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130-410: A third rail , also known as a live rail , electric rail or conductor rail , is a method of providing electric power to a railway locomotive or train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway track . It is used typically in a mass transit or rapid transit system, which has alignments in its own corridors, fully or almost fully segregated from

260-455: A conductor 's surface, since otherwise there would be a force along the surface of the conductor that would move the charge carriers to even the potential across the surface. The electric field was formally defined as the force exerted per unit charge, but the concept of potential allows for a more useful and equivalent definition: the electric field is the local gradient of the electric potential. Usually expressed in volts per metre,

390-461: A force on each other, an effect that was known, though not understood, in antiquity. A lightweight ball suspended by a fine thread can be charged by touching it with a glass rod that has itself been charged by rubbing with a cloth. If a similar ball is charged by the same glass rod, it is found to repel the first: the charge acts to force the two balls apart. Two balls that are charged with a rubbed amber rod also repel each other. However, if one ball

520-409: A steady state current, but instead blocks it. The inductor is a conductor, usually a coil of wire, that stores energy in a magnetic field in response to the current through it. When the current changes, the magnetic field does too, inducing a voltage between the ends of the conductor. The induced voltage is proportional to the time rate of change of the current. The constant of proportionality

650-531: A 750 V DC third-rail system. The section from Vuosaari to Vuosaari harbour is not electrified, as its only purpose is to connect to the Finnish rail network, whose gauge differs only by a couple of millimetres from that of the metro. The route has been previously used by diesel shunting locomotives moving new metro trains to the electrified section of the line. The new tramway in Bordeaux (France) uses

780-455: A British Class 442 EMU . In the event of a collision with a foreign object, the beveled end ramps of bottom running systems can facilitate the hazard of having the third rail penetrate the interior of a passenger car. This is believed to have contributed to the death of five passengers in the Valhalla train crash of 2015. Modern systems, such as ground-level power supply (first used in

910-423: A cap and linear welded along the centre line of the rail. Because aluminium has a higher coefficient of thermal expansion than steel, the aluminium and steel must be positively locked to provide a good current collection interface. A third method rivets aluminium bus strips to the web of the steel rail. As with overhead wires, the return current usually flows through one or both running rails, and leakage to ground

1040-456: A central role in many modern technologies, serving in electric power where electric current is used to energise equipment, and in electronics dealing with electrical circuits involving active components such as vacuum tubes , transistors , diodes and integrated circuits , and associated passive interconnection technologies. The study of electrical phenomena dates back to antiquity, with theoretical understanding progressing slowly until

1170-498: A changeover to third rail made at Drayton Park railway station . A third rail is still used in the tunnel section of the route, because the size of the tunnels leading to Moorgate station was too small to allow for overhead electrification. The North Downs Line is not electrified on those parts of the line where the North Downs service has exclusive use. The electrified portions of the line are: The Helsinki Metro uses

1300-432: A charge of Q coulombs every t seconds passing through an electric potential ( voltage ) difference of V is where Electric power is generally supplied to businesses and homes by the electric power industry . Electricity is usually sold by the kilowatt hour (3.6 MJ) which is the product of power in kilowatts multiplied by running time in hours. Electric utilities measure power using electricity meters , which keep

1430-516: A common reference point to which potentials may be expressed and compared is useful. While this could be at infinity, a much more useful reference is the Earth itself, which is assumed to be at the same potential everywhere. This reference point naturally takes the name earth or ground . Earth is assumed to be an infinite source of equal amounts of positive and negative charge and is therefore electrically uncharged—and unchargeable. Electric potential

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1560-472: A current of one amp. The capacitor is a development of the Leyden jar and is a device that can store charge, and thereby storing electrical energy in the resulting field. It consists of two conducting plates separated by a thin insulating dielectric layer; in practice, thin metal foils are coiled together, increasing the surface area per unit volume and therefore the capacitance . The unit of capacitance

1690-494: A dampened kite string and flown the kite in a storm-threatened sky . A succession of sparks jumping from the key to the back of his hand showed that lightning was indeed electrical in nature. He also explained the apparently paradoxical behavior of the Leyden jar as a device for storing large amounts of electrical charge in terms of electricity consisting of both positive and negative charges. In 1775, Hugh Williamson reported

1820-452: A grade-separated junction may be referred to as a grade separation or as an interchange – in contrast with an intersection , at-grade , a diamond crossing or a level crossing , which are not grade-separated. Roads with grade separation generally allow traffic to move freely, with fewer interruptions, and at higher overall speeds; this is why speed limits are typically higher for grade-separated roads. In addition, reducing

1950-476: A high traffic density. Because of mechanical limitations on the contact to the third rail, trains that use this method of power supply achieve lower speeds than those using overhead electric wires and a pantograph . Nevertheless, they may be preferred inside cities as there is no need for very high speed and they cause less visual pollution . The third rail is an alternative to overhead lines that transmit power to trains by means of pantographs attached to

2080-416: A highway) that they cross. However, grade-separated pedestrian crossings with steps introduce accessibility problems. Some crossings have lifts , but these can be time-consuming to use. Grade-separated roads that permit for higher speed limits can actually reduce safety due to 'weaving' (see below) as well as a perceived sense of safety. The term is most widely applied to describe a road junction in which

2210-425: A large lightning cloud may be as high as 100 MV and have discharge energies as great as 250 kWh. The field strength is greatly affected by nearby conducting objects, and it is particularly intense when it is forced to curve around sharply pointed objects. This principle is exploited in the lightning conductor , the sharp spike of which acts to encourage the lightning strike to develop there, rather than to

2340-615: A less stable train vehicle. Nevertheless, it was sometimes used at the beginning of the development of electric trains. The oldest electric railway in the world, Volk's Railway in Brighton, England, was originally electrified at 50 volts DC using this system (it is now a three-rail system). Other railway systems that used it were the Gross-Lichterfelde Tramway and the Ungerer Tramway . The third rail

