1.
Short circuit
–
A short circuit is an electrical circuit that allows a current to travel along an unintended path with no or a very low electrical impedance. The electrical opposite of a circuit is an open circuit. It is common to misuse short circuit to describe any electrical malfunction, a short circuit is an abnormal connection between two nodes of an electric circuit intended to be at different voltages. This results in an electric current limited only by the Thévenin equivalent resistance of the rest of the network and potentially causes circuit damage, overheating. Although usually the result of a fault, there are cases where short circuits are caused intentionally, for example, in circuit analysis, a short circuit is a connection between two nodes that forces them to be at the same voltage. In an ideal circuit, this means there is no resistance. In real circuits, the result is a connection with almost no resistance, in such a case, the current is limited by the rest of the circuit. A common type of short circuit occurs when the positive and negative terminals of a battery are connected with a low-resistance conductor, with low resistance in the connection, a high current exists, causing the cell to deliver a large amount of energy in a short time. Overloaded wires can also overheat, sometimes causing damage to the wires insulation, in mains circuits, short circuits may occur between two phases, between a phase and neutral or between a phase and earth. Such short circuits are likely to result in a high current. However, it is possible for circuits to arise between neutral and earth conductors, and between two conductors of the same phase. Such short circuits can be dangerous, particularly as they may not immediately result in a current and are therefore less likely to be detected. Possible effects include unexpected energisation of a circuit presumed to be isolated, to help reduce the negative effects of short circuits, power distribution transformers are deliberately designed to have a certain amount of leakage reactance. The leakage reactance helps limit both the magnitude and rate of rise of the fault current, a short circuit may lead to formation of an electric arc. The arc, a channel of hot ionized plasma, is highly conductive, surface erosion is a typical sign of electric arc damage. Even short arcs can remove significant amount of materials from the electrodes, a short circuit fault current can, within milliseconds, be thousands of times larger than the normal operating current of the system. Damage from short circuits can be reduced or prevented by employing fuses, circuit breakers, or other overload protection, overload protection must be chosen according to the current rating of the circuit. Circuits for large home appliances require protective devices set or rated for higher currents than lighting circuits, wire gauges specified in building and electrical codes are chosen to ensure safe operation in conjunction with the overload protection
Short circuit
–
Tree limbs cause a short circuit, triggering an
electrical arc during a storm
2.
Nail (fastener)
–
In woodworking and construction, a nail is a pin-shaped object of metal which is used as a fastener, as a peg to hang something, or sometimes as a decoration. Generally nails have a point on one end and a flattened head on the other. Nails are made in a variety of forms for specialized purposes. The most common is a wire nail, other types of nails include pins, tacks, brads, and cleats. Nails are typically driven into the workpiece by a hammer, a nail gun. A nail holds materials together by friction in the axial direction, the point of the nail is also sometimes bent over or clinched after driving to prevent pulling out. The history of the nail is divided roughly into three periods, Hand-wrought nail Cut nail Wire nail To make a wrought-iron nail, iron ore is heated with carbon. To make a nail, a blacksmith heats the rod in a forge, next, the smith cuts off the taper, and inserts it into a nail heading tool with a square hole. The top of the taper is hammered downward to form a head, Nails date back at least to Ancient Egypt — bronze nails found in Egypt have been dated 3400 BC. The Romans made extensive use of nails, the Roman army, for example, left behind seven tons of nails when it evacuated the fortress of Inchtuthil in Perthshire in the United Kingdom in 86 or 87 CE. The term penny, as it refers to nails, probably originated in medieval England to describe the price of a hundred nails, Nails themselves were sufficiently valuable and standardized to be used as an informal medium of exchange. Until around 1800 artisans known as nailers or nailors made nails by hand – note the surname Naylor, at the time of the American Revolution, England was the largest manufacturer of nails in the world. Nails were expensive and difficult to obtain in the American colonies, families often had small nail-manufacturing setups in their homes, during bad weather and at night, the entire family might work at making nails for their own use and for barter. Thomas Jefferson wrote in a letter, In our private pursuits it is an advantage that every honest employment is deemed honorable. I am myself a nail maker, the production of wrought-iron nails continued well into the 19th century, but ultimately was reduced to nails for purposes for which the softer cut nails were unsuitable, including horseshoe nails. Originally, nails were handmade, and the process was slow, nails were relatively few. This naturally produced a desire to create machines to speed up and these nails were known as cut nails or square nails because of their roughly rectangular cross section. Cut nails were one of the important factors in the increase in balloon framing beginning in the 1830s, though still used for historical renovations, and for heavy-duty applications, such as attaching boards to masonry walls, cut nails are much less common today than wire nails
Nail (fastener)
–
A pile of nails
Nail (fastener)
–
Nails can be
hammered or shot into materials such as
wood.
Nail (fastener)
–
Different types of nails: 1) Roofing nail, 2) Umbrella head roofing nail, 3) Brass escutcheon pin, 4) Finish nail, 5) Concrete nail, 6) Spiral-shank nail, and 7) Ring-shank nail (the barbs are a leftover component of the feed system of a nail gun)
Nail (fastener)
–
Horseshoe nails
3.
Electrical breakdown
–
Electrical breakdown or dielectric breakdown is a long reduction in the resistance of an electrical insulator when the voltage applied across it exceeds the breakdown voltage. This results in the insulator becoming electrically conductive, electrical breakdown may be a momentary event, or may lead to a discontinuous arc charge if protective devices fail to interrupt the current in a low power circuit. Under sufficient electrical stress, electrical breakdown can occur within solids, liquids, however, the specific breakdown mechanisms are significantly different for each, particularly in different kinds of dielectric medium. Electrical breakdown can occur across the insulators that suspend overhead power lines, within underground power cables. Breakdown mechanisms seem to differ in solids, liquids, and gasses, breakdown is influenced by electrode material, sharp curvature of conductor material, the size of the gap between the electrodes, and the density of the material in the gap. In solid materials a long-time partial discharge typically precedes breakdown, degrading the insulators, ultimately the partial discharge chars through a channel of carbonized material that conducts current across the gap. There are several mechanisms for breakdown in liquids, bubbles, small impurities. In liquefied gases used as coolants for superconductivity – such as Helium at 4.2 K or Nitrogen at 96 K – bubbles should induce breakdown, in oil-cooled and oil-insulated transformers the field strength for breakdown is about 20 kV/mm. Despite the purified oils used, small particle contaminants are blamed, thermal effects are proposed for gases by some authors, since breakdown in 50/60 Hz AC lines sometimes occurs long after the maximum voltage is reached. Electrical breakdown occurs within a gas when the strength of the gas is exceeded. Regions of intense voltage gradients can cause gas to partially ionize. This is done deliberately in low pressure such as in fluorescent lights or in an electrostatic precipitator. The voltage that leads to electrical breakdown of a gas is approximated by Paschens Law, partial electrical breakdown of the air causes the fresh air smell of ozone during thunderstorms or around high-voltage equipment. Although air is normally an excellent insulator, when stressed by a high voltage, air can begin to break down. Across relatively small gaps, breakdown voltage in air is a function of gap length times pressure, if the voltage is sufficiently high, complete electrical breakdown of the air will culminate in an electrical spark or an electric arc that bridges the entire gap. The color of the spark depends upon the gases that make up the gaseous media, while the small sparks generated by static electricity may barely be audible, larger sparks are often accompanied by a loud snap or bang. Lightning is an example of a spark that can be many miles long. If a fuse or circuit breaker fails to interrupt the current through a spark in a circuit, current may continue
Electrical breakdown
–
Electrical breakdown in an
electric discharge showing the ribbon-like
plasma filaments from a
Tesla coil.
Electrical breakdown
–
Voltage-current relation before breakdown
Electrical breakdown
–
Dielectric breakdown within a solid insulator can permanently change its appearance and properties.
4.
Gas
–
Gas is one of the four fundamental states of matter. A pure gas may be made up of atoms, elemental molecules made from one type of atom. A gas mixture would contain a variety of pure gases much like the air, what distinguishes a gas from liquids and solids is the vast separation of the individual gas particles. This separation usually makes a colorless gas invisible to the human observer, the interaction of gas particles in the presence of electric and gravitational fields are considered negligible as indicated by the constant velocity vectors in the image. One type of commonly known gas is steam, the gaseous state of matter is found between the liquid and plasma states, the latter of which provides the upper temperature boundary for gases. Bounding the lower end of the temperature scale lie degenerative quantum gases which are gaining increasing attention, high-density atomic gases super cooled to incredibly low temperatures are classified by their statistical behavior as either a Bose gas or a Fermi gas. For a comprehensive listing of these states of matter see list of states of matter. The only chemical elements which are stable multi atom homonuclear molecules at temperature and pressure, are hydrogen, nitrogen and oxygen. These gases, when grouped together with the noble gases. Alternatively they are known as molecular gases to distinguish them from molecules that are also chemical compounds. The word gas is a neologism first used by the early 17th-century Flemish chemist J. B. van Helmont, according to Paracelsuss terminology, chaos meant something like ultra-rarefied water. An alternative story is that Van Helmonts word is corrupted from gahst and these four characteristics were repeatedly observed by scientists such as Robert Boyle, Jacques Charles, John Dalton, Joseph Gay-Lussac and Amedeo Avogadro for a variety of gases in various settings. Their detailed studies ultimately led to a relationship among these properties expressed by the ideal gas law. Gas particles are separated from one another, and consequently have weaker intermolecular bonds than liquids or solids. These intermolecular forces result from interactions between gas particles. Like-charged areas of different gas particles repel, while oppositely charged regions of different gas particles attract one another, transient, randomly induced charges exist across non-polar covalent bonds of molecules and electrostatic interactions caused by them are referred to as Van der Waals forces. The interaction of these forces varies within a substance which determines many of the physical properties unique to each gas. A comparison of boiling points for compounds formed by ionic and covalent bonds leads us to this conclusion, the drifting smoke particles in the image provides some insight into low pressure gas behavior
Gas
–
Drifting
smoke particles provide clues to the movement of the surrounding gas.
Gas
–
Gas phase particles (
atoms,
molecules, or
ions) move around freely in the absence of an applied
electric field.
Gas
–
Shuttle imagery of re-entry phase.
Gas
–
21 April 1990 eruption of
Mount Redoubt,
Alaska, illustrating real gases not in thermodynamic equilibrium.
5.
Electrical discharge
–
Electric discharge describes any flow of electric charge through a gas, liquid or solid. Tiny pulses of current are used to detect ionizing radiation in a Geiger–Müller tube, a low steady current can be used to illustrate the spectrum of gases in a Gas-filled tube. A neon lamp is an example of a lamp, useful both for illumination and as a voltage regulator. A flashtube generates a pulse of intense light useful for photography by sending a heavy current through a gas arc discharge. Corona discharges are used in photocopiers, Electric discharges can convey substantial energy to the electrodes at the ends of the discharge. A spark gap is used in combustion engines to ignite the fuel/air mixture on every power stroke. Spark gaps are used to switch heavy currents in a Marx generator. In electric discharge machining, multiple tiny electric arcs are used to erode a conductive workpiece to a finished shape, arc welding is used to assemble heavy steel structures, where the base metal is heated to melting by the heat of the arc. An electric arc furnace sustains arc currents of tens of thousands of amperes and is used for steelmaking and production of alloys, st. Elmos fire Lightning Electric organ Electrical breakdown Electric discharge in gases E/N ratio Lichtenberg figure Space charge Debye sheath
Electrical discharge
6.
Electric current
–
An electric current is a flow of electric charge. In electric circuits this charge is carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an ionised gas. The SI unit for measuring a current is the ampere. Electric current is measured using a device called an ammeter, electric currents cause Joule heating, which creates light in incandescent light bulbs. They also create magnetic fields, which are used in motors, inductors and generators, the particles that carry the charge in an electric current are called charge carriers. In metals, one or more electrons from each atom are loosely bound to the atom and these conduction electrons are the charge carriers in metal conductors. The conventional symbol for current is I, which originates from the French phrase intensité de courant, current intensity is often referred to simply as current. The I symbol was used by André-Marie Ampère, after whom the unit of current is named, in formulating the eponymous Ampères force law. The notation travelled from France to Great Britain, where it became standard, in a conductive material, the moving charged particles which constitute the electric current are called charge carriers. In other materials, notably the semiconductors, the carriers can be positive or negative. Positive and negative charge carriers may even be present at the same time, a flow of positive charges gives the same electric current, and has the same effect in a circuit, as an equal flow of negative charges in the opposite direction. Since current can be the flow of positive or negative charges. The direction of current is arbitrarily defined as the same direction as positive charges flow. This is called the direction of current I. If the current flows in the direction, the variable I has a negative value. When analyzing electrical circuits, the direction of current through a specific circuit element is usually unknown. Consequently, the directions of currents are often assigned arbitrarily
Electric current
–
A simple electric circuit, where current is represented by the letter i. The relationship between the voltage (V), resistance (R), and current (I) is V=IR; this is known as
Ohm's Law.
7.
Electrical conductance
–
The electrical resistance of an electrical conductor is a measure of the difficulty to pass an electric current through that conductor. The inverse quantity is electrical conductance, and is the ease with which a current passes. Electrical resistance shares some parallels with the notion of mechanical friction. The SI unit of resistance is the ohm, while electrical conductance is measured in siemens. An object of uniform cross section has a proportional to its resistivity and length. All materials show some resistance, except for superconductors, which have a resistance of zero and this proportionality is called Ohms law, and materials that satisfy it are called ohmic materials. In other cases, such as a diode or battery, V and I are not directly proportional. The ratio V/I is sometimes useful, and is referred to as a chordal resistance or static resistance, since it corresponds to the inverse slope of a chord between the origin and an I–V curve. In other situations, the derivative d V d I may be most useful, in the hydraulic analogy, current flowing through a wire is like water flowing through a pipe, and the voltage drop across the wire is like the pressure drop that pushes water through the pipe. Conductance is proportional to how much flow occurs for a given pressure, the voltage drop, not the voltage itself, provides the driving force pushing current through a resistor. In hydraulics, it is similar, The pressure difference between two sides of a pipe, not the pressure itself, determines the flow through it, for example, there may be a large water pressure above the pipe, which tries to push water down through the pipe. But there may be a large water pressure below the pipe. If these pressures are equal, no water flows, in the same way, a long, thin copper wire has higher resistance than a short, thick copper wire. A pipe filled with hair restricts the flow of more than a clean pipe of the same shape. The difference between copper, steel, and rubber is related to their structure and electron configuration. In addition to geometry and material, there are other factors that influence resistance and conductance, such as temperature. Substances in which electricity can flow are called conductors, a piece of conducting material of a particular resistance meant for use in a circuit is called a resistor. Conductors are made of high-conductivity materials such as metals, in particular copper, Resistors, on the other hand, are made of a wide variety of materials depending on factors such as the desired resistance, amount of energy that it needs to dissipate, precision, and costs
Electrical conductance
–
A 6.5 MΩ
resistor, as identified by its
electronic color code (blue–green–black-yellow). An
ohmmeter could be used to verify this value.
