In physics, cryogenics is the production and behaviour of materials at low temperatures. A person who studies elements that have been subjected to cold temperatures is called a cryogenicist, it is not well-defined at what point on the temperature scale refrigeration ends and cryogenics begins, but scientists assume a gas to be cryogenic if it can be liquefied at or below −150 °C. The U. S. National Institute of Standards and Technology has chosen to consider the field of cryogenics as that involving temperatures below −180 °C; this is a logical dividing line, since the normal boiling points of the so-called permanent gases lie below −180 °C while the Freon refrigerants and other common refrigerants have boiling points above −180 °C. Discovery of superconducting materials with critical temperatures above the boiling point of liquid nitrogen has provided new interest in reliable, low cost methods of producing high temperature cryogenic refrigeration; the term "high temperature cryogenic" describes temperatures ranging from above the boiling point of liquid nitrogen, −195.79 °C, up to −50 °C, the defined upper limit of study referred to as cryogenics.
Cryogenicists use the Kelvin or Rankine temperature scale, both of which measure from absolute zero, rather than more usual scales such as Celsius or Fahrenheit, with their zeroes at arbitrary temperatures. Cryogenics The branches of engineering that involve the study of low temperatures, how to produce them, how materials behave at those temperatures. Cryobiology The branch of biology involving the study of the effects of low temperatures on organisms. Cryoconservation of animal genetic resources The conservation of genetic material with the intention of conserving a breed. Cryosurgery The branch of surgery applying cryogenic temperatures to destroy malignant tissue, e.g. cancer cells. Cryoelectronics The study of electronic phenomena at cryogenic temperatures. Examples include variable-range hopping. Cryotronics The practical application of cryoelectronics. Cryonics Cryopreserving humans and animals with the intention of future revival. "Cryogenics" is sometimes erroneously used to mean "Cryonics" in the press.
The word cryogenics stems from Greek κρύο – "cold" + γονική – "having to do with production". Cryogenic fluids with their boiling point in kelvins. Liquefied gases, such as liquid nitrogen and liquid helium, are used in many cryogenic applications. Liquid nitrogen is the most used element in cryogenics and is purchasable around the world. Liquid helium is commonly used and allows for the lowest attainable temperatures to be reached; these liquids may be stored in Dewar flasks, which are double-walled containers with a high vacuum between the walls to reduce heat transfer into the liquid. Typical laboratory Dewar flasks are spherical, made of glass and protected in a metal outer container. Dewar flasks for cold liquids such as liquid helium have another double-walled container filled with liquid nitrogen. Dewar flasks are named after James Dewar, the man who first liquefied hydrogen. Thermos bottles are smaller vacuum flasks fitted in a protective casing. Cryogenic barcode labels are used to mark Dewar flasks containing these liquids, will not frost over down to −195 degrees Celsius.
Cryogenic transfer pumps are the pumps used on LNG piers to transfer liquefied natural gas from LNG carriers to LNG storage tanks, as are cryogenic valves. The field of cryogenics advanced during World War II when scientists found that metals frozen to low temperatures showed more resistance to wear. Based on this theory of cryogenic hardening, the commercial cryogenic processing industry was founded in 1966 by Ed Busch. With a background in the heat treating industry, Busch founded a company in Detroit called CryoTech in 1966 which merged with 300 Below in 1999 to become the world's largest and oldest commercial cryogenic processing company. Busch experimented with the possibility of increasing the life of metal tools to anywhere between 200% and 400% of the original life expectancy using cryogenic tempering instead of heat treating; this evolved in the late 1990s into the treatment of other parts. Cryogens, such as liquid nitrogen, are further used for specialty chilling and freezing applications.
Some chemical reactions, like those used to produce the active ingredients for the popular statin drugs, must occur at low temperatures of −100 °C. Special cryogenic chemical reactors are used to remove reaction heat and provide a low temperature environment; the freezing of foods and biotechnology products, like vaccines, requires nitrogen in blast freezing or immersion freezing systems. Certain soft or elastic materials become hard and brittle at low temperatures, which makes cryogenic milling an option for some materials that cannot be milled at higher temperatures. Cryogenic processing is not a substitute for heat treatment, but rather an extension of the heating–quenching–tempering cycle; when an item is quenched, the final temperature is ambient. The only reason for this is. There is nothing metallurgically significant about ambient temperature; the cryogenic process continues this action from ambient temperature down to −320 °F. In most instances the cryogenic cycle is followed by a heat tempering procedure.