2470-495: A level crossing at the Kedzie station in an apparent attempt to urinate. The end ramps of conductor rails (where they are interrupted, or change sides) present a practical limitation on speed due to the mechanical impact of the shoe, and 161 km/h (100 mph) is considered the upper limit of practical third-rail operation. The world speed record for a third rail train is 175 km/h (109 mph) attained on 11 April 1988 by

2600-402: A millimetre per second, the electric field that drives them itself propagates at close to the speed of light , enabling electrical signals to pass rapidly along wires. Current causes several observable effects, which historically were the means of recognising its presence. That water could be decomposed by the current from a voltaic pile was discovered by Nicholson and Carlisle in 1800,

2730-553: A more reliable source of electrical energy than the electrostatic machines previously used. The recognition of electromagnetism , the unity of electric and magnetic phenomena, is due to Hans Christian Ørsted and André-Marie Ampère in 1819–1820. Michael Faraday invented the electric motor in 1821, and Georg Ohm mathematically analysed the electrical circuit in 1827. Electricity and magnetism (and light) were definitively linked by James Clerk Maxwell , in particular in his " On Physical Lines of Force " in 1861 and 1862. While

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2860-410: A novel system with a third rail in the centre of the track. The third rail is separated into 10 m (32 ft 9 + 3 ⁄ 4  in) long conducting and 3 m (9 ft 10 + 1 ⁄ 8  in) long isolation segments. Each conducting segment is attached to an electronic circuit which will make the segment live once it lies fully beneath the tram (activated by a coded signal sent by

2990-414: A partial grade separation will accomplish more improvement than for a road), and because at-grade railway connections often take up significant space on their own. However, they require significant engineering effort, and are very expensive and time-consuming to construct. Grade-separated pedestrian and cycling routes often require modest space since they do not typically intersect with the facility (such as

3120-425: A positive current is defined as having the same direction of flow as any positive charge it contains, or to flow from the most positive part of a circuit to the most negative part. Current defined in this manner is called conventional current . The motion of negatively charged electrons around an electric circuit , one of the most familiar forms of current, is thus deemed positive in the opposite direction to that of

3250-420: A positive or negative electric charge produces an electric field . The motion of electric charges is an electric current and produces a magnetic field . In most applications, Coulomb's law determines the force acting on an electric charge. Electric potential is the work done to move an electric charge from one point to another within an electric field, typically measured in volts . Electricity plays

3380-891: A practical generator, but it showed the possibility of generating electric power using magnetism, a possibility that would be taken up by those that followed on from his work. An electric circuit is an interconnection of electric components such that electric charge is made to flow along a closed path (a circuit), usually to perform some useful task. The components in an electric circuit can take many forms, which can include elements such as resistors , capacitors , switches , transformers and electronics . Electronic circuits contain active components , usually semiconductors , and typically exhibit non-linear behaviour, requiring complex analysis. The simplest electric components are those that are termed passive and linear : while they may temporarily store energy, they contain no sources of it, and exhibit linear responses to stimuli. The resistor

3510-406: A process now known as electrolysis . Their work was greatly expanded upon by Michael Faraday in 1833. Current through a resistance causes localised heating, an effect James Prescott Joule studied mathematically in 1840. One of the most important discoveries relating to current was made accidentally by Hans Christian Ørsted in 1820, when, while preparing a lecture, he witnessed the current in

3640-471: A rapidly changing one. Electric power is the rate at which electric energy is transferred by an electric circuit . The SI unit of power is the watt , one joule per second . Electric power, like mechanical power , is the rate of doing work , measured in watts , and represented by the letter P . The term wattage is used colloquially to mean "electric power in watts." The electric power in watts produced by an electric current I consisting of

3770-489: A resistance is directly proportional to the potential difference across it. The resistance of most materials is relatively constant over a range of temperatures and currents; materials under these conditions are known as 'ohmic'. The ohm , the unit of resistance, was named in honour of Georg Ohm , and is symbolised by the Greek letter Ω. 1 Ω is the resistance that will produce a potential difference of one volt in response to

3900-595: A running total of the electric energy delivered to a customer. Unlike fossil fuels , electricity is a low entropy form of energy and can be converted into motion or many other forms of energy with high efficiency. Electronics deals with electrical circuits that involve active electrical components such as vacuum tubes , transistors , diodes , sensors and integrated circuits , and associated passive interconnection technologies. The nonlinear behaviour of active components and their ability to control electron flows makes digital switching possible, and electronics

4030-431: A scientific curiosity into an essential tool for modern life. In 1887, Heinrich Hertz discovered that electrodes illuminated with ultraviolet light create electric sparks more easily. In 1905, Albert Einstein published a paper that explained experimental data from the photoelectric effect as being the result of light energy being carried in discrete quantized packets, energising electrons. This discovery led to

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4160-575: A series of experiments to the Royal Society on the shocks delivered by the electric eel ; that same year the surgeon and anatomist John Hunter described the structure of the fish's electric organs . In 1791, Luigi Galvani published his discovery of bioelectromagnetics , demonstrating that electricity was the medium by which neurons passed signals to the muscles. Alessandro Volta 's battery, or voltaic pile , of 1800, made from alternating layers of zinc and copper, provided scientists with

4290-515: A series of observations on static electricity around 600 BCE, from which he believed that friction rendered amber magnetic , in contrast to minerals such as magnetite , which needed no rubbing. Thales was incorrect in believing the attraction was due to a magnetic effect, but later science would prove a link between magnetism and electricity. According to a controversial theory, the Parthians may have had knowledge of electroplating , based on