Electrical conductance
–
Electromagnetism
Electrical conductance
–
Running current through a material with high resistance creates heat, in a phenomenon called
Joule heating. In this picture, a
cartridge heater, warmed by Joule heating, is
glowing red hot.
8.
Plasma (physics)
–
Plasma is one of the four fundamental states of matter, the others being solid, liquid, and gas. Yet unlike these three states of matter, plasma does not naturally exist on the Earth under normal surface conditions, the term was first introduced by chemist Irving Langmuir in the 1920s. However, true plasma production is from the separation of these ions and electrons that produces an electric field. Based on the environmental temperature and density either partially ionised or fully ionised forms of plasma may be produced. The positive charge in ions is achieved by stripping away electrons from atomic nuclei, the number of electrons removed is related to either the increase in temperature or the local density of other ionised matter. Plasma may be the most abundant form of matter in the universe, although this is currently tentative based on the existence. Plasma is mostly associated with the Sun and stars, extending to the rarefied intracluster medium, Plasma was first identified in a Crookes tube, and so described by Sir William Crookes in 1879. The nature of the Crookes tube cathode ray matter was identified by British physicist Sir J. J. The term plasma was coined by Irving Langmuir in 1928, perhaps because the glowing discharge molds itself to the shape of the Crookes tube and we shall use the name plasma to describe this region containing balanced charges of ions and electrons. Plasma is a neutral medium of unbound positive and negative particles. Although these particles are unbound, they are not ‘free’ in the sense of not experiencing forces, in turn this governs collective behavior with many degrees of variation. The average number of particles in the Debye sphere is given by the plasma parameter, bulk interactions, The Debye screening length is short compared to the physical size of the plasma. This criterion means that interactions in the bulk of the plasma are more important than those at its edges, when this criterion is satisfied, the plasma is quasineutral. Plasma frequency, The electron plasma frequency is compared to the electron-neutral collision frequency. When this condition is valid, electrostatic interactions dominate over the processes of ordinary gas kinetics, for plasma to exist, ionization is necessary. The term plasma density by itself refers to the electron density, that is. The degree of ionization of a plasma is the proportion of atoms that have lost or gained electrons, even a partially ionized gas in which as little as 1% of the particles are ionized can have the characteristics of a plasma. The degree of ionization, α, is defined as α = n i n i + n n, where n i is the number density of ions and n n is the number density of neutral atoms
Plasma (physics)
–
Plasma
Plasma (physics)
Plasma (physics)
Plasma (physics)
9.
Visible light
–
Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to light, which is visible to the human eye and is responsible for the sense of sight. Visible light is defined as having wavelengths in the range of 400–700 nanometres, or 4.00 × 10−7 to 7.00 × 10−7 m. This wavelength means a range of roughly 430–750 terahertz. The main source of light on Earth is the Sun, sunlight provides the energy that green plants use to create sugars mostly in the form of starches, which release energy into the living things that digest them. This process of photosynthesis provides virtually all the used by living things. Historically, another important source of light for humans has been fire, with the development of electric lights and power systems, electric lighting has effectively replaced firelight. Some species of animals generate their own light, a process called bioluminescence, for example, fireflies use light to locate mates, and vampire squids use it to hide themselves from prey. Visible light, as all types of electromagnetic radiation, is experimentally found to always move at this speed in a vacuum. In physics, the term sometimes refers to electromagnetic radiation of any wavelength. In this sense, gamma rays, X-rays, microwaves and radio waves are also light, like all types of light, visible light is emitted and absorbed in tiny packets called photons and exhibits properties of both waves and particles. This property is referred to as the wave–particle duality, the study of light, known as optics, is an important research area in modern physics. Generally, EM radiation, or EMR, is classified by wavelength into radio, microwave, infrared, the behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths, when EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. There exist animals that are sensitive to various types of infrared, infrared sensing in snakes depends on a kind of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, which is how these animals detect it, above the range of visible light, ultraviolet light becomes invisible to humans, mostly because it is absorbed by the cornea below 360 nanometers and the internal lens below 400. Furthermore, the rods and cones located in the retina of the eye cannot detect the very short ultraviolet wavelengths and are in fact damaged by ultraviolet. Many animals with eyes that do not require lenses are able to detect ultraviolet, by quantum photon-absorption mechanisms, various sources define visible light as narrowly as 420 to 680 to as broadly as 380 to 800 nm
Visible light
–
An example of refraction of light. The straw appears bent, because of refraction of light as it enters liquid from air.
Visible light
–
A triangular prism dispersing a beam of white light. The longer wavelengths (red) and the shorter wavelengths (blue) get separated.
Visible light
–
A
cloud illuminated by
sunlight
Visible light
–
A
city illuminated by
artificial lighting
10.
Glow discharge
–
A glow discharge is a plasma formed by the passage of electric current through a low-pressure gas. It is created by applying a voltage between two electrodes in a glass tube containing gas. When the voltage exceeds a value called the striking voltage, the gas in the tube ionizes, becoming a plasma. The color depends on the gas used, glow discharge is widely used as a source of light in devices such as neon lights, fluorescent lamps, and plasma-screen televisions. Analyzing the light produced by spectroscopy can reveal much about the interactions in the gas, so glow discharge is used in plasma physics. It is also used in the surface treatment technique called sputtering, the simplest type of glow discharge is a direct-current glow discharge. In its simplest form, it consists of two electrodes in a cell held at low pressure, the cell is typically filled with neon, but other gases can also be used. An electric potential of several hundred volts is applied between the two electrodes, a small fraction of the population of atoms within the cell is initially ionized through random processes. The ions are driven towards the cathode by the potential. The initial population of ions and electrons collides with other atoms, as long as the potential is maintained, a population of ions and electrons remains. Some of the kinetic energy is transferred to the cathode. This happens partially through the ions striking the cathode directly, the primary mechanism, however, is less direct. Ions strike the more numerous neutral gas atoms, transferring a portion of their energy to them and these neutral atoms then strike the cathode. Whichever species strike the cathode, collisions within the cathode redistribute this energy until a portion of the cathode is ejected and this process is known as sputtering. Once free of the cathode, atoms move into the bulk of the discharge through drift. The atoms can then be excited by collisions with ions, electrons, once excited, atoms will lose their energy fairly quickly. Of the various ways that this energy can be lost, the most important is radiatively, in atomic mass spectrometry, these ions are detected. Their mass identifies the type of atoms and their quantity reveals the amount of element in the sample
Glow discharge
–
NE-2 type neon lamp powered by
alternating current
Glow discharge
–
DC powered neon lamp, showing glow discharge surrounding only the cathode
Glow discharge
–
A 5651 voltage-regulator tube in operation
11.
Thermionic emission
–
Thermionic emission is the thermally induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the energy given to the carrier overcomes the work function of the material. The charge carriers can be electrons or ions, and in literature are sometimes referred to as thermions. After emission, a charge that is equal in magnitude and opposite in sign to the total charge emitted is initially left behind in the emitting region, the classical example of thermionic emission is the emission of electrons from a hot cathode into a vacuum in a vacuum tube. The hot cathode can be a metal filament, a metal filament. Vacuum emission from metals tends to become significant only for temperatures over 1,000 K, the term thermionic emission is now also used to refer to any thermally-excited charge emission process, even when the charge is emitted from one solid-state region into another. This process is important in the operation of a variety of electronic devices. The magnitude of the flow increases dramatically with increasing temperature. The phenomenon was reported in 1853 by Edmond Becquerel. It was rediscovered in 1873 by Frederick Guthrie in Britain, while doing work on charged objects, Guthrie discovered that a red-hot iron sphere with a negative charge would lose its charge. He also found that this did not happen if the sphere had a positive charge, other early contributors included Johann Wilhelm Hittorf, Eugen Goldstein, and Julius Elster and Hans Friedrich Geitel. Edison built several experimental lamp bulbs with a wire, metal plate, or foil inside the bulb that was separate from the filament. He connected a galvanometer, a used to measure current. If the foil was put at a negative potential relative to the filament and we now know that the filament was emitting electrons, which were attracted to a positively charged foil, but not a negatively charged one. This one-way current was called the Edison effect and he found that the current emitted by the hot filament increased rapidly with increasing voltage, and filed a patent application for a voltage-regulating device using the effect on November 15,1883. He found that sufficient current would pass through the device to operate a telegraph sounder and this was exhibited at the International Electrical Exposition in Philadelphia in September 1884. William Preece, a British scientist, took back with him several of the Edison effect bulbs and he presented a paper on them in 1885, where he referred to thermionic emission as the Edison Effect. The British physicist John Ambrose Fleming, working for the British Wireless Telegraphy Company, Fleming went on to develop the two-element vacuum tube known as the diode, which he patented on November 16,1904
Thermionic emission
–
Closeup of the
filament on a low pressure mercury
gas discharge lamp showing white thermionic emission mix coating on the central portion of the coil. Typically made of a mixture of
barium,
strontium and
calcium oxides, the coating is
sputtered away through normal use, often eventually resulting in lamp failure.
Thermionic emission
–
One of the bulbs with which Edison discovered thermionic emission. It consists of an evacuated glass light bulb containing a carbon filament (hairpin shape), with an additional metal plate attached to wires emerging from the base. Electrons released by the filament were attracted to the plate when it had a positive voltage.
12.
Electrode
–
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit. The word was coined by William Whewell at the request of the scientist Michael Faraday from the Greek words elektron, meaning amber, and hodos, an electrode in an electrochemical cell is referred to as either an anode or a cathode. The anode is now defined as the electrode at which electrons leave the cell and oxidation occurs, each electrode may become either the anode or the cathode depending on the direction of current through the cell. A bipolar electrode is an electrode that functions as the anode of one cell, a primary cell is a special type of electrochemical cell in which the reaction cannot be reversed, and the identities of the anode and cathode are therefore fixed. The anode is always the negative electrode, the cell can be discharged but not recharged. A secondary cell, for example a rechargeable battery, is a cell in which the reactions are reversible. When the cell is being charged, the anode becomes the positive and this is also the case in an electrolytic cell. When the cell is being discharged, it behaves like a cell, with the anode as the negative. In a vacuum tube or a semiconductor having polarity the anode is the positive electrode, the electrons enter the device through the cathode and exit the device through the anode. Many devices have other electrodes to control operation, e. g. base, gate, control grid. In a three-electrode cell, an electrode, also called an auxiliary electrode, is used only to make a connection to the electrolyte so that a current can be applied to the working electrode. The counter electrode is made of an inert material, such as a noble metal or graphite. In arc welding an electrode is used to conduct current through a workpiece to fuse two pieces together. Depending upon the process, the electrode is either consumable, in the case of gas metal arc welding or shielded metal arc welding, or non-consumable, such as in gas tungsten arc welding. For a direct current system the weld rod or stick may be a cathode for a filling type weld or an anode for other welding processes, for an alternating current arc welder the welding electrode would not be considered an anode or cathode. Electrodes are used to provide current through nonmetal objects to them in numerous ways. These electrodes are used for advanced purposes in research and investigation
Electrode
–
Electrodes used in
arc welding
13.
Arc suppression
–
Arc suppression is the reduction of sparks formed when current-carrying contacts are separated. The spark is a discharge of highly energized electrons and ions. In industrial, military and consumer electronic design, the method generally applies to devices such as electromechanical power switches, relays. In this context, arc suppression is contact protection, the break arc is typically more energetic and thus more destructive. The temperature of the electric arc is very high, causing the metal on the contact surfaces to melt, pool. The high temperature of the arc cracks the surrounding gas molecules creating ozone, carbon monoxide, the arc energy slowly destroys the contact metal, causing some material to escape into the air as fine particulate matter. This very activity causes the material in the contacts to degrade quickly, Arc suppression is an area of interest in engineering because of the destructive effects of the electrical arc to electromechanical power switches, relays and contactors’ points of contact. Common devices used to prevent arcs are capacitors, snubbers, diodes, Zener diodes, varistors, reduced heat generation resulting in less heat management measures such as venting and fans. Reduced Electromagnetic Interference from arcs - a common source of radiated EMI, contact protection Relays - Frequently Asked Questions Arc suppression articles on Institute of Electrical and Electronics Engineers website
Arc suppression
–
(left) Pristine contacts from a relay (right) The nearly destroyed contacts from a relay operated under power for nearly 100,000 cycles
Arc suppression
–
Screen captures from an oscilloscope measuring the arc energy: current shown by blue line (sinusoidal wave), 2V/div = 5A/div; voltage shown by red line,10V/div. (left) Unsuppressed AC power electrical arc (right) An identical arc with suppression.
14.