As all alloys do not have the same chemical constituents, the tempering procedure varies according to the material's chemical composition, t
A beam is a structural element that resists loads applied laterally to the beam's axis. Its mode of deflection is by bending; the loads applied to the beam result in reaction forces at the beam's support points. The total effect of all the forces acting on the beam is to produce shear forces and bending moments within the beam, that in turn induce internal stresses and deflections of the beam. Beams are characterized by their manner of support, profile and their material. Beams are traditionally descriptions of building or civil engineering structural elements, but any structures such as automotive automobile frames, aircraft components, machine frames, other mechanical or structural systems contain beam structures that are designed to carry lateral loads are analyzed in a similar fashion. Beams were squared timbers but are metal, stone, or combinations of wood and metal such as a flitch beam. Beams can carry vertical gravitational forces but are used to carry horizontal loads; the loads carried by a beam are transferred to columns, walls, or girders, which transfer the force to adjacent structural compression members and to ground.
In light frame construction, joists may rest on beams. In carpentry, a beam is called a plate as in a sill plate or wall plate, beam as in a summer beam or dragon beam. In engineering, beams are of several types: Simply supported – a beam supported on the ends which are free to rotate and have no moment resistance. Fixed – a beam supported on both ends and restrained from rotation. Over hanging – a simple beam extending beyond its support on one end. Double overhanging – a simple beam with both ends extending beyond its supports on both ends. Continuous – a beam extending over more than two supports. Cantilever – a projecting beam fixed only at one end. Trussed – a beam strengthened by adding a cable or rod to form a truss. In the beam equation I is used to represent the second moment of area, it is known as the moment of inertia, is the sum, about the neutral axis, of dA*r^2, where r is the distance from the neutral axis, dA is a small patch of area. Therefore, it encompasses not just how much area the beam section has overall, but how far each bit of area is from the axis, squared.
The greater I is. Internally, beams subjected to loads that do not induce torsion or axial loading experience compressive and shear stresses as a result of the loads applied to them. Under gravity loads, the original length of the beam is reduced to enclose a smaller radius arc at the top of the beam, resulting in compression, while the same original beam length at the bottom of the beam is stretched to enclose a larger radius arc, so is under tension. Modes of deformation where the top face of the beam is in compression, as under a vertical load, are known as sagging modes and where the top is in tension, for example over a support, is known as hogging; the same original length of the middle of the beam halfway between the top and bottom, is the same as the radial arc of bending, so it is under neither compression nor tension, defines the neutral axis. Above the supports, the beam is exposed to shear stress. There are some reinforced concrete beams in which the concrete is in compression with tensile forces taken by steel tendons.
These beams are known as prestressed concrete beams, are fabricated to produce a compression more than the expected tension under loading conditions. High strength steel tendons are stretched; when the concrete has cured, the tendons are released and the beam is under eccentric axial loads. This eccentric loading creates an internal moment, and, in turn, increases the moment carrying capacity of the beam, they are used on highway bridges. The primary tool for structural analysis of beams is the Euler–Bernoulli beam equation; this equation describes the elastic behaviour of slender beams where the cross sectional dimensions are small compared to the length of the beam. For beams that are not slender a different theory needs to be adopted to account for the deformation due to shear forces and, in dynamic cases, the rotary inertia; the beam formulation adopted here is that of Timoshenko and comparative examples can be found in NAFEMS Benchmark Challenge Number 7. Other mathematical methods for determining the deflection of beams include "method of virtual work" and the "slope deflection method".
Engineers are interested in determining deflections because the beam may be in direct contact with a brittle material such as glass. Beam deflections are minimized for aesthetic reasons. A visibly sagging beam if structurally safe, is unsightly and to be avoided. A stiffer beam creates less deflection. Mathematical methods for determining the beam forces include the "moment distribution method", the force or flexibility method and the direct stiffness method. Most beams in reinforced concrete buildings have rectangular cross sections, but a more efficient cross section for a beam is an I or H section, seen in steel construction; because of the parallel axis theorem and the fact that most of the material is away from the neutral axis, the second moment of area of the beam increases, which in turn increases the stiffness. An I-beam is only the most efficient shape in one direction of be
A stuffing box is an assembly, used to house a gland seal. It is used to prevent leakage of fluid, such as water or steam, between sliding or turning parts of machine elements. A stuffing box of a sailboat will have a stern tube that's bigger than the prop shaft, it will have packing nut threads or a gland nut. The packing creates the seal; the shaft is put in the gland nut. Through tightening it onto the stern tube, the packing is compressed, creating a seal against the shaft. Creating a proper plunger alignment is critical for correct flow and a long wear life. Stuffing box components are of stainless brass or other application-specific materials. A gland is a general type of stuffing box, used to seal a rotating or reciprocating shaft against a fluid; the most common example is in the head of a tap where the gland is packed with string, soaked in tallow or similar grease. The gland nut allows the packing material to be compressed to form a watertight seal and prevent water leaking up the shaft when the tap is turned on.