4420-472: A set of equations that could unambiguously describe the interrelationship between electric field, magnetic field, electric charge, and electric current. He could moreover prove that in a vacuum such a wave would travel at the speed of light , and thus light itself was a form of electromagnetic radiation. Maxwell's equations , which unify light, fields, and charge are one of the great milestones of theoretical physics. Grade separation In North America,

4550-437: A smooth engagement of the train's contact shoes. The position of contact between the train and the rail varies: some of the earliest systems used top contact, but later developments use side or bottom contact, which enabled the conductor rail to be covered, protecting track workers from accidental contact and protecting the conductor rail from frost, ice, snow and leaf-fall. Because third-rail systems, which are located close to

4680-477: A solid crystal (such as a germanium crystal) to detect a radio signal by the contact junction effect. In a solid-state component, the current is confined to solid elements and compounds engineered specifically to switch and amplify it. Current flow can be understood in two forms: as negatively charged electrons , and as positively charged electron deficiencies called holes . These charges and holes are understood in terms of quantum physics. The building material

4810-640: A third rail (current feed, outside the running rails) and fourth rail (current return, midway between the running rails), is used by a few steel-wheel systems; see fourth rail . The London Underground is the largest of these (see railway electrification in Great Britain ). The main reason for using the fourth rail to carry the return current is to avoid this current flowing through the original metal tunnel linings which were never intended to carry current, and which would suffer electrolytic corrosion should such currents flow in them. Another four-rail system

4940-508: A train from an external source was by using both rails on which a train runs, whereby each rail is a conductor for each polarity, and is insulated by the sleepers . This method is used by most scale model trains ; however, it does not work as well for large trains as the sleepers are not good insulators. Furthermore, the electric connection requires insulated wheels or insulated axles, but most insulation materials have poor mechanical properties compared with metals used for this purpose, leading to

5070-511: A wire disturbing the needle of a magnetic compass. He had discovered electromagnetism , a fundamental interaction between electricity and magnetics. The level of electromagnetic emissions generated by electric arcing is high enough to produce electromagnetic interference , which can be detrimental to the workings of adjacent equipment. In engineering or household applications, current is often described as being either direct current (DC) or alternating current (AC). These terms refer to how

5200-448: A wire suspended from a pivot above the magnet and dipped into the mercury. The magnet exerted a tangential force on the wire, making it circle around the magnet for as long as the current was maintained. Experimentation by Faraday in 1831 revealed that a wire moving perpendicular to a magnetic field developed a potential difference between its ends. Further analysis of this process, known as electromagnetic induction , enabled him to state

5330-402: A wire. The informal term static electricity refers to the net presence (or 'imbalance') of charge on a body, usually caused when dissimilar materials are rubbed together, transferring charge from one to the other. Charge can be measured by a number of means, an early instrument being the gold-leaf electroscope , which although still in use for classroom demonstrations, has been superseded by

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5460-404: Is a power surge or a break in the wires. Depending on train and track geometry, gaps in the conductor rail (e.g., at level crossings and junctions) could allow a train to stop in a position where all of its power pickup shoes are in gaps, so that no traction power is available. The train is then said to be "gapped". Another train must then be brought up behind the stranded train to push it on to

5590-584: Is a scalar quantity . That is, it has only magnitude and not direction. It may be viewed as analogous to height : just as a released object will fall through a difference in heights caused by a gravitational field, so a charge will 'fall' across the voltage caused by an electric field. As relief maps show contour lines marking points of equal height, a set of lines marking points of equal potential (known as equipotentials ) may be drawn around an electrostatically charged object. The equipotentials cross all lines of force at right angles. They must also lie parallel to

5720-411: Is a vector field . The study of electric fields created by stationary charges is called electrostatics . The field may be visualised by a set of imaginary lines whose direction at any point is the same as that of the field. This concept was introduced by Faraday, whose term ' lines of force ' still sometimes sees use. The field lines are the paths that a point positive charge would seek to make as it

5850-452: Is an important difference. Gravity always acts in attraction, drawing two masses together, while the electric field can result in either attraction or repulsion. Since large bodies such as planets generally carry no net charge, the electric field at a distance is usually zero. Thus gravity is the dominant force at distance in the universe, despite being much weaker. An electric field generally varies in space, and its strength at any one point

5980-401: Is charged by the glass rod, and the other by an amber rod, the two balls are found to attract each other. These phenomena were investigated in the late eighteenth century by Charles-Augustin de Coulomb , who deduced that charge manifests itself in two opposing forms. This discovery led to the well-known axiom: like-charged objects repel and opposite-charged objects attract . The force acts on

6110-473: Is defined as negative, and that by protons is positive. Before these particles were discovered, Benjamin Franklin had defined a positive charge as being the charge acquired by a glass rod when it is rubbed with a silk cloth. A proton by definition carries a charge of exactly 1.602 176 634 × 10  coulombs . This value is also defined as the elementary charge . No object can have a charge smaller than

6240-483: Is defined as the force (per unit charge) that would be felt by a stationary, negligible charge if placed at that point. The conceptual charge, termed a ' test charge ', must be vanishingly small to prevent its own electric field disturbing the main field and must also be stationary to prevent the effect of magnetic fields . As the electric field is defined in terms of force , and force is a vector , having both magnitude and direction , it follows that an electric field

6370-419: Is fully grade-separated, i.e. traffic on one road does not have to stop at yield lines or signals on one road, but may have to do so when switching to the other: On roadways with grade-separated interchanges, weaving is a result of placing an exit ramp a short distance after an entry ramp, causing conflicts between traffic attempting to leave the roadway at the next junction and traffic attempting to enter from

6500-531: Is integral to applications spanning transport , heating , lighting , communications , and computation , making it the foundation of modern industrial society. Long before any knowledge of electricity existed, people were aware of shocks from electric fish . Ancient Egyptian texts dating from 2750 BCE described them as the "protectors" of all other fish. Electric fish were again reported millennia later by ancient Greek , Roman and Arabic naturalists and physicians . Several ancient writers, such as Pliny