Fluorescent lamp
–
A fluorescent lamp or a fluorescent tube is a low pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor which produces ultraviolet light that then causes a phosphor coating on the inside of the lamp to glow. A fluorescent lamp converts electrical energy into useful light much more efficiently than incandescent lamps, the typical luminous efficacy of fluorescent lighting systems is 50–100 lumens per watt, several times the efficacy of incandescent bulbs with comparable light output. Compact fluorescent lamps are now available in the same sizes as incandescents and are used as an energy-saving alternative in homes. Because they contain mercury, many fluorescent lamps are classified as hazardous waste, the United States Environmental Protection Agency recommends that fluorescent lamps be segregated from general waste for recycling or safe disposal, and some jurisdictions require recycling of them. Fluorescence of certain rocks and other substances had been observed for hundreds of years before its nature was understood, by the middle of the 19th century, experimenters had observed a radiant glow emanating from partially evacuated glass vessels through which an electric current passed. The explanation relied on the nature of electricity and light phenomena as developed by the British scientists Michael Faraday in the 1840s and James Clerk Maxwell in the 1860s. Little more was done with this phenomenon until 1856 when German glassblower Heinrich Geissler created a vacuum pump that evacuated a glass tube to an extent not previously possible. Geissler invented the first gas-discharge lamp, the Geissler tube, consisting of an evacuated glass tube with a metal electrode at either end. When a high voltage was applied between the electrodes, the inside of the lit up with a glow discharge. By putting different chemicals inside, the tubes could be made to produce a variety of colors, more important, however, was its contribution to scientific research. One of the first scientists to experiment with a Geissler tube was Julius Plücker who systematically described in 1858 the luminescent effects that occurred in a Geissler tube and he also made the important observation that the glow in the tube shifted position when in proximity to an electromagnetic field. Alexandre Edmond Becquerel observed in 1859 that certain substances gave off light when they were placed in a Geissler tube and he went on to apply thin coatings of luminescent materials to the surfaces of these tubes. Fluorescence occurred, but the tubes were very inefficient and had an operating life. Inquiries that began with the Geissler tube continued as even better vacuums were produced, the most famous was the evacuated tube used for scientific research by William Crookes. That tube was evacuated by the highly effective mercury vacuum pump created by Hermann Sprengel, Research conducted by Crookes and others ultimately led to the discovery of the electron in 1897 by J. J. Thomson and X-rays in 1895 by Wilhelm Roentgen. While Becquerel was interested primarily in conducting research into fluorescence. Although Edison lost interest in fluorescent lighting, one of his employees was able to create a gas-based lamp that achieved a measure of commercial success
Fluorescent lamp
–
Fluorescent lamps
Fluorescent lamp
–
Top, two compact fluorescent lamps. Bottom, two fluorescent tube lamps. A matchstick, left, is shown for scale.
Fluorescent lamp
–
Compact fluorescent lamp with
electronic ballast
Fluorescent lamp
–
Typical F71T12 100 W bi-pin lamp used in tanning beds. The (Hg) symbol indicates that this lamp contains
mercury. In the US, this symbol is now required on all fluorescent lamps that contain mercury.
15.
Metal-halide lamp
–
It is a type of high-intensity discharge gas discharge lamp. Developed in the 1960s, they are similar to mercury vapor lamps, but contain additional metal halide compounds in the arc tube. The most common metal halide compound used is sodium iodide, once the arc tube reaches its running temperature, the sodium dissociates from the iodine, adding orange and reds to the lamps spectrum from the sodium D line as the metal ionizes. Lamp life is 6,000 to 15,000 hours, as one of the most efficient sources of high CRI white light, metal halides as of 2005 were the fastest growing segment of the lighting industry. They operate at a pressure between 4 and 20 atmospheres, and require special fixtures to operate safely, as well as an electrical ballast, metal atoms produce most of the light output. They require a period of several minutes to reach full light output. Because of their spectrum and good efficiency they are used for indoor growing applications and are quite popular with reef aquarists. Like other gas-discharge lamps such as the very-similar mercury-vapor lamps, metal-halide lamps produce light by ionizing a mixture of gasses in an electric arc. In a metal-halide lamp, the arc tube contains a mixture of argon or xenon, mercury. The particular mixture of metal halides influences the color temperature. When started, the gas in the lamp is ionized first. The arc-tube operates at anywhere from 5–50 atm or more and 1000–3000 °C, about 24% of the energy used by metal-halide lamps produces light, making them substantially more efficient than incandescent bulbs, which typically have efficiencies in the range 2–4%. Metal-halide lamps consist of an arc tube with electrodes, a bulb. Inside the fused quartz arc tube, two tungsten electrodes doped with thorium are sealed into each end and an AC voltage is applied to them through molybdenum foil seals fused in silica and it is the arc between the two electrodes where the light is actually created. Besides mercury vapor, the lamp contains iodides or bromides of different metals, iodine and bromine are of the halogen group of the periodic table, and so are termed halides when ionized. Scandium and sodium are also used in some types, with thallium, indium, more recent types use dysprosium for high color temperature and tin for lower color temperature. Holmium and thulium are used in high power movie lighting models. Gallium or lead are used in special high UV-A models for printing purposes, the mixture of the metals used defines the color of the lamp
Metal-halide lamp
–
Metal halide lamp bulb (type /O with arc tube shield.)
Metal-halide lamp
–
Metal halide streetlight
Metal-halide lamp
–
Metal halide floodlights at a baseball field
Metal-halide lamp
–
150 watt metal-halide bulb in fixture, about halfway through warmup
16.
Xenon arc lamp
–
A xenon arc lamp is a specialized type of gas discharge lamp, an electric light that produces light by passing electricity through ionized xenon gas at high pressure. It produces a white light that closely mimics natural sunlight. Xenon arc lamps are used in movie projectors in theaters, in searchlights, xenon headlamps in automobiles actually use metal-halide lamps where a xenon arc is only used during start-up. Xenon arc lamps can be divided into three categories, continuous-output xenon short-arc lamps, continuous-output xenon long-arc lamps, and xenon flash lamps. Each consists of a quartz or other heat resistant glass arc tube. The glass tube is first evacuated and then re-filled with xenon gas, for xenon flashtubes, a third trigger electrode usually surrounds the exterior of the arc tube. The lifetime of an arc lamp varies according to its design and power consumption. Xenon short-arc lamps were invented in the 1940s in Germany and introduced in 1951 by Osram, first launched in the 2 kW size, these lamps saw wide use in movie projection, where they replaced the older, more labor intensive carbon arc lamps. The white continuous light generated by the arc is spectrally similar to daylight. Today, almost all movie projectors in theaters employ these lamps, omnimax projection systems use single xenon lamps with ratings as high as 15 kW. As of 2016, laser illumination for digital projectors, is starting to establish a market presence and has been predicted will supersede the xenon arc lamp for this application. All modern xenon short-arc lamps use a fused quartz envelope with thoriated tungsten electrodes, fused quartz is the only economically feasible material currently available that can withstand the high pressure and high temperature present in an operating lamp, while still being optically clear. The thorium dopant in the electrodes greatly enhances their electron emission characteristics, because of the very high power levels involved, large lamps are water-cooled. In those used in IMAX projectors, the bodies are made from solid Invar. An O-ring seals off the tube, so that the electrodes do not contact the water. In order to achieve maximum efficiency, the gas inside short-arc lamps is maintained at an extremely high pressure — up to 30 atmospheres — which poses safety concerns. If a lamp is dropped, or ruptures while in service, to mitigate this, large xenon short-arc lamps are normally shipped in protective shields, which will contain the envelope fragments should breakage occur. Normally, the shield is removed once the lamp is installed in the lamp housing, when the lamp reaches the end of its useful life, the protective shield is put back on the lamp, and the spent lamp is then removed from the equipment and discarded
Xenon arc lamp
–
15 kW xenon short-arc lamp used in
IMAX projectors
Xenon arc lamp
–
An
Osram 100 W xenon/mercury short-arc lamp in reflector
Xenon arc lamp
–
Perspective view of 3 kW lamp showing plastic safety shield used during shipping.
Xenon arc lamp
–
A Cermax 2 kW xenon lamp from a video projector. A pair of heatsinks are clamped on the two metal bands around the perimeter, which also double to supply power to the lamp
17.
Movie projector
–
A movie projector is an opto-mechanical device for displaying motion picture film by projecting it onto a screen. Most of the optical and mechanical elements, except for the illumination, the first movie projector was the Zoopraxiscope, invented by British photographer Eadweard Muybridge in 1879. The zoopraxiscope projected images from rotating glass disks in rapid succession to give the impression of motion, the stop-motion images were initially painted onto the glass, as silhouettes. A second series of discs, made in 1892–94, used outline drawings printed onto the discs photographically, a more sophisticated movie projector was invented by Frenchman Louis Le Prince while working in Leeds. In 1888 Le Prince took out a patent for a 16-lens device that combined a motion picture camera with a projector, in 1888, he used an updated version of his camera to film the first ever motion picture, the Roundhay Garden Scene. The pictures were exhibited in Hunslet. The Lumière brothers invented the first successful movie projector and they made their first film, Sortie de lusine Lumière de Lyon, in 1894, which was publicly screened at LEden, La Ciotat a year later. The first commercial, public screening of cinematographic films happened in Paris on 28 December 1895, the cinematograph was also exhibited at the Paris Exhibition of 1900. At the Exhibition, films made by the Lumière Brothers were projected onto a screen measuring 16 by 21 meters. In 1999, digital projectors were being tried out in some movie theatres. These early projectors played the movie stored on a server and played back through the projector, due to their relatively low resolution, the images at the time showed pixelization blocks in some scenes, much like images on early widescreen televisions. By 2006, the advent of much higher 4K resolution digital projection had removed any traces of pixelization, the systems became more compact than the larger machines of four years earlier. By 2009, movie theatres started replacing the film projectors with digital projectors, in 2013, it was estimated that 92% of movie theatres in the United States had converted to digital, with 8% still playing film. In 2015, numerous popular filmmakers—including Quentin Tarantino and Christopher Nolan—lobbied large studios to commit to purchase an amount of 35 mm film from Kodak. The decision ensured that Kodaks 35mm film production would continue for several years, nowadays film projectors are considered obsolete as high-resolution digital projectors offer many advantages over traditional film units. For example, digital projectors contain no moving parts except fans, can be operated remotely and they also allow for much easier, less expensive, and more reliable storage and distribution of content, including the ability to display live broadcasts. According to the theory of the phi phenomenon, the brain constitutes an experience of apparent movement when presented with a sequence of near-identical still images. Persistence of vision should be compared with the phenomena of beta movement
Movie projector
–
35 mm movie projector in operation.
Movie projector
–
35 mm Kinoton FP30ST movie projector, with parts labeled. (Click thumbnail for larger text.)
Movie projector
–
Imaging lens Diastar of an Askania 35mm movie projector (
focal length: 400 mm)
Movie projector
–
Christie AW3 platter, BIG SKY Industries console, and Century SA projector.
18.
Lightning
–
Lightning is a sudden electrostatic discharge that occurs during a thunder storm. This discharge occurs between electrically charged regions of a cloud, between two clouds, or between a cloud and the ground. The charged regions in the atmosphere temporarily equalize themselves through this discharge referred to as an if it hits an object on the ground. Lightning causes light in the form of plasma, and sound in the form of thunder, Lightning may be seen and not heard when it occurs at a distance too great for the sound to carry as far as the light from the strike or flash. This article incorporates public domain material from the National Oceanic and Atmospheric Administration document Understanding Lightning, the details of the charging process are still being studied by scientists, but there is general agreement on some of the basic concepts of thunderstorm electrification. The main charging area in a thunderstorm occurs in the part of the storm where air is moving upward rapidly and temperatures range from -15 to -25 Celsius. At that place, the combination of temperature and rapid upward air movement produces a mixture of super-cooled cloud droplets, small ice crystals, the updraft carries the super-cooled cloud droplets and very small ice crystals upward. At the same time, the graupel, which is larger and denser. The differences in the movement of the precipitation cause collisions to occur, when the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged. The updraft carries the positively charged ice crystals upward toward the top of the storm cloud, the larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm. The result is that the part of the thunderstorm cloud becomes positively charged while the middle to lower part of the thunderstorm cloud becomes negatively charged. This part of the cloud is called the anvil. While this is the charging process for the thunderstorm cloud. In addition, there is a small but important positive charge buildup near the bottom of the cloud due to the precipitation. Many factors affect the frequency, distribution, strength and physical properties of a lightning flash in a particular region of the world. These factors include ground elevation, latitude, prevailing wind currents, relative humidity, proximity to warm and cold bodies of water, to a certain degree, the ratio between IC, CC and CG lightning may also vary by season in middle latitudes. Lightnings relative unpredictability limits a complete explanation of how or why it occurs, the actual discharge is the final stage of a very complex process. At its peak, a thunderstorm produces three or more strikes to the Earth per minute
Lightning
–
A lightning flash during a
thunderstorm
Lightning
–
Lightning in
Belfort,
France
Lightning
–
View of lightning from an airplane flying above a system.
Lightning
–
A lightning strike from cloud to ground in the California,
Mojave Desert
19.
Electric spark
–
Michael Faraday described this phenomenon as the beautiful flash of light attending the discharge of common electricity. The rapid transition from a non-conducting to a conductive state produces an emission of light. A spark is created when the electric field exceeds the dielectric breakdown strength of the intervening medium. For air, the strength is about 30 kV/cm at sea level. At the beginning stages, free electrons in the gap are accelerated by the electrical field, as they collide with air molecules, they create additional ions and newly freed electrons which are also accelerated. The exponentially-increasing electrons and ions rapidly cause regions of the air in the gap to become conductive in a process called dielectric breakdown. Once the gap breaks down, current flow is limited by the charge or by the impedance of the external power supply. If the power supply continues to supply current, the spark will evolve into a continuous discharge called an electric arc, an electric spark can also occur within insulating liquids or solids, but the breakdown mechanisms are significantly different than for sparks in gases. Lightning is an example of a spark in nature, while electric sparks, large or small, occur in or near many man-made objects. Around 600 BC, Greek philosopher Thales of Miletus observed that amber could be electrified when rubbed with a cloth and attract other objects, in 1671, Leibniz discovered that sparks were associated with electrical phenomena. In 1708, Samuel Wall performed experiments with amber rubbed with cloth to produce sparks, in 1752, Thomas-François Dalibard and Benjamin Franklin independently demonstrated that lightning and electricity were equivalent. In Franklins famous kite experiment, he successfully extracted sparks from a cloud during a thunderstorm, electric sparks are used in spark plugs in gasoline internal combustion engines to ignite fuel and air mixtures. The electric discharge in a spark plug occurs between a central electrode and a grounded terminal on the base of the plug. The voltage for the spark is provided by a coil or magneto that is connected to the spark plug with an insulated wire. Flame igniters use electric sparks to initiate combustion in some furnaces, auto reignition is a safety feature that is used in some flame igniters that senses the electrical conductivity of the flame and uses this information to determine whether a burner flame is lit. This information is used to stop an ignition device from sparking after the flame is lit or restart the flame if it goes out, a spark-gap transmitter uses an electric spark gap to generate radio frequency electromagnetic radiation that can be used as transmitters for wireless communication. Spark gap transmitters were used in the first three decades of radio from 1887–1916. They were later supplanted by vacuum tube systems and by 1940 were no longer used for communication, the wide use of spark-gap transmitters led to the nickname sparks for a ships radio officer
Electric spark
–
Spark plug electric discharge
Electric spark
–
Benjamin Franklin drawing an electric spark to his finger from a key suspended from a kite string.
