick-break" switch overcame the problem of a switch's contacts developing electric arcing whenever the circuit was opened or closed. Arcing would cause pitting on one contact and the build-up of residue on the other, the switch's useful life would be diminished. Holmes' invention ensured that the contacts would separate or come together quickly, however much or little pressure was exerted by the user on the switch actuator.
The action of this "quick break" mechanism meant that there was insufficient time for an arc to form, the switch would thus have a long working life. This "quick break" technology is still in use in every ordinary light switch in the world today, numbering in the billions, as well as in many other forms of electric switch; the toggle light switch was invented in 1897 by William J. Newton; as a component of an electrical wiring or home wiring system, the installation of light switches is regulated by some authority concerned with safety and standards. In different countries the standard dimensions of the wall mounting hardware may differ. Since the face-plates used must cover this hardware, these standards determine the minimum sizes of all wall mounted equipment. Hence, the shape and size of the boxes and face-plates, as well as what is integrated, varies from country to country; the dimensions, mechanical designs, the general appearance of light switches have changed over time. Switches remain in service for many decades being changed only when a portion of a house is rewired.
It is not unusual to see century-old light switches still in functional use. Manufacturers introduce various new forms and styles, but for the most part decoration and fashion concerns are limited to the face-plates or wall-plates; the "modern" dimmer switch with knob is at least forty years old, in the newest construction the familiar toggle and rocker switch formats predominate. The direction which represents "on" varies by country. In the US and Canada, it is usual for the "on" position of a toggle switch to be "up", whereas in many other countries such as the UK, Ireland and New Zealand it is "down"; the switches may be multiple, designed for indoor or outdoor use. Optional extras may include dimmer-controls, environmental protection and security protection. In residential and light commercial lighting systems, the light switch directly controls the circuit feeding the lamps. In larger lighting systems, for example warehouses or outdoor lighting systems, the required current may be too high for a manual switch.
In these systems light switches control lighting contactors, a relay that allows the manual light switch to operate on a lower voltage or with smaller wiring than would be required in the main lighting circuit. In the UK, putting 13 amp BS 1363 sockets on a lighting circuit is discouraged, but 2 amp or 5 amp BS 546 outlets are put on lighting circuits to allow control of free-standing lamps from the room's light switches. In North American site-built and mobile homes living rooms and bedrooms have a switched receptacle for a floor or table lamp; the contacts of a switch are under their greatest stress while closing. As the switch is closed, the resistance between the contacts changes from infinite to zero. At infinite resistance, no current flows and no power is dissipated. At zero resistance, there is no voltage drop and no power is dissipated. However, while the contacts change state, there is a brief instant of partial contact when resistance is neither zero nor infinite, electrical power is converted into heat.
If the heating is excessive, the contacts may be damaged, or may weld themselves closed. A switch should be designed to make its transition as swiftly as possible; this is achieved by the initial operation of the switch lever mechanism storing potential energy as mechanical stress in a spring
ArXiv is a repository of electronic preprints approved for posting after moderation, but not full peer review. It consists of scientific papers in the fields of mathematics, astronomy, electrical engineering, computer science, quantitative biology, mathematical finance and economics, which can be accessed online. In many fields of mathematics and physics all scientific papers are self-archived on the arXiv repository. Begun on August 14, 1991, arXiv.org passed the half-million-article milestone on October 3, 2008, had hit a million by the end of 2014. By October 2016 the submission rate had grown to more than 10,000 per month. ArXiv was made possible by the compact TeX file format, which allowed scientific papers to be transmitted over the Internet and rendered client-side. Around 1990, Joanne Cohn began emailing physics preprints to colleagues as TeX files, but the number of papers being sent soon filled mailboxes to capacity. Paul Ginsparg recognized the need for central storage, in August 1991 he created a central repository mailbox stored at the Los Alamos National Laboratory which could be accessed from any computer.
Additional modes of access were soon added: FTP in 1991, Gopher in 1992, the World Wide Web in 1993. The term e-print was adopted to describe the articles, it began as a physics archive, called the LANL preprint archive, but soon expanded to include astronomy, computer science, quantitative biology and, most statistics. Its original domain name was xxx.lanl.gov. Due to LANL's lack of interest in the expanding technology, in 2001 Ginsparg changed institutions to Cornell University and changed the name of the repository to arXiv.org. It is now hosted principally with eight mirrors around the world, its existence was one of the precipitating factors that led to the current movement in scientific publishing known as open access. Mathematicians and scientists upload their papers to arXiv.org for worldwide access and sometimes for reviews before they are published in peer-reviewed journals. Ginsparg was awarded a MacArthur Fellowship in 2002 for his establishment of arXiv; the annual budget for arXiv is $826,000 for 2013 to 2017, funded jointly by Cornell University Library, the Simons Foundation and annual fee income from member institutions.