6630-730: Is known as a flying junction and one which is not a level junction . In 1897, the London and South Western Railway (LSWR) made use of a flying junction at Worting Junction south of Basingstoke to allow traffic on the Salisbury and Southampton routes to converge without conflicting movements; this became known as "Battledown Flyover". Also in Britain, the Southern Railway later made extensive use of flying junctions on other parts of its busy former LSWR main line. Today in Britain,

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6760-523: Is line M1 of the Milan Metro , where current is drawn by a lateral, flat bar with side contact, with return via a central rail with top contact. Along some sections on the northern part of the line an overhead line is also in place, to allow line M2's trains (that use pantographs and higher voltage, and have no contact shoes) to access a depot located on line M1. In depots, line M1 trains use pantographs because of safety reasons, with transition made near

6890-406: Is mediated by the magnetic field each current produces and forms the basis for the international definition of the ampere . This relationship between magnetic fields and currents is extremely important, for it led to Michael Faraday's invention of the electric motor in 1821. Faraday's homopolar motor consisted of a permanent magnet sitting in a pool of mercury . A current was allowed through

7020-415: Is most often a crystalline semiconductor . Solid-state electronics came into its own with the emergence of transistor technology. The first working transistor, a germanium -based point-contact transistor , was invented by John Bardeen and Walter Houser Brattain at Bell Labs in 1947, followed by the bipolar junction transistor in 1948. By modern convention, the charge carried by electrons

7150-659: Is not considered serious. Where trains run on rubber tyres, as on parts of the Lyon Metro , Paris Métro , Mexico City Metro , Santiago Metro , Sapporo Municipal Subway , and on all of the Montreal Metro and some automated guideway transit systems (e.g. the Astram Line ), a live rail must be provided to feed the current. The return is effected through the rails of the conventional track between these guide bars ( see rubber-tyred metro ). Another design, with

7280-403: Is perhaps the simplest of passive circuit elements: as its name suggests, it resists the current through it, dissipating its energy as heat. The resistance is a consequence of the motion of charge through a conductor: in metals, for example, resistance is primarily due to collisions between electrons and ions. Ohm's law is a basic law of circuit theory , stating that the current passing through

7410-531: Is subjected to transients , such as when first energised. The concept of the electric field was introduced by Michael Faraday . An electric field is created by a charged body in the space that surrounds it, and results in a force exerted on any other charges placed within the field. The electric field acts between two charges in a similar manner to the way that the gravitational field acts between two masses , and like it, extends towards infinity and shows an inverse square relationship with distance. However, there

7540-406: Is termed the inductance . The unit of inductance is the henry , named after Joseph Henry , a contemporary of Faraday. One henry is the inductance that will induce a potential difference of one volt if the current through it changes at a rate of one ampere per second. The inductor's behaviour is in some regards converse to that of the capacitor: it will freely allow an unchanging current, but opposes

7670-415: Is the farad , named after Michael Faraday , and given the symbol F : one farad is the capacitance that develops a potential difference of one volt when it stores a charge of one coulomb. A capacitor connected to a voltage supply initially causes a current as it accumulates charge; this current will however decay in time as the capacitor fills, eventually falling to zero. A capacitor will therefore not permit

7800-529: Is therefore 0 at all places inside the body. This is the operating principal of the Faraday cage , a conducting metal shell which isolates its interior from outside electrical effects. The principles of electrostatics are important when designing items of high-voltage equipment. There is a finite limit to the electric field strength that may be withstood by any medium. Beyond this point, electrical breakdown occurs and an electric arc causes flashover between

7930-459: Is usually located outside the two running rails, but on some systems it is mounted between them. The electricity is transmitted to the train by means of a sliding shoe , which is held in contact with the rail. On many systems, an insulating cover is provided above the third rail to protect employees working near the track; sometimes the shoe is designed to contact the side (called "side running") or bottom (called "bottom running" or "under-running") of

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8060-409: Is usually measured in volts , and one volt is the potential for which one joule of work must be expended to bring a charge of one coulomb from infinity. This definition of potential, while formal, has little practical application, and a more useful concept is that of electric potential difference , and is the energy required to move a unit charge between two specified points. An electric field has

8190-536: Is widely used in information processing , telecommunications , and signal processing . Interconnection technologies such as circuit boards , electronics packaging technology, and other varied forms of communication infrastructure complete circuit functionality and transform the mixed components into a regular working system . Today, most electronic devices use semiconductor components to perform electron control. The underlying principles that explain how semiconductors work are studied in solid state physics , whereas

8320-526: The Great Central Railway , built between 1896 and 1899, was the first fully grade-separated railway of this type in the UK. This also applies to light rail and even to street cars . Attempts have been made to increase the capacity of railways by making tracks cross in a grade-separated manner, as opposed to the traditional use of flat crossings to change tracks. A grade-separated rail interchange

8450-721: The Neo-Latin word electricus ("of amber" or "like amber", from ἤλεκτρον, elektron , the Greek word for "amber") to refer to the property of attracting small objects after being rubbed. This association gave rise to the English words "electric" and "electricity", which made their first appearance in print in Thomas Browne 's Pseudodoxia Epidemica of 1646. Isaac Newton made early investigations into electricity, with an idea of his written down in his book Opticks arguably

8580-627: The New York metropolitan area ; the SEPTA Market–Frankford Line in Philadelphia ; and London's Docklands Light Railway . Electric traction trains (using electric power generated at a remote power station and transmitted to the trains) are considerably more cost-effective than diesel or steam units, where separate power units must be carried on each train. This advantage is especially marked in urban and rapid transit systems with