Electric spark
–
Gas stove burner - the electric spark flame igniter is shown at the left.
Electric spark
–
Spark transmitter used for ship to shore communication up to 10 km (c. 1900)."
20.
Humphry Davy
–
He also studied the forces involved in these separations, inventing the new field of electrochemistry. Berzelius called Davys 1806 Bakerian Lecture On Some Chemical Agencies of Electricity one of the best memoirs which has ever enriched the theory of chemistry and he was a Baronet, President of the Royal Society, Member of the Royal Irish Academy, and Fellow of the Geological Society. He also invented the Davy Lamp and an early form of incandescent light bulb. Davy was born in Penzance in Cornwall in England on 17 December 1778 and his family moved to Varfell, near Ludgvan, when he was nine, and in term-time Davy boarded with John Tonkin, his mothers godfather. Davy said, I consider it fortunate I was left much to myself as a child, after Davys father died in 1794, Tonkin apprenticed him to John Bingham Borlase, a surgeon with a practice in Penzance. Davys indenture is dated 10 February 1795, in the apothecarys dispensary, Davy became a chemist, and a garret in Tonkins house was where he conducted his earliest chemical experiments. Davys friends said, This boy Humphry is incorrigible and he will blow us all into the air. His elder sister complained of the ravages made on her dresses by corrosive substances, John Ayrton Paris remarked that poetry written by the young Davy bear the stamp of lofty genius. Davys first preserved poem entitled The Sons of Genius is dated 1795, other poems written in the following years, especially On the Mounts Bay and St Michaels Mount, are descriptive verses, showing sensibility but no true poetic imagination. Three of Davys paintings from around 1796 have been donated to the Penlee House museum at Penzance, one is of the view from above Gulval showing the church, Mounts Bay and the Mount, while the other two depict Loch Lomond in Scotland. While writing verses at the age of 17 in honour of his first love, he was discussing the question of the materiality of heat with his Quaker friend. Dunkin remarked, I tell thee what, Humphry, thou art the most quibbling hand at a dispute I ever met with in my life and it was a crude form of analogous experiment exhibited by Davy in the lecture-room of the Royal Institution that elicited considerable attention. As professor at the Royal Institution, Davy repeated many of the experiments he learned from his friend and mentor. Davies Giddy met Davy in Penzance carelessly swinging on the half-gate of Dr Borlases house and this led to an introduction to Dr Edwards, who lived at Hayle Copper House. Edwards was a lecturer in chemistry in the school of St. Bartholomews Hospital, galvanic corrosion was not understood at that time, but the phenomenon prepared Davys mind for subsequent experiments on ships copper sheathing. Gregory Watt, son of James Watt, visited Penzance for his healths sake, Davy was acquainted with the Wedgwood family, who spent a winter at Penzance. Thomas Beddoes and John Hailstone were engaged in a controversy on the rival merits of the Plutonian. They travelled together to examine the Cornish coast accompanied by Davies Gilbert, Beddoes, who had established at Bristol a Pneumatic Institution, needed an assistant to superintend the laboratory
Humphry Davy
–
Sir Humphry Davy, Bt by
Thomas Phillips National Portrait Gallery, London
Humphry Davy
–
James Watt in 1792 by
Carl Frederik von Breda
Humphry Davy
–
1802 satirical cartoon by
James Gillray showing a
Royal Institution lecture on pneumatics, with Davy holding the bellows and
Count Rumford looking on at extreme right. Dr Garnett is the lecturer, holding the victim's nose.
Humphry Davy
–
Sodium metal, about 10 g, under oil
21.
William Nicholson (chemist)
–
William Nicholson was a renowned English chemist and writer on natural philosophy and chemistry, as well as a translator, journalist, publisher, scientist, inventor, patent agent and civil engineer. Nicholson was educated in Yorkshire, and after leaving school, he made two voyages as a midshipman in the service of the British East India Company and his first ship was reportedly called The Boston and the second voyage was on board The Gatton. Subsequently, having become acquainted with Josiah Wedgwood in 1775, he moved to Amsterdam, meanwhile, he devoted himself to the preparation of An Introduction to Natural Philosophy, which was published in 1781 and was at once successful. A translation of Voltaires Elements of the Newtonian Philosophy soon followed and he gave much attention to the construction of various machines for comb-cutting, file-making, cylinder printing and other uses—he also invented an areometer. On 12 December 1783, Nicholson was elected to the Chapter Coffee House Philosophical Society and he was proposed by Jean-Hyacinthe Magellan and seconded by horologist John Whitehurst. Nicholson communicated to the Royal Society in 1789 two papers on electrical subjects, the journal included the first comprehensive descriptions of aerodynamics with George Cayleys On Aerial Navigation, which inspired the Wright brothers a hundred years later. In 1799 he established a school in Londons Soho Square, where he taught philosophy and chemistry. In May 1800 he with Anthony Carlisle discovered electrolysis, the decomposition of water into hydrogen and oxygen by voltaic current, the two were then appointed to a chemical investigation committee of the new Royal Institution. But his own interests shortly turned elsewhere, during the later years of his life, Nicholsons attention was chiefly directed to water supply engineering at Portsmouth, at Gosport and in Hammersmith. William Nicholson died in Bloomsbury at the age of 61 on 21 May 1815, attribution This article incorporates text from a publication now in the public domain, Nicholson, William
William Nicholson (chemist)
–
William Nicholson
22.
Royal Society
–
Founded in November 1660, it was granted a royal charter by King Charles II as The Royal Society. The society is governed by its Council, which is chaired by the Societys President, according to a set of statutes and standing orders. The members of Council and the President are elected from and by its Fellows, the members of the society. As of 2016, there are about 1,600 fellows, allowed to use the postnominal title FRS, there are also royal fellows, honorary fellows and foreign members, the last of which are allowed to use the postnominal title ForMemRS. The Royal Society President is Venkatraman Ramakrishnan, who took up the post on 30 November 2015, since 1967, the society has been based at 6–9 Carlton House Terrace, a Grade I listed building in central London which was previously used by the Embassy of Germany, London. The Royal Society started from groups of physicians and natural philosophers, meeting at variety of locations and they were influenced by the new science, as promoted by Francis Bacon in his New Atlantis, from approximately 1645 onwards. A group known as The Philosophical Society of Oxford was run under a set of rules still retained by the Bodleian Library, after the English Restoration, there were regular meetings at Gresham College. It is widely held that these groups were the inspiration for the foundation of the Royal Society, I will not say, that Mr Oldenburg did rather inspire the French to follow the English, or, at least, did help them, and hinder us. But tis well known who were the men that began and promoted that design. This initial royal favour has continued and, since then, every monarch has been the patron of the society, the societys early meetings included experiments performed first by Hooke and then by Denis Papin, who was appointed in 1684. These experiments varied in their area, and were both important in some cases and trivial in others. The Society returned to Gresham in 1673, there had been an attempt in 1667 to establish a permanent college for the society. Michael Hunter argues that this was influenced by Solomons House in Bacons New Atlantis and, to a lesser extent, by J. V. The first proposal was given by John Evelyn to Robert Boyle in a letter dated 3 September 1659, he suggested a scheme, with apartments for members. The societys ideas were simpler and only included residences for a handful of staff and these plans were progressing by November 1667, but never came to anything, given the lack of contributions from members and the unrealised—perhaps unrealistic—aspirations of the society. During the 18th century, the gusto that had characterised the early years of the society faded, with a number of scientific greats compared to other periods. The pointed lightning conductor had been invented by Benjamin Franklin in 1749, during the same time period, it became customary to appoint society fellows to serve on government committees where science was concerned, something that still continues. The 18th century featured remedies to many of the early problems
Royal Society
–
The entrance to the Royal Society in Carlton House Terrace, London
Royal Society
–
The President, Council, and Fellows of the Royal Society of London for Improving Natural Knowledge
Royal Society
–
John Evelyn, who helped to found the Royal Society
Royal Society
–
Mace granted by Charles II
23.
Current (electricity)
–
An electric current is a flow of electric charge. In electric circuits this charge is carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an ionised gas. The SI unit for measuring a current is the ampere. Electric current is measured using a device called an ammeter, electric currents cause Joule heating, which creates light in incandescent light bulbs. They also create magnetic fields, which are used in motors, inductors and generators, the particles that carry the charge in an electric current are called charge carriers. In metals, one or more electrons from each atom are loosely bound to the atom and these conduction electrons are the charge carriers in metal conductors. The conventional symbol for current is I, which originates from the French phrase intensité de courant, current intensity is often referred to simply as current. The I symbol was used by André-Marie Ampère, after whom the unit of current is named, in formulating the eponymous Ampères force law. The notation travelled from France to Great Britain, where it became standard, in a conductive material, the moving charged particles which constitute the electric current are called charge carriers. In other materials, notably the semiconductors, the carriers can be positive or negative. Positive and negative charge carriers may even be present at the same time, a flow of positive charges gives the same electric current, and has the same effect in a circuit, as an equal flow of negative charges in the opposite direction. Since current can be the flow of positive or negative charges. The direction of current is arbitrarily defined as the same direction as positive charges flow. This is called the direction of current I. If the current flows in the direction, the variable I has a negative value. When analyzing electrical circuits, the direction of current through a specific circuit element is usually unknown. Consequently, the directions of currents are often assigned arbitrarily
Current (electricity)
–
A simple electric circuit, where current is represented by the letter i. The relationship between the voltage (V), resistance (R), and current (I) is V=IR; this is known as
Ohm's Law.
24.
Carbon
–
Carbon is a chemical element with symbol C and atomic number 6. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds, three isotopes occur naturally, 12C and 13C being stable, while 14C is a radioactive isotope, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity, Carbon is the 15th most abundant element in the Earths crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is the second most abundant element in the body by mass after oxygen. The atoms of carbon can bond together in different ways, termed allotropes of carbon, the best known are graphite, diamond, and amorphous carbon. The physical properties of carbon vary widely with the allotropic form, for example, graphite is opaque and black while diamond is highly transparent. Graphite is soft enough to form a streak on paper, while diamond is the hardest naturally occurring material known, graphite is a good electrical conductor while diamond has a low electrical conductivity. Under normal conditions, diamond, carbon nanotubes, and graphene have the highest thermal conductivities of all known materials, all carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. They are chemically resistant and require high temperature to react even with oxygen, the most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and transition metal carbonyl complexes. The largest sources of carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil. For this reason, carbon has often referred to as the king of the elements. The allotropes of carbon graphite, one of the softest known substances, and diamond. It bonds readily with other small atoms including other carbon atoms, Carbon is known to form almost ten million different compounds, a large majority of all chemical compounds. Carbon also has the highest sublimation point of all elements, although thermodynamically prone to oxidation, carbon resists oxidation more effectively than elements such as iron and copper that are weaker reducing agents at room temperature. Carbon is the element, with a ground-state electron configuration of 1s22s22p2. Its first four ionisation energies,1086.5,2352.6,4620.5 and 6222.7 kJ/mol, are higher than those of the heavier group 14 elements. Carbons covalent radii are normally taken as 77.2 pm,66.7 pm and 60.3 pm, although these may vary depending on coordination number, in general, covalent radius decreases with lower coordination number and higher bond order. Carbon compounds form the basis of all life on Earth
Carbon
–
Graphite (left) and diamond (right), the two most well-known allotropes of carbon
Carbon
–
Spectral lines of carbon
Carbon
–
A large sample of glassy carbon.
Carbon
–
Graphite ore
25.
Charcoal
–
Charcoal is a lightweight, black residue, consisting of carbon and any remaining ash, obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis- the heating of wood or other substances in the absence of oxygen, the whole pile is covered with turf or moistened clay. The firing is begun at the bottom of the flue, the success of the operation depends upon the rate of the combustion. The operation is so delicate that it was left to colliers. They often lived alone in small huts in order to tend their wood piles, for example, in the Harz Mountains of Germany, charcoal burners lived in conical huts called Köten which are still much in evidence today. The massive production of charcoal was a cause of deforestation. The increasing scarcity of easily harvested wood was a factor behind the switch to fossil fuel equivalents, mainly coal. Charcoal made at 300 °C is brown, soft and friable, and readily inflames at 380 °C, made at higher temperatures it is hard and brittle, in Finland and Scandinavia, the charcoal was considered the by-product of wood tar production. The best tar came from pine, thus pinewoods were cut down for tar pyrolysis, the residual charcoal was widely used as substitute for metallurgical coke in blast furnaces for smelting. Tar production led to deforestation, it has been estimated all Finnish forests are younger than 300 years. The end of tar production at the end of the 19th century resulted in rapid re-forestation, the charcoal briquette was first invented and patented by Ellsworth B. A. Zwoyer of Pennsylvania in 1897 and was produced by the Zwoyer Fuel Company. The process was popularized by Henry Ford, who used wood. Ford Charcoal went on to become the Kingsford Company, Charcoal has been made by various methods. The traditional method in Britain used a clamp and this is essentially a pile of wooden logs leaning against a chimney. The chimney consists of 4 wooden stakes held up by some rope, the logs are completely covered with soil and straw allowing no air to enter. It must be lit by introducing some burning fuel into the chimney, if the soil covering gets torn by the fire, additional soil is placed on the cracks. Once the burn is complete, the chimney is plugged to prevent air from entering, the true art of this production method is in managing the sufficient generation of heat, and its transfer to wood parts in the process of being carbonised. A strong disadvantage of this method is the huge amount of emissions that are harmful to human health
Charcoal
–
Dry charcoal
Charcoal
–
Charcoal burning
Charcoal
–
Wood pile before covering it with
turf or soil, and firing it (around 1890)
Charcoal
–
An abandoned charcoal kiln near Walker, Arizona, USA.