This model arose in 2010, when Cornell sought to broaden the financial funding of the project by asking institutions to make annual voluntary contributions based on the amount of download usage by each institution. Each member institution pledges a five-year funding commitment to support arXiv. Based on institutional usage ranking, the annual fees are set in four tiers from $1,000 to $4,400. Cornell's goal is to raise at least $504,000 per year through membership fees generated by 220 institutions. In September 2011, Cornell University Library took overall administrative and financial responsibility for arXiv's operation and development. Ginsparg was quoted in the Chronicle of Higher Education as saying it "was supposed to be a three-hour tour, not a life sentence". However, Ginsparg remains on the arXiv Scientific Advisory Board and on the arXiv Physics Advisory Committee. Although arXiv is not peer reviewed, a collection of moderators for each area review the submissions; the lists of moderators for many sections of arXiv are publicly available, but moderators for most of the physics sections remain unlisted.
Additionally, an "endorsement" system was introduced in 2004 as part of an effort to ensure content is relevant and of interest to current research in the specified disciplines. Under the system, for categories that use it, an author must be endorsed by an established arXiv author before being allowed to submit papers to those categories. Endorsers are not asked to review the paper for errors, but to check whether the paper is appropriate for the intended subject area. New authors from recognized academic institutions receive automatic endorsement, which in practice means that they do not need to deal with the endorsement system at all. However, the endorsement system has attracted criticism for restricting scientific inquiry. A majority of the e-prints are submitted to journals for publication, but some work, including some influential papers, remain purely as e-prints and are never published in a peer-reviewed journal. A well-known example of the latter is an outline of a proof of Thurston's geometrization conjecture, including the Poincaré conjecture as a particular case, uploaded by Grigori Perelman in November 2002.
Perelman appears content to forgo the traditional peer-reviewed journal process, stating: "If anybody is interested in my way of solving the problem, it's all there – let them go and read about it". Despite this non-traditional method of publication, other mathematicians recognized this work by offering the Fields Medal and Clay Mathematics Millennium Prizes to Perelman, both of which he refused. Papers can be submitted in any of several formats, including LaTeX, PDF printed from a word processor other than TeX or LaTeX; the submission is rejected by the arXiv software if generating the final PDF file fails, if any image file is too large, or if the total size of the submission is too large. ArXiv now allows one to store and modify an incomplete submission, only finalize the submission when ready; the time stamp on the article is set. The standard access route is through one of several mirrors. Sev
In electronics, a crossbar switch is a collection of switches arranged in a matrix configuration. A crossbar switch has multiple input and output lines that form a crossed pattern of interconnecting lines between which a connection may be established by closing a switch located at each intersection, the elements of the matrix. A crossbar switch consisted of crossing metal bars that provided the input and output paths. Implementations achieved the same switching topology in solid state semiconductor chips; the cross-point switch is one of the principal switch architectures, together with a rotary switch, memory switch, a crossover switch. A crossbar switch is an assembly of individual switches between a set of inputs and a set of outputs; the switches are arranged in a matrix. If the crossbar switch has M inputs and N outputs a crossbar has a matrix with M × N cross-points or places where the connections cross. At each crosspoint is a switch. A given crossbar is non-blocking switch. Non-blocking switch means that other concurrent connections do not prevent connecting other inputs to other outputs.
Collections of crossbars can be blocking switches. A crossbar switching system is called a coordinate switching system. Crossbar switches are used in information processing applications such as telephony and circuit switching, but they are used in applications such as mechanical sorting machines; the matrix layout of a crossbar switch is used in some semiconductor memory devices. Here the bars are thin metal wires, the switches are fusible links; the fuses are read using low voltage. Such devices are called programmable read-only memory. At the 2008 NSTI Nanotechnology Conference a paper was presented that discussed a nanoscale crossbar implementation of an adding circuit used as an alternative to logic gates for computation. Matrix arrays are fundamental to modern flat-panel displays. Thin-film-transistor LCDs have a transistor at each crosspoint, so they could be considered to include a crossbar switch as part of their structure. For video switching in home and professional theater applications, a crossbar switch is used to distribute the output of multiple video appliances to every monitor or every room throughout a building.
In a typical installation, all the video sources are located on an equipment rack, are connected as inputs to the matrix switch. Where central control of the matrix is practical, a typical rack-mount matrix switch offers front-panel buttons to allow manual connection of inputs to outputs. An example of such a usage might be a sports bar, where numerous programs are displayed simultaneously. Ordinarily, a sports bar would install a separate desk top box for each display for which independent control is desired; the matrix switch enables the operator to route signals at will, so that only enough set top boxes are needed to cover the total number of unique programs to be viewed, while making it easier to control sound from any program in the overall sound system. Such switches are used in high-end home theater applications. Video sources shared include set-top receivers or DVD changers; the outputs are wired to televisions in individual rooms. The matrix switch is controlled via an Ethernet or RS-232 connection by a whole-house automation controller, such as those made by AMX, Crestron, or Control4, which provides the user interface that enables the user in each room to select which appliance to watch.