8710-814: The Pyrenees also features a third rail. Many suburban lines that ran out of the Paris Saint Lazare station used third-rail (bottom contact) feed. To mitigate investment costs, the Rotterdam Metro , basically a third-rail-powered system, has been given some outlying branches built on surface tracks as light rail (called sneltram  [ nl ] in Dutch), with numerous level crossings protected with barriers and traffic lights. These branches have overhead wires. The RandstadRail project also requires Rotterdam Metro trains to run under wires along

8840-423: The electromagnetic force , one of the four fundamental forces of nature. Experiment has shown charge to be a conserved quantity , that is, the net charge within an electrically isolated system will always remain constant regardless of any changes taking place within that system. Within the system, charge may be transferred between bodies, either by direct contact, or by passing along a conducting material, such as

8970-546: The loading gauge . There is also a risk of pedestrians walking onto the tracks at level crossings and accidentally touching the third rail, unless grade separation is fully implemented. In the United States, a 1992 Supreme Court of Illinois decision affirmed a $ 1.5 million verdict against the Chicago Transit Authority for failing to stop an intoxicated person from walking onto the tracks at

9100-522: The quantum revolution. Einstein was awarded the Nobel Prize in Physics in 1921 for "his discovery of the law of the photoelectric effect". The photoelectric effect is also employed in photocells such as can be found in solar panels . The first solid-state device was the " cat's-whisker detector " first used in the 1900s in radio receivers. A whisker-like wire is placed lightly in contact with

9230-452: The strong interaction , but unlike that force it operates over all distances. In comparison with the much weaker gravitational force , the electromagnetic force pushing two electrons apart is 10 times that of the gravitational attraction pulling them together. Charge originates from certain types of subatomic particles , the most familiar carriers of which are the electron and proton . Electric charge gives rise to and interacts with

9360-451: The tramway of Bordeaux in 2003), avoid the safety problem by segmenting the powered rail, with each segment being powered only when fully covered by the vehicle which utilizes its power. Third-rail systems using top contact are prone to accumulations of snow, or ice formed from refrozen snow, and this can interrupt operations. Some systems operate dedicated de-icing trains to deposit an oily fluid or antifreeze (such as propylene glycol ) on

9490-541: The 17th and 18th centuries. The development of the theory of electromagnetism in the 19th century marked significant progress, leading to electricity's industrial and residential application by electrical engineers by the century's end. This rapid expansion in electrical technology at the time was the driving force behind the Second Industrial Revolution , with electricity's versatility driving transformations in both industry and society. Electricity

9620-545: The 1936 discovery of the Baghdad Battery , which resembles a galvanic cell , though it is uncertain whether the artifact was electrical in nature. Electricity would remain little more than an intellectual curiosity for millennia until 1600, when the English scientist William Gilbert wrote De Magnete , in which he made a careful study of electricity and magnetism, distinguishing the lodestone effect from static electricity produced by rubbing amber. He coined

9750-583: The A4/M5 junction west of Bristol . Weaving can often cause side-on collisions on very fast roads with top speeds of up to 200 kilometres per hour, as well as the problem of blind spots. Where junctions have unusual designs weaving can be a problem other than on the main road. An example of this can be found at Junction 7 of the M6, where traffic joining the roundabout from the M6 Eastbound off-slip must weave with

9880-677: The Belgian high-speed section and Brussels Midi station or 1.5 kV DC on the railway lines in the south of France for seasonal services. As originally delivered, the Class 373 units were additionally fitted with 750 V DC collection shoes , designed for the journey in London via the suburban commuter lines to Waterloo . A switch between third-rail and overhead collection was performed while running at speed, initially at Continental Junction near Folkestone, and later on at Fawkham Junction after

10010-458: The Eastbound off-slip must leave. Weaving can be alleviated by using collector/distributor roads or braided ramps to separate entering and exiting traffic. In railway construction, grade separation also means the avoidance of level crossings by making any roads or footpaths crossing the line either pass under or over the railway on bridges . This greatly improves safety and is crucial to

10140-656: The Elder and Scribonius Largus , attested to the numbing effect of electric shocks delivered by electric catfish and electric rays , and knew that such shocks could travel along conducting objects. Patients with ailments such as gout or headache were directed to touch electric fish in the hope that the powerful jolt might cure them. Ancient cultures around the Mediterranean knew that certain objects, such as rods of amber , could be rubbed with cat's fur to attract light objects like feathers. Thales of Miletus made

10270-593: The Southern Region third rail network from Farringdon southwards and on overhead line northwards to Bedford , Cambridge and Peterborough . The changeover is made whilst stationary at Farringdon when heading southbound, and at City Thameslink when heading northbound. On the Moorgate to Hertford and Welwyn suburban service routes, the East Coast Main Line sections are 25 kV AC, with

10400-461: The actual spacing depends on the carrying capacity, maximum speed, and service frequency of the line. One method for reducing current losses (and thus increase the spacing of feeder/substations, a major cost in third rail electrification) is to use a composite conductor rail of a hybrid aluminium/steel design. The aluminium is a better conductor of electricity, and a running face of stainless steel gives better wear. There are several ways of attaching

10530-408: The beginning of the field theory of the electric force. Further work was conducted in the 17th and early 18th centuries by Otto von Guericke , Robert Boyle , Stephen Gray and C. F. du Fay . Later in the 18th century, Benjamin Franklin conducted extensive research in electricity, selling his possessions to fund his work. In June 1752 he is reputed to have attached a metal key to the bottom of

10660-419: The building it serves to protect. The concept of electric potential is closely linked to that of the electric field. A small charge placed within an electric field experiences a force, and to have brought that charge to that point against the force requires work . The electric potential at any point is defined as the energy required to bring a unit test charge from an infinite distance slowly to that point. It