26.
Vasily V. Petrov
–
Vasily Vladimirovich Petrov was a Russian experimental physicist, self-taught electrical technician, academician of Russian Academy of Sciences. Vasily Petrov was born in the town of Oboyan in the family of a priest and he studied at a public school in Kharkov, and then at the St. Petersburg Teachers College. In 1788, he gained a position as mathematics and physics teacher at Kolyvansko-Voskresenskoe College of Mining, in 1791, he was transferred to Saint Petersburg to teach mathematics and Russian at the military Engineering College, in the Izmailovsky regiment. In 1793, Petrov was invited to teach mathematics and physics at the St. Petersburg Medical and Surgery School, in 1795, he was promoted to the rank of Extraordinary Professor. During the next few years, he built up a physics laboratory. His first published book, A collection of new physical-chemical experiments, the bulk of this work was dedicated to the description of experiments related to combustion, as evidence against the then-popular phlogiston theory. The chapters, describing luminosity of phosphors of mineral and organic origins have elicited vivid interest in scientific circles. In 1802, Petrov discovered the electric arc effect, thanks to his building the worlds largest and most powerful Voltaic pile at the time, Petrov was forgotten soon after his death and his works fell into oblivion. A copy of News of Galvanic-Voltaic Experiments was discovered by chance in a library in the town of Vilno near the end of the 19th century, the book was the first time in world literature that a series of important physical phenomena related to electricity were described in detail. It was not until the late 1880s that technology based on Petrovs experiments was developed with the goal of industrial usage, menschutkin, Boris N. Vasilij Vladimirovic Petrov and His Physico-Chemical Work. Rise of Science in Russia 1700-1917
Vasily V. Petrov
–
Vasily Petrov
Vasily V. Petrov
–
An
electric arc.
27.
Voltaic pile
–
The voltaic pile was the first electrical battery that could continuously provide an electric current to a circuit. It was invented by Alessandro Volta, who published his experiments in 1799, the entire 19th century electrical industry was powered by batteries related to Voltas until the advent of the dynamo in the 1870s. Voltas invention built on Luigi Galvanis 1780s discovery of how a circuit of two metals and a leg can cause the frogs leg to respond. Volta demonstrated in 1794 that when two metals and brine-soaked cloth or cardboard are arranged in a circuit they produce an electric current, in 1800, Volta stacked several pairs of alternating copper and zinc discs separated by cloth or cardboard soaked in brine to increase the electrolyte conductivity. When the top and bottom contacts were connected by a wire, an electric current flowed through the voltaic pile, on 20 March 1800, Volta wrote to the London Royal Society to describe the technique for producing electric current using his device. On learning of the pile, William Nicholson and Anthony Carlisle used it to discover the electrolysis of water. He also used the voltaic pile to decompose chemicals and to produce new chemicals, William Hyde Wollaston showed that electricity from voltaic piles had identical effects to those of electricity produced by friction. In 1802 Vasily Petrov used voltaic piles in the discovery and research of electric arc effects, sir Humphry Davy and Andrew Crosse were among the first to develop large voltaic piles. Davy used a 2000-pair pile made for the Royal Institution in 1808 to demonstrate carbon arc discharge and isolate five new elements, barium, calcium, boron, strontium and magnesium. Because Volta believed that the electromotive force occurred at the contact between the two metals, Voltas piles had a different design than the modern design illustrated on this page. His piles had one extra disc of copper at the top, in contact with the zinc, expanding on Volta’s work and the electro-magnetism work of his mentor Humphry Davy, Michael Faraday utilized both magnets and the voltaic pile in his experiments with electricity. Faraday believed that all “electricities” being studied at the time were one and his work to prove this theory led him to propose two laws of electrochemistry which stood in direct conflict with the current scientific beliefs of the day as laid down by Volta thirty years earlier. Because of their contributions to the understanding of field of study, Faraday. The words electrode and electrolyte, used above to describe Voltas work, are due to Faraday, indeed, Volta himself experimented with a pile whose cardboard discs had dried out, most likely accidentally. The first to publish was Johann Wilhelm Ritter in 1802, albeit in an obscure journal, one form of dry pile is the Zamboni pile. The dry pile could be referred to as the ancestor of the dry cell. The strength of the pile is expressed in terms of its force, or emf. Alessandro Voltas theory of contact tension considered that the emf, which drives the current through a circuit containing a voltaic cell
Voltaic pile
–
A voltaic pile on display in the
Tempio Voltiano (the Volta Temple) near Volta's home in
Como.
Voltaic pile
–
Schematic diagram of a
copper –
zinc voltaic pile. The copper and zinc discs were separated by cardboard or felt spacers soaked in salt water (the electrolyte). Volta's original piles contained an additional zinc disk at the bottom, and an additional copper disk at the top. These were later shown to be unnecessary.
28.
Hertha Marks Ayrton
–
Phoebe Sarah Hertha Ayrton, was a British engineer, mathematician, physicist and inventor. Known in adult life as Hertha Ayrton, born Phoebe Sarah Marks, she was awarded the Hughes Medal by the Royal Society for her work on electric arcs and ripples in sand and water. Hertha Ayrton was born Phoebe Sarah Marks in Portsea, Hampshire, England and her father died in 1861, leaving Sarahs mother with seven children and an eighth expected. Sarah then took up some of the responsibility for caring for the younger children, at the age of nine, Sarah was invited by her aunts, who ran a school in northwest London, to live with her cousins and be educated with them. She was known to her peers and teachers as a fiery and her cousins introduced Ayrton to science and mathematics. By age 16, she was working as a governess, Ayrton attended Girton College, Cambridge, where she studied mathematics and was coached by physicist Richard Glazebrook. George Eliot supported Ayrtons application to Girton College, during her time at Cambridge, Ayrton constructed a sphygmomanometer, led the choral society, founded the Girton fire brigade, and, together with Charlotte Scott, formed a mathematical club. In 1880, Ayrton passed the Mathematical Tripos, but Cambridge did not grant her a degree because, at the time, Cambridge gave only certificates. Ayrton passed an examination at the University of London, which awarded her a Bachelor of Science degree in 1881. Upon her return to London, Ayrton earned money by teaching and embroidery, ran a club for working girls, in 1884 Ayrton patented a line-divider, an engineering drawing instrument for dividing a line into any number of equal parts and for enlarging and reducing figures. The line-divider was her first major invention and, while its use was likely for artists for enlarging and diminishing, it was also useful to architects. Ayrtons 1884 patent was the first of many – from 1884 until her death, Hertha registered 26 patents, five on mathematical dividers,13 on arc lamps and electrodes, the rest on the propulsion of air. On 6 May 1885 she married her teacher, and thereafter assisted him with experiments in physics. She also began her own investigation into the characteristics of the electric arc, in the late nineteenth century, electric arc lighting was in wide use for public lighting. The tendency of electric arcs to flicker and hiss was a major problem, in 1895, Hertha Ayrton wrote a series of articles for the Electrician, explaining that these phenomena were the result of oxygen coming into contact with the carbon rods used to create the arc. In 1899, she was the first woman ever to read her own paper before the Institution of Electrical Engineers and her paper was entitled The Hissing of the Electric Arc. Shortly thereafter, Ayrton was elected the first female member of the IEE and she petitioned to present a paper before the Royal Society but was not allowed because of her sex and The Mechanism of the Electric Arc was read by John Perry in her stead in 1901. By the late century, Ayrtons work in the field of electrical engineering was recognised more widely, domestically and internationally
Hertha Marks Ayrton
–
Hertha Ayrton
29.
Pantograph (transport)
–
A pantograph is an apparatus mounted on the roof of an electric train, tram or electric bus to collect power through contact with an overhead catenary wire. It is a type of current collector. Typically, a wire is used, with the return current running through the track. The term stems from the resemblance of some styles to the mechanical pantographs used for copying handwriting, the pantograph was invented in 1879 by Walter Reichel, chief engineer at Siemens & Halske in Germany. A flat slide-pantograph was invented in 1895 at the Baltimore and Ohio Railroad The familiar diamond-shaped roller pantograph was invented by John Q. Brown of the Key System shops for their commuter trains ran between San Francisco and the East Bay section of the San Francisco Bay Area in California. They appear in photographs of the first day of service,26 October 1903, for many decades thereafter, the same diamond shape was used by electric-rail systems around the world and remains in use by some today. The most common type of today is the so-called half-pantograph. Louis Faiveley invented this type of pantograph in 1955, the half-pantograph can be seen in use on everything from very fast trains to low-speed urban tram systems. The electric transmission system for electric rail systems consists of an upper. The pantograph is spring-loaded and pushes a contact shoe up against the underside of the wire to draw the current needed to run the train. The steel rails of the act as the electrical return. As the train moves, the shoe slides along the wire and can set up standing waves in the wires which break the contact. This means that on some systems adjacent pantographs are not permitted, pantographs are the successor technology to trolley poles, which were widely used on early streetcar systems. However, many of these networks, including Torontos, are undergoing upgrades to accommodate pantograph operation, as a precaution against loss of pressure in the second case, the arm is held in the down position by a catch. For high-voltage systems, the air supply is used to blow out the electric arc when roof-mounted circuit breakers are used. Pantographs may have either a single or a double arm, double-arm pantographs are usually heavier, requiring more power to raise and lower, but may also be more fault-tolerant. On railways of the former USSR, the most widely used pantographs are those with a double arm, some streetcars use double-arm pantographs, among them the Russian KTM-5, KTM-8, LVS-86 and many other Russian-made trams, as well as some Euro-PCC trams in Belgium
Pantograph (transport)
–
The diamond-shaped, electric-rod pantograph of the Swiss
cogwheel locomotive of the
Schynige Platte railway in
Schynige Platte, built in 1911
Pantograph (transport)
–
The (asymmetrical) 'Z'-shaped pantograph of the electrical pickup on the
Berlin Straßenbahn. This pantograph uses a single-arm design
Pantograph (transport)
–
Close up of a single arm pantograph on a
British Rail Class 333.
Pantograph (transport)
–
High-performance pantograph used for measurements on the
ICE S
30.
Overhead line
–
An overhead line or overhead wire is used to transmit electrical energy to trams, trolleybuses, or trains. Overhead line is designed on the principle of one or more overhead wires situated over rail tracks, the feeder stations are usually fed from a high-voltage electrical grid. Electric trains that collect their current from overhead lines use a device such as a pantograph and it presses against the underside of the lowest overhead wire, the contact wire. Current collectors are electrically conductive and allow current to flow through to the train or tram, non-electric locomotives may pass along these tracks without affecting the overhead line, although there may be difficulties with overhead clearance. Alternative electrical power transmission schemes for trains include third rail, ground-level power supply, batteries and this article does not cover regenerative braking, where the traction motors act as generators to retard movement and return power to the overhead. To achieve good high-speed current collection, it is necessary to keep the wire geometry within defined limits. This is usually achieved by supporting the wire from a second wire known as the messenger wire or catenary. This wire approximates the path of a wire strung between two points, a catenary curve, thus the use of catenary to describe this wire or sometimes the whole system. This wire is attached to the wire at regular intervals by vertical wires known as droppers or drop wires. It is supported regularly at structures, by a pulley, link, the whole system is then subjected to a mechanical tension. As the contact wire makes contact with the pantograph, the insert on top of the pantograph is worn down. The straight wire between supports will cause the wire to cross over the whole surface of the pantograph as the train travels around the curve, causing uniform wear. On straight track, the wire is zigzagged slightly to the left. The movement of the wire across the head of the pantograph is called the sweep. The zigzagging of the line is not required for trolley poles. Depot areas tend to have only a wire and are known as simple equipment or trolley wire. When overhead line systems were first conceived, good current collection was only at low speeds. Compound equipment - uses a second wire, known as the auxiliary
Overhead line
–
Overhead lines on
Swiss Federal Railways
Overhead line
–
Overhead lines in China
Overhead line
–
Overhead lines in Denmark near
Roskilde. For aesthetic reasons the support structure is constructed from hollow
Corten steel masts.
Overhead line
–
Brussels-South, overhead wires suspended across multiple tracks.
31.
London Underground
–
The London Underground is a public rapid transit system serving London and some parts of the adjacent counties of Buckinghamshire, Essex and Hertfordshire in the United Kingdom. The network has expanded to 11 lines, and in 2015–16 carried 1.34 billion passengers, the 11 lines collectively handle approximately 4.8 million passengers a day. The system has 270 stations and 250 miles of track, despite its name, only 45% of the system is actually underground in tunnels, with much of the network in the outer environs of London being on the surface. In addition, the Underground does not cover most southern parts of Greater London, the current operator, London Underground Limited, is a wholly owned subsidiary of Transport for London, the statutory corporation responsible for the transport network in Greater London. As of 2015, 92% of operational expenditure is covered by passenger fares, the Travelcard ticket was introduced in 1983 and Oyster, a contactless ticketing system, in 2003. Contactless card payments were introduced in 2014, the LPTB was a prominent patron of art and design, commissioning many new station buildings, posters and public artworks in a modernist style. Other famous London Underground branding includes the roundel and Johnston typeface, to prepare construction, a short test tunnel was built in 1855 in Kibblesworth, a small town with geological properties similar to London. This test tunnel was used for two years in the development of the first underground train, and was later, in 1861, the worlds first underground railway, it opened in January 1863 between Paddington and Farringdon using gas-lit wooden carriages hauled by steam locomotives. It was hailed as a success, carrying 38,000 passengers on the opening day, the Metropolitan District Railway opened in December 1868 from South Kensington to Westminster as part of a plan for an underground inner circle connecting Londons main-line termini. The Metropolitan and District railways completed the Circle line in 1884, built using the cut and this opened in 1890 with electric locomotives that hauled carriages with small opaque windows, nicknamed padded cells. The Waterloo and City Railway opened in 1898, followed by the Central London Railway in 1900, the Metropolitan Railway protested about the change of plan, but after arbitration by the Board of Trade, the DC system was adopted. When the Bakerloo was so named in July 1906, The Railway Magazine called it an undignified gutter title, by 1907 the District and Metropolitan Railways had electrified the underground sections of their lines. In January 1913, the UERL acquired the Central London Railway, the Bakerloo line was extended north to Queens Park to join a new electric line from Euston to Watford, but World War I delayed construction and trains reached Watford Junction in 1917. During air raids in 1915 people used the stations as shelters. An extension of the Central line west to Ealing was also delayed by the war, the Metropolitan promoted housing estates near the railway with the Metro-land brand and nine housing estates were built near stations on the line. Electrification was extended north from Harrow to Rickmansworth, and branches opened from Rickmansworth to Watford in 1925, the Piccadilly line was extended north to Cockfosters and took over District line branches to Harrow and Hounslow. In 1933, most of Londons underground railways, tramway and bus services were merged to form the London Passenger Transport Board, the Waterloo & City Railway, which was by then in the ownership of the main line Southern Railway, remained with its existing owners. In the same year that the London Passenger Transport Board was formed, in the following years, the outlying lines of the former Metropolitan Railway closed, the Brill Tramway in 1935, and the line from Quainton Road to Verney Junction in 1936
London Underground
–
A deep-level
Central line train at
Lancaster Gate bound for
Ealing Broadway
London Underground
–
London Underground
London Underground
–
A larger sub-surface
Metropolitan line train at
Farringdon bound for
Aldgate
London Underground
–
The Metropolitan Railway opened using GWR broad gauge locomotives
32.