The actual user interface varies by system brand, might include a combination of on-screen menus, touch-screens, handheld remote controls. The system is necessary to enable the user to select the program they wish to watch from the same room they will watch it from, otherwise it would be necessary for them to walk to the equipment rack; the special crossbar switches used in distributing satellite TV signals are called multiswitches. A crossbar switch consisted of metal bars associated with each input and output, together with some means of controlling movable contacts at each cross-point. In the part of the 20th century, these literal crossbar switches declined and the term came to be used figuratively for rectangular array switches in general. Modern crossbar switches are implemented with semiconductor technology. An important emerging class of optical crossbars is being implemented with MEMS technology. A type of middle 19th-century telegraph exchange consisted of a grid of vertical and horizontal brass bars with a hole at each intersection.
The operator inserted a brass pin to connect one telegraph line to another. A telephony crossbar switch is an electromechanical device for switching telephone calls; the first design of what is now called a crossbar switch was the Bell company Western Electric's coordinate selector of 1915. To save money on control systems, this system was organized on the stepping switch or selector principle rather than the link principle, it was little used in America, but the Televerket Swedish governmental agency manufactured its own design, used it in Sweden from 1926 until the digitalization in the 1980s in small and medium-sized A204 model switches. The system design used in AT&T Corporation's 1XB crossbar exchanges, which entered revenue service from 1938, developed by Bell Telephone Labs, was inspired by the Swedish design but was based on the rediscovered link principle. In 1945, a similar design by Swedish Televerket was i
In physics, the Faraday effect or Faraday rotation is a magneto-optical phenomenon—that is, an interaction between light and a magnetic field in a medium. The Faraday effect causes a rotation of the plane of polarization, linearly proportional to the component of the magnetic field in the direction of propagation. Formally, it is a special case of gyroelectromagnetism obtained when the dielectric permittivity tensor is diagonal. Discovered by Michael Faraday in 1845, the Faraday effect was the first experimental evidence that light and electromagnetism are related; the theoretical basis of electromagnetic radiation was completed by James Clerk Maxwell in the 1860s and 1870s. This effect occurs in most optically transparent dielectric materials under the influence of magnetic fields; the Faraday effect is caused by left and right circularly polarized waves propagating at different speeds, a property known as circular birefringence. Since a linear polarization can be decomposed into the superposition of two equal-amplitude circularly polarized components of opposite handedness and different phase, the effect of a relative phase shift, induced by the Faraday effect, is to rotate the orientation of a wave's linear polarization.
The Faraday effect has applications in measuring instruments. For instance, the Faraday effect has been used to measure optical rotatory power and for remote sensing of magnetic fields; the Faraday effect is used in spintronics research to study the polarization of electron spins in semiconductors. Faraday rotators can be used for amplitude modulation of light, are the basis of optical isolators and optical circulators. By 1845, it was known through the work of Fresnel and others that different materials are able to modify the direction of polarization of light when appropriately oriented, making polarized light a powerful tool to investigate the properties of transparent materials. Faraday believed that light was an electromagnetic phenomenon, as such should be affected by electromagnetic forces, he spent considerable effort looking for evidence of electric forces affecting the polarization of light through what are now known as electro-optic effects, starting with decomposing electrolytes.
However, his experimental methods were not sensitive enough, the effect was only measured thirty years by John Kerr. Faraday attempted to look for the effects of magnetic forces on light passing through various substances. After several unsuccessful trials, he happened to test a piece of "heavy" glass, containing traces of lead, that he had made during his earlier work on glass manufacturing. Faraday observed that when a beam of polarized light passed through the glass in the direction of an applied magnetic force, the polarization of light rotated by an angle, proportional to the strength of the force, he was able to reproduce the effect in several other solids and gases by procuring stronger electromagnets. The discovery is well documented in Faraday's daily notebook. On 13 Sept. 1845, in paragraph #7504, under the rubric Heavy Glass, he wrote: … BUT, when the contrary magnetic poles were on the same side, there was an effect produced on the polarized ray, thus magnetic force and light were proved to have relation to each other.