The gland at the rotating shaft of a centrifugal pump may be packed in a similar way and graphite grease used to accommodate continuous operation. The linear seal around the piston rod of a double acting steam piston is known as a gland in marine applications; the shaft of a handpump or wind pump is sealed with a gland where the shaft exits the borehole. Other types of sealed connections without moving parts are sometimes called glands. On a boat having an inboard motor that turns a shaft attached to an external propeller, the shaft passes though a stuffing box called a "packing box" or "stern gland" in this application; the stuffing box prevents sea water from entering the boat's hull. In many small fiberglass boats, for example, the stuffing box is mounted inboard near the point the shaft exits the hull; the "box" is a cylindrical assembly of bronze, comprising a sleeve threaded on one end to accept adjusting and locking nuts. A special purpose heavy-duty rubber hose attaches the stuffing box to a stern tube called a shaft log, that projects inward from the hull.
Marine-duty hose clamps secure the hose to the stern tube and the aft portion of the stuffing box sleeve. A sound stuffing box installation is critical to safety because failure can admit a catastrophic volume of water into the boat. In a common type of stuffing box, rings of braided fiber, known as shaft packing or gland packing, form a seal between the shaft and the stuffing box. A traditional variety of shaft packing comprises a square cross-section rope made of flax or hemp impregnated with wax and lubricants. A turn of the adjusting nut compresses the shaft packing. Ideally, the compression is just enough to make the seal both watertight when the shaft is stationary and drip when the shaft is turning; the drip rate must be at once sufficient to lubricate and cool the shaft and packing, but not so much as could sink an unattended boat. The market offers improved shaft packing materials that aim to be drip-less when the shaft is turning as well as when stationary. There are pack-less sealing systems that employ engineered materials such as carbon composites and PTFE.
In a steam engine, where the piston rod reciprocates through the cylinder cover, a stuffing box provided in the cylinder cover prevents the leakage of steam from the cylinder. Axlebox Bilge Compression seal fitting Journal bearing Journal box Labyrinth seal Calder, Nigel. Boatowner's electrical manual. Camden, Maine: International Marine/McGraw-Hill. ISBN 0-07-143238-8 Servicing Your Stuffing Box, by Don Casey Step-by-Step Instructions For Servicing Your Stuffing Box, by Capt. Vincent Daniello, August 16, 2012
An I-beam known as H-beam, w-beam, universal beam, rolled steel joist, or double-T, is a beam with an I or H-shaped cross-section. The horizontal elements of the "I" are known as flanges, while the vertical element is termed the "web". I-beams are made of structural steel and are used in construction and civil engineering; the web resists shear forces, while the flanges resist most of the bending moment experienced by the beam. Beam theory shows that the I-shaped section is a efficient form for carrying both bending and shear loads in the plane of the web. On the other hand, the cross-section has a reduced capacity in the transverse direction, is inefficient in carrying torsion, for which hollow structural sections are preferred; the method of producing an I-beam, as rolled from a single piece of steel, was patented by Alphonse Halbou of the company Forges de la Providence in 1849. Bethlehem Steel was a leading supplier of rolled structural steel of various cross-sections in American bridge and skyscraper work of the mid-twentieth century.