10790-515: The capacity of a road compared to an identical road with at-grade junctions. For instance, it is extremely uncommon to find an at-grade junction on a British motorway ; it is all but impossible on a U.S. Interstate Highway , though a few do exist. If traffic can traverse the junction from any direction without being forced to come to a halt, then the junction is described as fully grade separated or free-flowing . These junctions connect two freeways: These junctions connect two roads, but only one

10920-426: The charged particles themselves, hence charge has a tendency to spread itself as evenly as possible over a conducting surface. The magnitude of the electromagnetic force, whether attractive or repulsive, is given by Coulomb's law , which relates the force to the product of the charges and has an inverse-square relation to the distance between them. The electromagnetic force is very strong, second only in strength to

11050-450: The charged parts. Air, for example, tends to arc across small gaps at electric field strengths which exceed 30 kV per centimetre. Over larger gaps, its breakdown strength is weaker, perhaps 1 kV per centimetre. The most visible natural occurrence of this is lightning , caused when charge becomes separated in the clouds by rising columns of air, and raises the electric field in the air to greater than it can withstand. The voltage of

11180-725: The complexity of traffic movements reduces the risk of accidents . Grade-separated road junctions are typically space-intensive, complicated, and costly, due to the need for large physical structures such as tunnels, ramps, and bridges. Their height can be obtrusive, and this, combined with the large traffic volumes that grade-separated roads attract, tend to make them unpopular to nearby landowners and residents. For these reasons, proposals for new grade-separated roads can receive significant public opposition. Rail-over-rail grade separations take up less space than road grade separations: because shoulders are not needed, there are generally fewer branches and side road connections to accommodate (because

11310-420: The conductor rail to prevent the frozen build-up. The third rail can also be heated to alleviate the problem of ice. Unlike overhead line equipment, third-rail systems are not susceptible to strong winds or freezing rain , which can bring down overhead wires and hence disable all trains. Thunderstorms can also disable the power with lightning strikes on systems with overhead wires , disabling trains if there

11440-465: The conductor rail, or a jumper cable may be used to supply enough power to the train to get one of its contact shoes back on the live rail. Avoiding this problem requires a minimum length of trains that can be run on a line. Locomotives have either had the backup of an on-board diesel engine system (e.g., British Rail Class 73 ), or have been connected to shoes on the rolling stock (e.g. Metropolitan Railway ). The first idea for feeding electricity to

11570-555: The conductor rail. The traction current is returned to the generating station through the running rails. In North America, the conductor rail is usually made of high conductivity steel or steel bolted to aluminium to increase the conductivity. Elsewhere in the world, extruded aluminium conductors with stainless steel contact surface or cap, is the preferred technology due to its lower electrical resistance, longer life, and lighter weight. The running rails are electrically connected using wire bonds or other devices, to minimise resistance in

11700-411: The current varies in time. Direct current, as produced by example from a battery and required by most electronic devices, is a unidirectional flow from the positive part of a circuit to the negative. If, as is most common, this flow is carried by electrons, they will be travelling in the opposite direction. Alternating current is any current that reverses direction repeatedly; almost always this takes

11830-742: The depots away from revenue tracks. Third rail electrification is less visually obtrusive than overhead electrification . Several systems use a third rail for part of the route, and other motive power such as overhead catenary or diesel power for the remainder. These may exist because of the connection of separately owned railways using the different motive systems, local ordinances, or other historical reasons. Several types of British trains have been able to operate on both overhead and third-rail systems, including British Rail Class 313 , 319 , 325 , 350 , 365 , 375/6 , 377/2 , 377/5 , 377/7 , 378/2 , 387 , 373 , 395 , 700 and 717 EMUs, as well as Class 92 locomotives. Network Rail claims to run

11960-516: The design and construction of electronic circuits to solve practical problems are part of electronics engineering . Faraday's and Ampère's work showed that a time-varying magnetic field created an electric field, and a time-varying electric field created a magnetic field. Thus, when either field is changing in time, a field of the other is always induced. These variations are an electromagnetic wave . Electromagnetic waves were analysed theoretically by James Clerk Maxwell in 1864. Maxwell developed

12090-571: The direct flow of traffic on one or more of the roads is not disrupted. Instead of a direct connection, traffic must use on and off ramps ( United States , Australia , New Zealand ) or slip roads ( United Kingdom , Ireland ) to access the other roads at the junction. The road which carries on through the junction can also be referred to as grade separated . Typically, large freeways , highways , motorways , or dual carriageways are chosen to be grade separated, through their entire length or for part of it. Grade separation drastically increases

12220-478: The early 19th century had seen rapid progress in electrical science, the late 19th century would see the greatest progress in electrical engineering . Through such people as Alexander Graham Bell , Ottó Bláthy , Thomas Edison , Galileo Ferraris , Oliver Heaviside , Ányos Jedlik , William Thomson, 1st Baron Kelvin , Charles Algernon Parsons , Werner von Siemens , Joseph Swan , Reginald Fessenden , Nikola Tesla and George Westinghouse , electricity turned from

12350-427: The effect was reciprocal: a current exerts a force on a magnet, and a magnetic field exerts a force on a current. The phenomenon was further investigated by Ampère , who discovered that two parallel current-carrying wires exerted a force upon each other: two wires conducting currents in the same direction are attracted to each other, while wires containing currents in opposite directions are forced apart. The interaction

12480-401: The electric circuit. Contact shoes can be positioned below, above, or beside the third rail, depending on the type of third rail used: these third rails are referred to as bottom-contact, top-contact, or side-contact, respectively. The conductor rails have to be interrupted at level crossings , crossovers , and substation gaps. Tapered rails are provided at the ends of each section to allow

12610-448: The electronic electrometer . The movement of electric charge is known as an electric current , the intensity of which is usually measured in amperes . Current can consist of any moving charged particles; most commonly these are electrons, but any charge in motion constitutes a current. Electric current can flow through some things, electrical conductors , but will not flow through an electrical insulator . By historical convention,