… He summarized the results of his experiments on 30 Sept. 1845, in paragraph #7718, famously writing: … Still, I have at last succeeded in illuminating a magnetic curve or line of force, in magnetizing a ray of light. … The linear polarized light, seen to rotate in the Faraday effect can be seen as consisting of the superposition of a right- and a left- circularly polarized beam. We can look at the effects of each component separately, see what effect this has on the result. In circularly polarized light the direction of the electric field rotates at the frequency of the light, either clockwise or counter-clockwise. In a material, this electric field causes a force on the charged particles comprising the material; the motion thus effected will be circular, circularly moving charges will create their own field in addition to the external magnetic field. There will thus be two different cases: the created field will be parallel to the external field for one polarization, in the opposing direction for the other polarization direction – thus the net B field is enhanced in one direction and diminished in the opposite direction.
This changes the dynamics of the interaction for each beam and one of the beams will be slowed down more than the other, causing a phase difference between the left- and right-polarized beam. When the two beams are added after this phase shift, the result is again a linearly polarized beam, but with a rotation in the polarization direction; the direction of polarization rotation depends on the properties of the material through which the light is shone. A full treatment would have to take into account the effect of the external and radiation-induced fields on the wave function of the electrons, calculate the effect of this change on the refractive index of the material for each polarization, to see whether the right- or left circular polarization is slowed down more. Formally, the magnetic permeability is treated as a non-diagonal tensor as expressed by the equation: B = | μ 1 − i
An optical attenuator, or fiber optic attenuator, is a device used to reduce the power level of an optical signal, either in free space or in an optical fiber. The basic types of optical attenuators are fixed, step-wise variable, continuously variable. Optical attenuators are used in fiber optic communications, either to test power level margins by temporarily adding a calibrated amount of signal loss, or installed permanently to properly match transmitter and receiver levels. Sharp bends can cause losses. If a received signal is too strong a temporary fix is to wrap the cable around a pencil until the desired level of attenuation is achieved. However, such arrangements are unreliable; the power reduction is done by such means as absorption, diffusion, deflection and dispersion, etc. Optical attenuators work by absorbing the light, like sunglasses absorb extra light energy, they have a working wavelength range in which they absorb all light energy equally. They should not reflect the light or scatter the light in an air gap, since that could cause unwanted back reflection in the fiber system.
Another type of attenuator utilizes a length of high-loss optical fiber, that operates upon its input optical signal power level in such a way that its output signal power level is less than the input level. Optical attenuators can take a number of different forms and are classified as fixed or variable attenuators. What's more, they can be classified as LC, SC, ST, FC, MU, E2000 etc. according to the different types of connectors. Fixed optical attenuators used in fiber optic systems may use a variety of principles for their functioning. Preferred attenuators use either doped fibers, or mis-aligned splices,or total power since both of these are reliable and inexpensive. Inline style attenuators are incorporated into patch cables; the alternative build out style attenuator is a small male-female adapter that can be added onto other cables. Non-preferred attenuators use gap loss or reflective principles; such devices can be sensitive to: modal distribution, contamination, temperature, damage due to power bursts, may cause back reflections, may cause signal dispersion etc.
Loopback fiber optic attenuator is designed for testing and the burn-in stage of boards or other equipment. Available in SC/UPC, SC/APC, LC/UPC, LC/APC, MTRJ, MPO for singlemode application.900um fiber cable inside of the black shell for LC and SC type. No black shell for MTRJ and MPO type. Built-in variable optical attenuators may electrically controlled. A manual device is useful for one-time set up of a system, is a near-equivalent to a fixed attenuator, may be referred to as an "adjustable attenuator". In contrast, an electrically controlled attenuator can provide adaptive power optimization. Attributes of merit for electrically controlled devices, include speed of response and avoiding degradation of the transmitted signal. Dynamic range is quite restricted, power feedback may mean that long term stability is a minor issue. Speed of response is a major issue in dynamically reconfigurable systems, where a delay of one millionth of a second can result in the loss of large amounts of transmitted data.
Typical technologies employed for high speed response include liquid crystal variable attenuator, or lithium niobate devices. There is a class of built-in attenuators, technically indistinguishable from test attenuators, except they are packaged for rack mounting, have no test display. Variable optical test attenuators use a variable neutral density filter. Despite high cost, this arrangement has the advantages of being stable, wavelength insensitive, mode insensitive, offering a large dynamic range. Other schemes such as LCD, variable air gap etc. have been tried over the years, but with limited success. They may be either manually or motor controlled. Motor control give regular users a distinct productivity advantage, since used test sequences can be run automatically. Attenuator instrument calibration is a major issue; the user would like an absolute port to port calibration. Calibration should be at a number of wavelengths and power levels, since the device is not always linear; however a number of instruments do not in fact offer these basic features in an attempt to reduce cost.
The most accurate variable attenuator instruments have thousands of calibration points, resulting in excellent overall accuracy in use. Test sequences that use variable attenuators, can be time consuming. Therefore, automation is to achieve useful benefits. Both bench and handheld style devices are available. Attenuation Optical fiber cable Optical fiber connector Gap loss - attenuation sources and causes Optical power meter This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C"