Arc welding
–
Arc welding is a process that is used to join metal to metal by using electricity to create enough heat to melt metal, and the melted metals when cool result in a binding of the metals. It is a type of welding that uses a power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can use either direct or alternating current, and consumable or non-consumable electrodes, the welding region is usually protected by some type of shielding gas, vapor, or slag. Arc welding processes may be manual, semi-automatic, or fully automated, first developed in the late part of the 19th century, arc welding became commercially important in shipbuilding during the Second World War. Today it remains an important process for the fabrication of steel structures and vehicles, to supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used. The most common classification is constant current power supplies and constant voltage power supplies, in arc welding, the voltage is directly related to the length of the arc, and the current is related to the amount of heat input. This is important because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate. In these processes, arc length is constant, since any fluctuation in the distance between the wire and the base material is quickly rectified by a large change in current. The direction of current used in arc welding also plays an important role in welding, consumable electrode processes such as shielded metal arc welding and gas metal arc welding generally use direct current, but the electrode can be charged either positively or negatively. In welding, the positively charged anode will have a greater concentration and, as a result. If the electrode is charged, it will melt more quickly, increasing weld penetration. Alternatively, a negatively charged electrode results in more shallow welds, non-consumable electrode processes, such as gas tungsten arc welding, can use either type of direct current, as well as alternating current. Alternating current rapidly moves between two, resulting in medium-penetration welds. Duty cycle is an equipment specification which defines the number of minutes, within a 10-minute period. For example, an 80 A welder with a 60% duty cycle must be rested for at least 4 minutes after 6 minutes of continuous welding, failure to observe duty cycle limitations could damage the welder. Commercial- or professional-grade welders typically have a 100% duty cycle, one of the most common types of arc welding is shielded metal arc welding, which is also known as manual metal arc welding or stick welding. An electric current is used to strike an arc between the material and a consumable electrode rod or stick. The electrode core itself acts as filler material, making a separate filler unnecessary, the process is very versatile, requiring little operator training and inexpensive equipment
Arc welding
–
Gas metal arc welding
Arc welding
–
Engine driven welder capable of AC/DC welding.
Arc welding
–
A diesel powered welding generator (the electric generator is on the left) as used in
Indonesia.
Arc welding
–
Shielded metal arc welding
33.
Field emission
–
Field electron emission is emission of electrons induced by an electrostatic field. The most common context is field emission from a surface into vacuum. However, field emission can take place from solid or liquid surfaces, into vacuum, air, the field-induced promotion of electrons from the valence to conduction band of semiconductors can also be regarded as a form of field emission. When field emission is used without qualifiers it typically means cold emission, Field emission in pure metals occurs in high electric fields, the gradients are typically higher than 1 gigavolt per metre and strongly dependent upon the work function. Examples of applications for surface field emission include construction of bright electron sources for electron microscopes or the discharge of induced charges from spacecraft. Devices which eliminate induced charges are termed charge-neutralizers, Field emission was explained by quantum tunneling of electrons in the late 1920s. This was one of the triumphs of the nascent quantum mechanics, the theory of field emission from bulk metals was proposed by Ralph H. Fowler and Lothar Wolfgang Nordheim. A family of approximate equations, Fowler–Nordheim equations, is named after them, in some respects, field electron emission is a paradigm example of what physicists mean by tunneling. Unfortunately, it is also an example of the intense mathematical difficulties that can arise. Simple solvable models of the tunneling barrier lead to equations that get predictions of current density too low by a factor of 100 or more. To get even an approximate solution, it is necessary to use special approximate methods known in physics as semi-classical or quasi-classical methods, a consequence of these difficulties has been a heritage of misunderstanding and disinformation that still persists in some current field emission research literature. This article tries to present an account of field emission for the 21st century. Field electron emission, field-induced electron emission, field emission and electron field emission are general names for this experimental phenomenon, the first name is used here. Fowler–Nordheim tunneling is the tunneling of electrons through a rounded triangular barrier created at the surface of an electron conductor by applying a very high electric field. Individual electrons can escape by Fowler-Nordheim tunneling from many materials in different circumstances. Many solid and liquid materials can emit electrons in a CFE regime if a field of an appropriate size is applied. Fowler–Nordheim-type equations are a family of approximate equations derived to describe CFE from the electron states in bulk metals. The different members of the family represent different degrees of approximation to reality, there is no theoretical reason to believe that Fowler-Nordheim-type equations validly describe field emission from materials other than bulk crystalline solids
Field emission
–
Fig. 1. P-T energy-space diagram, showing the region in P-T energy space where traveling-wave electron states exist.
34.
Amperes
–
The ampere, often shortened to amp, is a unit of electric current. In the International System of Units the ampere is one of the seven SI base units and it is named after André-Marie Ampère, French mathematician and physicist, considered the father of electrodynamics. SI defines the ampere in terms of base units by measuring the electromagnetic force between electrical conductors carrying electric current. The ampere was then defined as one coulomb of charge per second, in SI, the unit of charge, the coulomb, is defined as the charge carried by one ampere during one second. In the future, the SI definition may shift back to charge as the base unit, ampères force law states that there is an attractive or repulsive force between two parallel wires carrying an electric current. This force is used in the definition of the ampere. The SI unit of charge, the coulomb, is the quantity of electricity carried in 1 second by a current of 1 ampere, conversely, a current of one ampere is one coulomb of charge going past a given point per second,1 A =1 C s. In general, charge Q is determined by steady current I flowing for a time t as Q = It, constant, instantaneous and average current are expressed in amperes and the charge accumulated, or passed through a circuit over a period of time is expressed in coulombs. The relation of the ampere to the coulomb is the same as that of the watt to the joule, the ampere was originally defined as one tenth of the unit of electric current in the centimetre–gram–second system of units. That unit, now known as the abampere, was defined as the amount of current that generates a force of two dynes per centimetre of length between two wires one centimetre apart. The size of the unit was chosen so that the derived from it in the MKSA system would be conveniently sized. The international ampere was a realization of the ampere, defined as the current that would deposit 0.001118 grams of silver per second from a silver nitrate solution. Later, more accurate measurements revealed that this current is 0.99985 A, at present, techniques to establish the realization of an ampere have a relative uncertainty of approximately a few parts in 107, and involve realizations of the watt, the ohm and the volt. Rather than a definition in terms of the force between two current-carrying wires, it has proposed that the ampere should be defined in terms of the rate of flow of elementary charges. Since a coulomb is equal to 6. 2415093×1018 elementary charges. The proposed change would define 1 A as being the current in the direction of flow of a number of elementary charges per second. In 2005, the International Committee for Weights and Measures agreed to study the proposed change, the new definition was discussed at the 25th General Conference on Weights and Measures in 2014 but for the time being was not adopted. The current drawn by typical constant-voltage energy distribution systems is usually dictated by the power consumed by the system, for this reason the examples given below are grouped by voltage level
Amperes
–
Demonstration model of a moving iron ammeter. As the current through the coil increases, the plunger is drawn further into the coil and the pointer deflects to the right.
35.
Emergence
–
Emergence is central in theories of integrative levels and of complex systems. For instance, the phenomenon of life as studied in biology is an emergent property of chemistry, in philosophy, theories that emphasize emergent properties have been called emergentism. Almost all accounts of emergentism include a form of epistemic or ontological irreducibility to the lower levels, in philosophy, emergence is often understood to be a claim about the etiology of a systems properties. An emergent property of a system, in context, is one that is not a property of any component of that system. Nicolai Hartmann, one of the first modern philosophers to write on emergence and this idea of emergence has been around since at least the time of Aristotle. John Stuart Mill and Julian Huxley are two of many scientists and philosophers who have written on the concept, further, every resultant is clearly traceable in its components, because these are homogeneous and commensurable. The emergent is unlike its components insofar as these are incommensurable, economist Jeffrey Goldstein provided a current definition of emergence in the journal Emergence. Goldstein initially defined emergence as, the arising of novel and coherent structures, patterns, Systems scientist Peter Corning also says that living systems cannot be reduced to underlying laws of physics, Rules, or laws, have no causal efficacy, they do not in fact “generate” anything. They serve merely to describe regularities and consistent relationships in nature and these patterns may be very illuminating and important, but the underlying causal agencies must be separately specified. But that aside, the game of chess illustrates, why any laws or rules of emergence and evolution are insufficient. Even in a game, you cannot use the rules to predict “history” – i. e. the course of any given game. Indeed, you cannot even reliably predict the move in a chess game. Because the “system” involves more than the rules of the game and it also includes the players and their unfolding, moment-by-moment decisions among a very large number of available options at each choice point. The game of chess is inescapably historical, even though it is constrained and shaped by a set of rules. Moreover, and this is a key point, the game of chess is also shaped by teleonomic, cybernetic and it is not simply a self-ordered process, it involves an organized, “purposeful” activity. Usage of the notion emergence may generally be subdivided into two perspectives, that of weak emergence and strong emergence, in terms of physical systems, weak emergence is a type of emergence in which the emergent property is amenable to computer simulation. This is opposed to the notion of strong emergence, in which the emergent property cannot be simulated by a computer. Some common points between the two notions are that emergence concerns new properties produced as the system grows, which is to say ones which are not shared with its components or prior states, also, it is assumed that the properties are supervenient rather than metaphysically primitive
Emergence
–
The formation of complex symmetrical and
fractal patterns by
Snowflakes is an example of emergence in a physical system.
Emergence
–
A
termite "cathedral" mound produced by a
termite colony is a classic example of emergence in nature.
Emergence
–
Ripple patterns in a
sand dune created by wind or water is an example of an emergent structure in nature.
Emergence
–
Giant's Causeway in Northern Ireland is an example of a complex emergent structure created by natural processes.
36.
Electric field
–
An electric field is a vector field that associates to each point in space the Coulomb force that would be experienced per unit of electric charge, by an infinitesimal test charge at that point. Electric fields are created by electric charges and can be induced by time-varying magnetic fields, the electric field combines with the magnetic field to form the electromagnetic field. The electric field, E, at a point is defined as the force, F. A particle of charge q would be subject to a force F = q E and its SI units are newtons per coulomb or, equivalently, volts per metre, which in terms of SI base units are kg⋅m⋅s−3⋅A−1. Electric fields are caused by electric charges or varying magnetic fields, in the special case of a steady state, the Maxwell-Faraday inductive effect disappears. The resulting two equations, taken together, are equivalent to Coulombs law, written as E =14 π ε0 ∫ d r ′ ρ r − r ′ | r − r ′ |3 for a charge density ρ. Notice that ε0, the permittivity of vacuum, must be substituted if charges are considered in non-empty media, the equations of electromagnetism are best described in a continuous description. A charge q located at r 0 can be described mathematically as a charge density ρ = q δ, conversely, a charge distribution can be approximated by many small point charges. Electric fields satisfy the principle, because Maxwells equations are linear. This principle is useful to calculate the field created by point charges. Q n are stationary in space at r 1, r 2, in that case, Coulombs law fully describes the field. If a system is static, such that magnetic fields are not time-varying, then by Faradays law, in this case, one can define an electric potential, that is, a function Φ such that E = − ∇ Φ. This is analogous to the gravitational potential, Coulombs law, which describes the interaction of electric charges, F = q = q E is similar to Newtons law of universal gravitation, F = m = m g. This suggests similarities between the electric field E and the gravitational field g, or their associated potentials, mass is sometimes called gravitational charge because of that similarity. Electrostatic and gravitational forces both are central, conservative and obey an inverse-square law, a uniform field is one in which the electric field is constant at every point. It can be approximated by placing two conducting plates parallel to other and maintaining a voltage between them, it is only an approximation because of boundary effects. Assuming infinite planes, the magnitude of the electric field E is, electrodynamic fields are E-fields which do change with time, for instance when charges are in motion. The electric field cannot be described independently of the field in that case
Electric field
–
Electric field lines emanating from a point positive
electric charge suspended over an infinite sheet of conducting material.
37.