12740-416: The electrons. However, depending on the conditions, an electric current can consist of a flow of charged particles in either direction, or even in both directions at once. The positive-to-negative convention is widely used to simplify this situation. The process by which electric current passes through a material is termed electrical conduction , and its nature varies with that of the charged particles and

12870-428: The elementary charge, and any amount of charge an object may carry is a multiple of the elementary charge. An electron has an equal negative charge, i.e. −1.602 176 634 × 10  coulombs . Charge is possessed not just by matter , but also by antimatter , each antiparticle bearing an equal and opposite charge to its corresponding particle. The presence of charge gives rise to an electrostatic force: charges exert

13000-622: The fact that there are already overhead electric wires on part of the route for freight and Regional Eurostar services, led to the change. Also in London, the West London Line changes power supply between Shepherd's Bush and Willesden Junction , where it meets the North London Line. South of the changeover point, the WLL is third rail electrified, north of there, it is overhead . The cross-city Thameslink service runs on

13130-573: The faster speed requires grade separation. Therefore, many high speed lines are elevated, especially in Taiwan and Japan , where population density alongside high speed lines is higher than in France, Italy or Germany. In the United States, a flying junction on the Nickel Plate Road through Cleveland , Ohio , United States was completed in 1913. The most frequent use was later found on

13260-495: The form of a sine wave . Alternating current thus pulses back and forth within a conductor without the charge moving any net distance over time. The time-averaged value of an alternating current is zero, but it delivers energy in first one direction, and then the reverse. Alternating current is affected by electrical properties that are not observed under steady state direct current, such as inductance and capacitance . These properties however can become important when circuitry

13390-508: The former Pennsylvania Railroad main lines. The lines are included as part of the Northeast Corridor and Keystone Corridor now owned by Amtrak . The most complex of these junctions, near Philadelphia Zoo , handles railway traffic for Amtrak, SEPTA , New Jersey Transit , Norfolk Southern , CSX Transportation , and Conrail . In what is known as "area 1520" , which includes the former Soviet Union and other regions using

13520-531: The former mainline railways to The Hague and Hook of Holland. Electricity Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge . Electricity is related to magnetism , both being part of the phenomenon of electromagnetism , as described by Maxwell's equations . Common phenomena are related to electricity, including lightning , static electricity , electric heating , electric discharges and many others. The presence of either

13650-460: The ground, present electric shock hazards, high voltages (above 1500 V) are not considered safe. A very high current must therefore be used to transfer adequate power to the train, resulting in high resistive losses , and requiring relatively closely spaced feed points ( electrical substations ). The electrified rail threatens electrocution of anyone wandering or falling onto the tracks. This can be avoided by using platform screen doors , or

13780-495: The main lines to serve north and mid Kent. As a consequence, these trains are dual-voltage enabled, as the majority of the routes along which they travel are third-rail electrified. In London, the North London Line changes from third rail to overhead electrification between Richmond and Stratford at Acton Central . The entire route originally used third rail, but several technical electrical earthing problems, plus

13910-400: The material through which they are travelling. Examples of electric currents include metallic conduction, where electrons flow through a conductor such as metal, and electrolysis , where ions (charged atoms ) flow through liquids, or through plasmas such as electrical sparks. While the particles themselves can move quite slowly, sometimes with an average drift velocity only fractions of

14040-550: The opening of the first section of the Channel Tunnel Rail Link . Between Kensington Olympia railway station and North Pole depot , further switchovers were necessary. The dual-voltage system did cause some problems. Failure to retract the shoes when entering France caused severe damage to the trackside equipment, causing SNCF to install a pair of concrete blocks at the Calais end of both tunnels to break off

14170-412: The outside environment. Third-rail systems are usually supplied from direct current electricity. Modern tram systems with street-running avoid the risk of electrocution by the exposed electric rail by implementing a segmented ground-level power supply , where each segment is electrified only while covered by a vehicle which is using its power. The third-rail system of electrification is not related to

14300-491: The previous junction. This situation is most prevalent either where the junction designer has placed the on-slip to the road before the off-slip at a junction (for example, the cloverleaf interchange ), or in urban areas with many close-spaced junctions. The ring road of Coventry , England , is a notorious example, as are parts of the southern M25, the London orbital motorway , the M6/M5 junction north-west of Birmingham , and

14430-428: The principle, now known as Faraday's law of induction , that the potential difference induced in a closed circuit is proportional to the rate of change of magnetic flux through the loop. Exploitation of this discovery enabled him to invent the first electrical generator in 1831, in which he converted the mechanical energy of a rotating copper disc to electrical energy. Faraday's disc was inefficient and of no use as

14560-401: The risk can be reduced by placing the conductor rail on the side of the track away from the platform, when allowed by the station layout. The risk can also be reduced by having a coverboard , supported by brackets , to protect the third rail from contact, although many systems do not use one. Where coverboards are used, they reduce the structure gauge near the top of rail. This in turn reduces

14690-557: The safe operation of high-speed lines. The construction of new level crossings is generally not permitted, especially for high speed railway lines and level crossings are increasingly less common due to the increase of both road and rail traffic. Efforts to remove level crossings are done in the UK by Network Rail and in Melbourne as part of the Level Crossing Removal Project . The London Extension of

14820-747: The same gauge, the most complicated grade-separation railpoint is found at Liubotyn in Ukraine . Footbridges and subways (called underpasses in North America as well as in the United Kingdom when referring to roads) may be employed to allow pedestrians and cyclists to cross busy or fast streets. They are often used over and under motorways since at grade pedestrian crossings are generally not permitted. Same can be said for railways. Though introduced to Central Park in New York City in

14950-454: The special property that it is conservative , which means that the path taken by the test charge is irrelevant: all paths between two specified points expend the same energy, and thus a unique value for potential difference may be stated. The volt is so strongly identified as the unit of choice for measurement and description of electric potential difference that the term voltage sees greater everyday usage. For practical purposes, defining

15080-415: The stainless steel to the aluminium. The oldest is a co-extruded method, where the stainless steel is extruded with the aluminium. This method has suffered, in isolated cases, from de-lamination (where the stainless steel separates from the aluminium); this is said to have been eliminated in the latest co-extruded rails. A second method is an aluminium core, upon which two stainless steel sections are fitted as

15210-450: The third rail shoes if they had not been retracted. An accident occurred in the UK when a Eurostar driver failed to retract the pantograph before entering the third-rail system, damaging a signal gantry and the pantograph. On 14 November 2007, Eurostar's passenger operations were transferred to St Pancras railway station and maintenance operations to Temple Mills depot, making the 750 V DC third rail collection equipment redundant and

15340-560: The third rail shoes were removed. The trains themselves are no longer fitted with a speedometer capable of measuring the speed in miles per hour (the indication used to automatically change when the collector shoes were deployed). In 2009, Southeastern began operating domestic services over High Speed 1 trackage from St Pancras using its new Class 395 EMUs. These services operate on the High Speed line as far as Ebbsfleet International or Ashford International , before transferring to

15470-669: The third rail used in dual-gauge railways. Third-rail systems are a means of providing electric traction power to trains using an additional rail (called a "conductor rail") for the purpose. On most systems, the conductor rail is placed on the sleeper ends outside the running rails, but in some systems a central conductor rail is used. The conductor rail is supported on ceramic insulators (known as "pots"), at top contact or insulated brackets , at bottom contact, typically at intervals of around 10 feet (3.0 m). The trains have metal contact blocks called collector shoes (also known as contact shoes or pickup shoes) which make contact with

15600-443: The third rail, allowing the protective cover to be mounted directly to its top surface. When the shoe slides along the top surface, it is referred to as "top running". When the shoe slides along the bottom surface, it is less affected by the build-up of snow, ice, or leaves, and reduces the chances of a person being electrocuted by coming in contact with the rail. Examples of systems using under-running third rail include Metro-North in

15730-400: The tightly grouped nest of flying junctions to the north of Clapham Junction railway station —although technically a combination of many junctions—handle more than 4,000 trains per day (about one train every 15 seconds). Virtually all major railway lines no longer cross (forming an 'X' shape ) at flat level (although many diverge - i.e. 'Y' shape). On almost all high-speed railway lines,

15860-548: The traffic already on the roundabout wishing to use the M6 Westbound on-slip. This is as a result of the slip roads on the west side of the junction connecting to the roundabout on the inside of the eastern arc rather than the outside of the western arc as is normal. The two slip-roads are connected by a single lane on the inside of the roundabout, which traffic wishing to use the Westbound on-slip must join, and traffic from

15990-569: The train) and switch it off before it becomes exposed again. This system (called Alimentation par Sol (APS), meaning 'current supply via ground') is used in various locations around the city but especially in the historic centre: elsewhere the trams use the conventional overhead lines (see also ground-level power supply ). In summer 2006 it was announced that two new French tram systems would be using APS over part of their networks. These will be Angers and Reims , with both systems expected to open around 2009–2010. The French Culoz–Modane railway

16120-479: The trains. Whereas overhead-wire systems can operate at 25 kV or more, using alternating current (AC), the smaller clearance around a live rail imposes a maximum of about 1200 V, with some systems using 1500 V ( Line 4, Guangzhou Metro , Line 5, Guangzhou Metro , Line 3, Shenzhen Metro ), and direct current (DC) is used. Trains on some lines or networks use both power supply modes (see § Mixed systems below). All third-rail systems throughout

16250-418: The two forces of nature then known. The force on the compass needle did not direct it to or away from the current-carrying wire, but acted at right angles to it. Ørsted's words were that "the electric conflict acts in a revolving manner." The force also depended on the direction of the current, for if the flow was reversed, then the force did too. Ørsted did not fully understand his discovery, but he observed

16380-417: The vector direction of the field is the line of greatest slope of potential, and where the equipotentials lie closest together. Ørsted's discovery in 1821 that a magnetic field existed around all sides of a wire carrying an electric current indicated that there was a direct relationship between electricity and magnetism. Moreover, the interaction seemed different from gravitational and electrostatic forces,

16510-436: The world are energised with DC supplies. Some of the reasons for this are historical. Early traction engines were DC motors, and the then-available rectifying equipment was large, expensive and impractical to install onboard trains. Also, transmission of the relatively high currents required results in higher losses with AC than DC. Substations for a DC system will have to be (typically) about 2 kilometres (1.2 miles) apart, though

16640-543: The world's largest third-rail network. On the southern region of British Rail, freight yards had overhead wires to avoid the electrocution hazards of a third rail. The locomotives were fitted with a pantograph as well as pick-up shoes. The Class 373 used for international high-speed rail services operated by Eurostar through the Channel Tunnel runs on overhead wires at 25 kV AC for most of its journey, with sections of 3 kV DC on Belgian lines between

16770-636: Was electrified with 1500 V DC third rail, later converted to overhead wires at the same voltage. Stations had overhead wires from the beginning. The French branch line which serves Chamonix and the Mont Blanc region ( Saint-Gervais-le-Fayet to Vallorcine ) is third rail (top contact) and metre gauge. It continues in Switzerland, partly with the same third-rail system, partly with an overhead line. The 63 km (39 mi) long Train Jaune line in

16900-543: Was forced to move within the field; they are however an imaginary concept with no physical existence, and the field permeates all the intervening space between the lines. Field lines emanating from stationary charges have several key properties: first, that they originate at positive charges and terminate at negative charges; second, that they must enter any good conductor at right angles, and third, that they may never cross nor close in on themselves. A hollow conducting body carries all its charge on its outer surface. The field

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