Electrodes
–
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit. The word was coined by William Whewell at the request of the scientist Michael Faraday from the Greek words elektron, meaning amber, and hodos, an electrode in an electrochemical cell is referred to as either an anode or a cathode. The anode is now defined as the electrode at which electrons leave the cell and oxidation occurs, each electrode may become either the anode or the cathode depending on the direction of current through the cell. A bipolar electrode is an electrode that functions as the anode of one cell, a primary cell is a special type of electrochemical cell in which the reaction cannot be reversed, and the identities of the anode and cathode are therefore fixed. The anode is always the negative electrode, the cell can be discharged but not recharged. A secondary cell, for example a rechargeable battery, is a cell in which the reactions are reversible. When the cell is being charged, the anode becomes the positive and this is also the case in an electrolytic cell. When the cell is being discharged, it behaves like a cell, with the anode as the negative. In a vacuum tube or a semiconductor having polarity the anode is the positive electrode, the electrons enter the device through the cathode and exit the device through the anode. Many devices have other electrodes to control operation, e. g. base, gate, control grid. In a three-electrode cell, an electrode, also called an auxiliary electrode, is used only to make a connection to the electrolyte so that a current can be applied to the working electrode. The counter electrode is made of an inert material, such as a noble metal or graphite. In arc welding an electrode is used to conduct current through a workpiece to fuse two pieces together. Depending upon the process, the electrode is either consumable, in the case of gas metal arc welding or shielded metal arc welding, or non-consumable, such as in gas tungsten arc welding. For a direct current system the weld rod or stick may be a cathode for a filling type weld or an anode for other welding processes, for an alternating current arc welder the welding electrode would not be considered an anode or cathode. Electrodes are used to provide current through nonmetal objects to them in numerous ways. These electrodes are used for advanced purposes in research and investigation
Electrodes
–
Electrodes used in
arc welding
38.
Tungsten
–
Tungsten, also known as wolfram, is a chemical element with symbol W and atomic number 74. The word tungsten comes from the Swedish language tung sten, which translates to heavy stone. Its name in Swedish is volfram, however, in order to distinguish it from scheelite, a hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth almost exclusively in chemical compounds. It was identified as a new element in 1781, and first isolated as a metal in 1783 and its important ores include wolframite and scheelite. The free element is remarkable for its robustness, especially the fact that it has the highest melting point of all the elements discovered, melting at 3,422 °C,6,192 °F. Its high density is 19.3 times that of water, comparable to that of uranium and gold, polycrystalline tungsten is an intrinsically brittle and hard material, making it difficult to work. However, pure single-crystalline tungsten is more ductile, and can be cut with a hard-steel hacksaw, tungstens many alloys have numerous applications, including incandescent light bulb filaments, X-ray tubes, electrodes in TIG welding, superalloys, and radiation shielding. Tungstens hardness and high density give it military applications in penetrating projectiles, tungsten compounds are also often used as industrial catalysts. Tungsten is the metal from the third transition series that is known to occur in biomolecules. It is the heaviest element known to be essential to any living organism, tungsten interferes with molybdenum and copper metabolism and is somewhat toxic to animal life. In its raw form, tungsten is a hard metal that is often brittle. If made very pure, tungsten retains its hardness, and becomes malleable enough that it can be worked easily and it is worked by forging, drawing, or extruding. Tungsten objects are commonly formed by sintering. Of all metals in pure form, tungsten has the highest melting point, lowest vapor pressure, although carbon remains solid at higher temperatures than tungsten, carbon sublimes, rather than melts, so tungsten is considered to have a higher melting point. Tungsten has the lowest coefficient of expansion of any pure metal. The low thermal expansion and high melting point and tensile strength of tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d electrons, alloying small quantities of tungsten with steel greatly increases its toughness. Tungsten exists in two crystalline forms, α and β. The former has a cubic structure and is the more stable form
Tungsten
–
Tungsten, 74 W
Tungsten
–
Wolframite, scale in cm
Tungsten
–
Close-up of a tungsten filament inside a
halogen lamp
Tungsten
–
Tungsten carbide ring (jewelry)
39.
Temperature
–
A temperature is an objective comparative measurement of hot or cold. It is measured by a thermometer, several scales and units exist for measuring temperature, the most common being Celsius, Fahrenheit, and, especially in science, Kelvin. Absolute zero is denoted as 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, the kinetic theory offers a valuable but limited account of the behavior of the materials of macroscopic bodies, especially of fluids. Temperature is important in all fields of science including physics, geology, chemistry, atmospheric sciences, medicine. The Celsius scale is used for temperature measurements in most of the world. Because of the 100 degree interval, it is called a centigrade scale.15, the United States commonly uses the Fahrenheit scale, on which water freezes at 32°F and boils at 212°F at sea-level atmospheric pressure. Many scientific measurements use the Kelvin temperature scale, named in honor of the Scottish physicist who first defined it and it is a thermodynamic or absolute temperature scale. Its zero point, 0K, is defined to coincide with the coldest physically-possible temperature and its degrees are defined through thermodynamics. The temperature of zero occurs at 0K = −273. 15°C. For historical reasons, the triple point temperature of water is fixed at 273.16 units of the measurement increment, Temperature is one of the principal quantities in the study of thermodynamics. There is a variety of kinds of temperature scale and it may be convenient to classify them as empirically and theoretically based. Empirical temperature scales are historically older, while theoretically based scales arose in the middle of the nineteenth century, empirically based temperature scales rely directly on measurements of simple physical properties of materials. For example, the length of a column of mercury, confined in a capillary tube, is dependent largely on temperature. Such scales are only within convenient ranges of temperature. For example, above the point of mercury, a mercury-in-glass thermometer is impracticable. A material is of no use as a thermometer near one of its phase-change temperatures, in spite of these restrictions, most generally used practical thermometers are of the empirically based kind. Especially, it was used for calorimetry, which contributed greatly to the discovery of thermodynamics, nevertheless, empirical thermometry has serious drawbacks when judged as a basis for theoretical physics. Theoretically based temperature scales are based directly on theoretical arguments, especially those of thermodynamics, kinetic theory and they rely on theoretical properties of idealized devices and materials
Temperature
–
Annual mean temperature around the world
Temperature
–
Body temperature variation
Temperature
–
A typical Celsius thermometer measures a winter day temperature of -17 °C.
Temperature
–
Plots of pressure vs temperature for three different gas samples extrapolated to absolute zero.
40.
Melting
–
Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the energy of the solid increases, typically by the application of heat or pressure. At the melting point, the ordering of ions or molecules in the solid breaks down to an ordered state. Substances in the state generally have reduced viscosity as the temperature increases. An exception to this principle is the element sulfur, whose viscosity increases to a point due to polymerization, some organic compounds melt through mesophases, states of partial order between solid and liquid. Melting is therefore classified as a phase transition. Melting occurs when the Gibbs free energy of the liquid becomes lower than the solid for that material, the temperature at which this occurs is dependent on the ambient pressure. Low-temperature helium is the only exception to the general rule. Helium-3 has an enthalpy of fusion at temperatures below 0.3 K. Helium-4 also has a very slightly negative enthalpy of fusion below 0.8 K. This means that, at appropriate constant pressures, heat must be removed from these substances in order to melt them, among the theoretical criteria for melting, the Lindemann and of Born criteria are those most frequently used as a basis to analyse the melting conditions. The Lindemann melting criterion is supported by experimental data both for crystalline materials and for transitions in amorphous materials. The Born criterion is based on rigidity catastrophe caused by the elastic shear modulus. Under a standard set of conditions, the point of a substance is a characteristic property. The melting point is equal to the freezing point. However, under carefully created conditions, supercooling or superheating past the melting or freezing point can occur, water on a very clean glass surface will often supercool several degrees below the freezing point without freezing. Fine emulsions of pure water have been cooled to −38 degrees Celsius without nucleation to form ice, nucleation occurs due to fluctuations in the properties of the material. If the material is still there is often nothing to trigger this change. Thermodynamically, the liquid is in the metastable state with respect to the crystalline phase
Melting
–
State
Melting
–
Ice melting
41.
Evaporation
–
Evaporation is a type of vaporization of a liquid that occurs from the surface of a liquid into a gaseous phase that is not saturated with the evaporating substance. The other type of vaporization is boiling, which is characterized by bubbles of saturated vapor forming in the liquid phase, steam produced in a boiler is another example of evaporation occurring in a saturated vapor phase. Evaporation that occurs directly from the solid phase below the melting point, on average, a fraction of the molecules in a glass of water have enough heat energy to escape from the liquid. The water in the glass will be cooled by the evaporation until an equilibrium is reached where the air supplies the amount of heat removed by the evaporating water, in an enclosed environment the water would evaporate until the air is saturated. With sufficient temperature, the liquid would turn into vapor quickly, when the molecules collide, they transfer energy to each other in varying degrees, based on how they collide. Sometimes the transfer is so one-sided for a molecule near the surface that it ends up with energy to escape. Evaporation is an part of the water cycle. The sun drives evaporation of water from oceans, lakes, moisture in the soil, in hydrology, evaporation and transpiration are collectively termed evapotranspiration. Evaporation of water occurs when the surface of the liquid is exposed, allowing molecules to escape and form water vapor, when only a small proportion of the molecules meet these criteria, the rate of evaporation is low. Since the kinetic energy of a molecule is proportional to its temperature, as the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid decreases. This phenomenon is also called evaporative cooling and this is why evaporating sweat cools the human body. Evaporation also tends to proceed quickly with higher flow rates between the gaseous and liquid phase and in liquids with higher vapor pressure. For example, laundry on a line will dry more rapidly on a windy day than on a still day. Three key parts to evaporation are heat, atmospheric pressure, on a molecular level, there is no strict boundary between the liquid state and the vapor state. Instead, there is a Knudsen layer, where the phase is undetermined, because this layer is only a few molecules thick, at a macroscopic scale a clear phase transition interface cannot be seen. It is just that the process is slower and thus significantly less visible. If evaporation takes place in an area, the escaping molecules accumulate as a vapor above the liquid. Many of the return to the liquid, with returning molecules becoming more frequent as the density
Evaporation
–
Aerosol of microscopic water droplets suspended in the air above a hot tea cup after that water vapor has sufficiently cooled and condensed.
Water vapor is an invisible
gas, but the
clouds of condensed water droplets refract and disperse the sun light and so are visible.
Evaporation
–
Vapor pressure of water vs. temperature. 760
Torr = 1
atm.
42.
Direct current
–
Direct current is a flow of electrical charge carriers that always takes place in the same direction. The current need not always have the magnitude, but if it is to be defined as dc. This contrasts with alternating current which varies the direction of flow, sources of direct current include power supplies, electrochemical cells and batteries, and photovoltaic cells and panels. The intensity, or amplitude, of a direct current might fluctuate with time, in some such cases the dc has an ac component superimposed on it. An example of this is the output of a cell that receives a modulated light communications signal. A source of dc is sometimes called a dc generator, batteries and various other sources of dc produce a constant voltage. This is called pure dc and can be represented by a straight, the peak and effective values are the same. The peak to peak value is zero because the instantaneous amplitude never changes, in some instances the value of a dc voltage pulsates or oscillates rapidly with time, in a manner similar to the changes in an ac wave. The unfiltered output of a wave or a full wave rectifier. In 1820, Hans Christian Orsted discovered that electrical current creates a magnetic field and this discovery made scientists relate magnetism to the electric phenomena. In 1879, Thomas Edison invented the light bulb. He improved a 50-year-old idea using lower current electricity, a vacuum inside the globe and a small carbonized filament. At that time, the idea of lightning was not new. Edison not only invented an incandescent electric light, but an electric lighting system contained all the necessary elements to make the incandescent light safe, economical. Prior to 1879, direct current electricity had been used in lighting for the outdoors and it was in the 1880s when the modern electric utility industry began. It was an evolution from street lighting systems and from gas and it was located in Lower Manhattan, on Pearl Street. This station provided light and electricity to customers in a one square mile range, the station was called Thomas Edisons Pearl Street Electricity Generating Station. This station introduced four elements of an electric utility system, Efficient distribution, competitive price, reliable central generation
Direct current
–
Direct Current (red curve). The horizontal axis measures time; the vertical, current or voltage.
43.
Alternating current
–
Alternating current, is an electric current which periodically reverses direction, whereas direct current flows only in one direction. A common source of DC power is a cell in a flashlight. The abbreviations AC and DC are often used to mean simply alternating and direct, the usual waveform of alternating current in most electric power circuits is a sine wave. In certain applications, different waveforms are used, such as triangular or square waves, audio and radio signals carried on electrical wires are also examples of alternating current. These types of alternating current carry information encoded onto the AC signal and these currents typically alternate at higher frequencies than those used in power transmission. Electrical energy is distributed as alternating current because AC voltage may be increased or decreased with a transformer, use of a higher voltage leads to significantly more efficient transmission of power. The power losses in a conductor are a product of the square of the current and this means that when transmitting a fixed power on a given wire, if the current is halved, the power loss will be four times less. Power is often transmitted at hundreds of kilovolts, and transformed to 100–240 volts for domestic use, high voltages have disadvantages, such as the increased insulation required, and generally increased difficulty in their safe handling. In a power plant, energy is generated at a convenient voltage for the design of a generator, near the loads, the transmission voltage is stepped down to the voltages used by equipment. Consumer voltages vary somewhat depending on the country and size of load, the voltage delivered to equipment such as lighting and motor loads is standardized, with an allowable range of voltage over which equipment is expected to operate. Standard power utilization voltages and percentage tolerance vary in the different mains power systems found in the world, high-voltage direct-current electric power transmission systems have become more viable as technology has provided efficient means of changing the voltage of DC power. HVDC systems, however, tend to be expensive and less efficient over shorter distances than transformers. Three-phase electrical generation is very common, the simplest way is to use three separate coils in the generator stator, physically offset by an angle of 120° to each other. Three current waveforms are produced that are equal in magnitude and 120° out of phase to each other, if coils are added opposite to these, they generate the same phases with reverse polarity and so can be simply wired together. In practice, higher pole orders are commonly used, for example, a 12-pole machine would have 36 coils. The advantage is that lower rotational speeds can be used to generate the same frequency, for example, a 2-pole machine running at 3600 rpm and a 12-pole machine running at 600 rpm produce the same frequency, the lower speed is preferable for larger machines. If the load on a system is balanced equally among the phases. Even in the worst-case unbalanced load, the current will not exceed the highest of the phase currents
Alternating current
–
High voltage transmission lines deliver power from
electric generation plants over long distances using alternating current. These particular lines are located in eastern
Utah.
Alternating current
–
Alternating current (green curve). The horizontal axis measures time; the vertical, current or voltage.
Alternating current
–
The prototype of ZBD. transformer is on display at the Széchenyi István Memorial Exhibition,
Nagycenk,
Hungary
Alternating current
44.
Cathode
–
A cathode is the electrode from which a conventional current leaves a polarized electrical device. A conventional current describes the direction in which positive electronic charges move, Electrons have a negative charge, so the movement of electrons is opposite to the conventional current flow. Consequently, the mnemonic cathode current departs also means that flow into the devices cathode. Cathode polarity with respect to the anode can be positive or negative and this outward current is carried internally by positive ions moving from the electrolyte to the positive cathode. It is continued externally by electrons moving inwards, this negative charge moving inwards constituting positive current flowing outwards, for example, the Daniell galvanic cells copper electrode is the positive terminal and the cathode. In a recharging battery, or an electrolytic cell performing electrolysis, for example, reversing the current direction in a Daniell galvanic cell would produce an electrolytic cell, where the copper electrode is the positive terminal and the anode. In a diode, the cathode is the terminal at the pointed end of the arrow symbol. Note, electrode naming for diodes is always based on the direction of the forward current, an electrode through which current flows the other way is termed an anode. The use of West to mean the out direction may appear unnecessarily contrived, previously, as related in the first reference cited above, Faraday had used the more straightforward term exode. The reference he used to effect was the Earths magnetic field direction. The flow of electrons is almost always from anode to cathode outside of the cell or device, regardless of the cell or device type, an exception is when a diode reverse-conducts, either by accident or by design. In chemistry, a cathode is the electrode of a cell at which reduction occurs. Another mnemonic is to note the cathode has a c, as does reduction, perhaps most useful would be to remember cathode corresponds to cation and anode corresponds to anion. The cathode can be negative like when the cell is electrolytic, the cathode supplies electrons to the positively charged cations which flow to it from the electrolyte. The cathodic current, in electrochemistry, is the flow of electrons from the interface to a species in solution. The anodic current is the flow of electrons into the anode from a species in solution, in an electrolytic cell, the cathode is where the negative polarity is applied to drive the cell. Common results of reduction at the cathode are hydrogen gas or pure metal from metal ions, when discussing the relative reducing power of two redox agents, the couple for generating the more reducing species is said to be more cathodic with respect to the more easily reduced reagent. When metal ions are reduced from ionic solution, they form a metal surface on the cathode
Cathode
–
Glow from the directly heated cathode of a 1 kW power
tetrode tube in a radio transmitter. The cathode filament is not directly visible
Cathode
–
Diagram of a
copper cathode in a
galvanic cell (e.g., a battery). A positive current i flows out of the cathode.
Cathode
–
Two indirectly-heated cathodes (orange heater strip) in ECC83 dual triode tube
Cathode
–
Cold cathode (lefthand electrode) in
neon lamp
45.
Megaampere
–
The ampere, often shortened to amp, is a unit of electric current. In the International System of Units the ampere is one of the seven SI base units and it is named after André-Marie Ampère, French mathematician and physicist, considered the father of electrodynamics. SI defines the ampere in terms of base units by measuring the electromagnetic force between electrical conductors carrying electric current. The ampere was then defined as one coulomb of charge per second, in SI, the unit of charge, the coulomb, is defined as the charge carried by one ampere during one second. In the future, the SI definition may shift back to charge as the base unit, ampères force law states that there is an attractive or repulsive force between two parallel wires carrying an electric current. This force is used in the definition of the ampere. The SI unit of charge, the coulomb, is the quantity of electricity carried in 1 second by a current of 1 ampere, conversely, a current of one ampere is one coulomb of charge going past a given point per second,1 A =1 C s. In general, charge Q is determined by steady current I flowing for a time t as Q = It, constant, instantaneous and average current are expressed in amperes and the charge accumulated, or passed through a circuit over a period of time is expressed in coulombs. The relation of the ampere to the coulomb is the same as that of the watt to the joule, the ampere was originally defined as one tenth of the unit of electric current in the centimetre–gram–second system of units. That unit, now known as the abampere, was defined as the amount of current that generates a force of two dynes per centimetre of length between two wires one centimetre apart. The size of the unit was chosen so that the derived from it in the MKSA system would be conveniently sized. The international ampere was a realization of the ampere, defined as the current that would deposit 0.001118 grams of silver per second from a silver nitrate solution. Later, more accurate measurements revealed that this current is 0.99985 A, at present, techniques to establish the realization of an ampere have a relative uncertainty of approximately a few parts in 107, and involve realizations of the watt, the ohm and the volt. Rather than a definition in terms of the force between two current-carrying wires, it has proposed that the ampere should be defined in terms of the rate of flow of elementary charges. Since a coulomb is equal to 6. 2415093×1018 elementary charges. The proposed change would define 1 A as being the current in the direction of flow of a number of elementary charges per second. In 2005, the International Committee for Weights and Measures agreed to study the proposed change, the new definition was discussed at the 25th General Conference on Weights and Measures in 2014 but for the time being was not adopted. The current drawn by typical constant-voltage energy distribution systems is usually dictated by the power consumed by the system, for this reason the examples given below are grouped by voltage level
Megaampere
–
Demonstration model of a moving iron ammeter. As the current through the coil increases, the plunger is drawn further into the coil and the pointer deflects to the right.
46.
Negative resistance
–
This is in contrast to an ordinary resistor in which an increase of applied voltage causes a proportional increase in current due to Ohms law, resulting in a positive resistance. While a positive resistance consumes power from current passing through it, under certain conditions it can increase the power of an electrical signal, amplifying it. Negative resistance is a property which occurs in a few nonlinear electronic components. The term negative resistance refers to negative resistance, Δ v / Δ i <0. They are used in oscillators and amplifiers, particularly at microwave frequencies. Most microwave energy is produced with negative resistance devices. They can also have hysteresis and be bistable, and so are used in switching, examples of devices with negative differential resistance are tunnel diodes, Gunn diodes, and gas discharge tubes such as neon lamps. In addition, circuits containing amplifying devices such as transistors and op amps with positive feedback can have negative differential resistance and these are used in oscillators and active filters. Because they are nonlinear, negative resistance devices have a complicated behavior than the positive ohmic resistances usually encountered in electric circuits. Therefore, there is no real negative resistor analogous to a positive resistor, the resistance between two terminals of an electrical device or circuit is determined by its current–voltage curve, giving the current i through it for any given voltage v across it. Most materials, including the ordinary resistances encountered in electrical circuits, obey Ohms law, so the I–V curve of an ohmic resistance is a straight line through the origin with positive slope. The resistance is the ratio of voltage to current, the slope of the line and is constant. Negative resistance occurs in a few nonlinear devices, in a nonlinear component the I–V curve is not a straight line, so it does not obey Ohms law. Resistance can still be defined, but the resistance is not constant and it is the inverse slope of the line from the origin through the point on the I–V curve. Thus power sources formally have negative static resistance, however this term is never used in practice, because the term resistance is only applied to passive components. Static resistance determines the power dissipation in a component, passive devices, which consume electric power, have positive static resistance, while active devices, which produce electric power, do not. Differential resistance is only relevant to time-varying currents, points on the curve where the slope is negative, meaning an increase in voltage causes a decrease in current, have negative differential resistance. Devices of this type can amplify signals, and are what is meant by the term negative resistance
Negative resistance
Negative resistance
–
A
Gunn diode, a
semiconductor device with negative differential resistance used in
electronic oscillators to generate
microwaves
Negative resistance
–
An
oscillator consisting of a
Gunn diode inside a
cavity resonator. The negative resistance of the diode excites
microwave oscillations in the cavity, which radiate through the aperture into a
waveguide (not shown).
Negative resistance
–
8–12 GHz microwave amplifier consisting of two cascaded tunnel diode reflection amplifiers
47.
Electrical impedance
–
Electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied. In quantitative terms, it is the ratio of the voltage to the current in an alternating current circuit. Impedance extends the concept of resistance to AC circuits, and possesses both magnitude and phase, unlike resistance, which has only magnitude. When a circuit is driven with direct current, there is no distinction between impedance and resistance, the latter can be thought of as impedance with zero phase angle. The impedance caused by two effects is collectively referred to as reactance and forms the imaginary part of complex impedance whereas resistance forms the real part. The symbol for impedance is usually Z and it may be represented by writing its magnitude, however, cartesian complex number representation is often more powerful for circuit analysis purposes. The term impedance was coined by Oliver Heaviside in July 1886, arthur Kennelly was the first to represent impedance with complex numbers in 1893. Impedance is defined as the frequency ratio of the voltage to the current. In other words, it is the voltage–current ratio for a complex exponential at a particular frequency ω. In general, impedance will be a number, with the same units as resistance. For a sinusoidal current or voltage input, the form of the complex impedance relates the amplitude and phase of the voltage. In particular, The magnitude of the impedance is the ratio of the voltage amplitude to the current amplitude. The phase of the impedance is the phase shift by which the current lags the voltage. The reciprocal of impedance is admittance, Impedance is represented as a complex quantity Z and the term complex impedance may be used interchangeably. J is the unit, and is used instead of i in this context to avoid confusion with the symbol for electric current. In Cartesian form, impedance is defined as Z = R + j X where the part of impedance is the resistance R. Where it is needed to add or subtract impedances, the form is more convenient, but when quantities are multiplied or divided. A circuit calculation, such as finding the total impedance of two impedances in parallel, may require conversion between forms several times during the calculation, conversion between the forms follows the normal conversion rules of complex numbers
Electrical impedance
–
A graphical representation of the
complex impedance plane
48.
Electrical ballast
–
An electrical ballast is a device placed in line with the load to limit the amount of current in an electrical circuit. It may be a fixed or variable resistor, an electrical ballast is a device which limits the current through an electrical load. These are most often used when a load has its terminal decline when current through the load increases. If such a device were connected to a power supply. To prevent this, a ballast provides a resistance or reactance that limits the current. The ballast provides for the operation of the negative-resistance device by limiting current. A gas-discharge lamp is an example of a device which, under conditions, has negative differential resistance. In such a situation, every increase in the lamp current tends to reduce the voltage dropped across it. Let d I represent the change in current I, and d V represent the change in voltage V, each variation can be positive if its variable increases. The differential resistance is the ratio between d V and d I, d V d I and it can be positive or negative. This is quite a different concept from the resistance, which is considered positive. In the case of a lamp, the differential resistance really becomes negative because the positive variation for the current causes a negative variation for the voltage across the lamp. For a mechanical analogy to negative resistance behavior, and how a ballast can limit current, applying a force to the weight initially does not cause it to move because the static friction of the weight against the surface is more than the applied force. If we flood the surface with depth of viscous oil. The transition between static friction and dynamic friction as the moves is the negative resistance region - less force creates more movement. Ballasts can also be used simply to limit the current in an ordinary, prior to the advent of solid-state ignition, automobile ignition systems commonly included a ballast resistor to regulate the voltage applied to the ignition system. Series resistors are used as ballasts to control the current through LEDs, for simple, low-powered loads such as a neon lamp or a LED, a fixed resistor is commonly used. Because the resistance of the ballast resistor is large it determines the current in the circuit, the term also referred to a automobile engine component that lowered the supply voltage to the ignition system after the engine had been started
Electrical ballast
–
"Choke ballast" (inductor) used in older lighting. This example is from a tanning bed. Requires a lamp starter (below) and capacitor.
Electrical ballast
–
Modern ballast for powering 4 F32T8 office lamps
Electrical ballast
–
Lamp starter, required with some inductor type ballasts. Connects both ends of the lamp together to "preheat" the lamp ends for 1 second before lighting.
Electrical ballast
–
Several typical magnetic ballasts for
fluorescent lamps. The top is a high-power factor rapid start series ballast for two 30-40 W lamps. The middle is a low power factor preheat ballast for a single 30-40 W lamp while the bottom ballast is a simple inductor used with a 15 W preheat lamp.
49.
Calcium oxide
–
Calcium oxide, commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature, the broadly used term lime connotes calcium-containing inorganic materials, in which carbonates, oxides and hydroxides of calcium, silicon, magnesium, aluminium, and iron predominate. By contrast, quicklime specifically applies to the chemical compound calcium oxide. Calcium oxide that survives processing without reacting in building such as cement is called free lime. Both it and a chemical derivative are important commodity chemicals, Calcium oxide is usually made by the thermal decomposition of materials, such as limestone or seashells, that contain calcium carbonate in a lime kiln. This is accomplished by heating the material to above 825 °C, annual worldwide production of quicklime is around 283 million tonnes. China is by far the worlds largest producer, with a total of around 170 million tonnes per year, the United States is the next largest, with around 20 million tonnes per year. Approximately 1.8 t of limestone is required per 1.0 t of quicklime, quicklime has a high affinity for water and is a more efficient desiccant than silica gel. The reaction of quicklime with water is associated with an increase in volume by a factor of at least 2.5, the major use of quicklime is in the Basic oxygen steelmaking process. Its usage varies from about 30–50 kg/t of steel, the quicklime neutralizes the acidic oxides, SiO2, Al2O3, and Fe2O3, to produce a basic molten slag. Ground quicklime is used in the production of aerated concrete blocks, quicklime and hydrated lime can considerably increase the load carrying capacity of clay-containing soils. They do this by reacting with finely divided silica and alumina to produce calcium silicates and aluminates, small quantities of quicklime are used in other processes, e. g. the production of glass, calcium aluminate cement, and organic chemicals. The hydrate can be reconverted to quicklime by removing the water by heating it to redness to reverse the hydration reaction, one litre of water combines with approximately 3.1 kilograms of quicklime to give calcium hydroxide plus 3.54 MJ of energy. This process can be used to provide a convenient portable source of heat, light, When quicklime is heated to 2,400 °C, it emits an intense glow. This form of illumination is known as a limelight, and was used broadly in theatrical productions prior to the invention of electric lighting, cement, Calcium oxide is a key ingredient for the process of making cement. As a cheap and widely available alkali, about 50% of the total quicklime production is converted to calcium hydroxide before use. Both quick- and hydrated lime are used in the treatment of drinking water, petroleum industry, Water detection pastes contain a mix of calcium oxide and phenolphthalein. Should this paste come into contact with water in a storage tank
Calcium oxide
Calcium oxide
–
Calcium oxide
. PMID 26601188. doi:10.1126/sciadv.1400111.