Today, rolled cross-sections have been displaced in such work by fabricated cross-sections. There are two standard I-beam forms: Rolled I-beam, formed by hot rolling, cold rolling or extrusion. Plate girder, formed by welding plates. I-beams are made of structural steel but may be formed from aluminium or other materials. A common type of I-beam is the rolled steel joist —sometimes incorrectly rendered as reinforced steel joist. British and European standards specify Universal Beams and Universal Columns; these sections have parallel flanges, as opposed to the varying thickness of RSJ flanges which are now rolled in the UK. Parallel flanges are easier to do away with the need for tapering washers. UCs have equal or near-equal width and depth and are more suited to being oriented vertically to carry axial load such as columns in multi-storey construction, while UBs are deeper than they are wide are more suited to carrying bending load such as beam elements in floors. I-joists—I-beams engineered from wood with fiberboard and/or laminated veneer lumber—are becoming popular in construction residential, as they are both lighter and less prone to warping than solid wooden joists.
However, there has been some concern as to their rapid loss of strength in a fire. I-beams are used in the construction industry and are available in a variety of standard sizes. Tables are available to allow easy selection of a suitable steel I-beam size for a given applied load. I-beams may be used both as columns. I-beams may be used both on their own, or acting compositely with another material concrete. Design may be governed by any of the following criteria: deflection: the stiffness of the I-beam will be chosen to minimize deformation vibration: the stiffness and mass are chosen to prevent unacceptable vibrations in settings sensitive to vibrations, such as offices and libraries bending failure by yielding: where the stress in the cross section exceeds the yield stress bending failure by lateral torsional buckling: where a flange in compression tends to buckle sideways or the entire cross-section buckles torsionally bending failure by local buckling: where the flange or web is so slender as to buckle locally local yield: caused by concentrated loads, such as at the beam's point of support shear failure: where the web fails.
Slender webs will fail by buckling, rippling in a phenomenon termed tension field action, but shear failure is resisted by the stiffness of the flanges buckling or yielding of components: for example, of stiffeners used to provide stability to the I-beam's web. A beam under bending sees high stresses along the axial fibers that are farthest from the neutral axis. To prevent failure, most of the material in the beam must be located in these regions. Comparatively little material is needed in the area close to the neutral axis; this observation is the basis of the I-beam cross-section. The ideal beam is the one with the least cross-sectional area needed to achieve a given section modulus. Since the section modulus depends on the value of the moment of inertia, an efficient beam must have most of its material located as far from the neutral axis as possible; the farther a given amount of material is from the neutral axis, the larger is the section modulus and hence a larger bending moment can be resisted.
When designing a symmetric I-beam to resist stresses due to bending the usual starting point is the required section modulus. If the allowable stress is σ m a x and the maximum expected bending moment is M m a x the required section modulus is given by S = M m a x σ m a x = I c where I is the moment of inertia of the beam cross-section and c is the distance of the top of the beam from the neu
A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting expansion to one dimension or two. There is a similar effect in water waves constrained within a canal, or guns that have barrels which restrict hot gas expansion to maximize energy transfer to their bullets. Without the physical constraint of a waveguide, wave amplitudes decrease according to the inverse square law as they expand into three dimensional space. There are different types of waveguides for each type of wave; the original and most common meaning is a hollow conductive metal pipe used to carry high frequency radio waves microwaves. The geometry of a waveguide reflects its function. Slab waveguides confine energy in one fiber or channel waveguides in two dimensions; the frequency of the transmitted wave dictates the shape of a waveguide: an optical fiber guiding high-frequency light will not guide microwaves of a much lower frequency. Some occurring structures can act as waveguides.
The SOFAR channel layer in the ocean can guide the sound of whale song across enormous distances. Waves propagate in all directions in open space as spherical waves; the power of the wave falls with the distance R from the source as the square of the distance. A waveguide confines the wave to propagate in one dimension, so that, under ideal conditions, the wave loses no power while propagating. Due to total reflection at the walls, waves are confined to the interior of a waveguide; the uses of waveguides for transmitting signals were known before the term was coined. The phenomenon of sound waves guided through a taut wire have been known for a long time, as well as sound through a hollow pipe such as a cave or medical stethoscope. Other uses of waveguides are in transmitting power between the components of a system such as radio, radar or optical devices. Waveguides are the fundamental principle of guided wave testing, one of the many methods of non-destructive evaluation. Specific examples: Optical fibers transmit light and signals for long distances with low attenuation and a wide usable range of wavelengths.
In a microwave oven a waveguide transfers power from the magnetron, where waves are formed, to the cooking chamber. In a radar, a waveguide transfers radio frequency energy to and from the antenna, where the impedance needs to be matched for efficient power transmission. Rectangular and Circular waveguides are used to connect feeds of parabolic dishes to their electronics, either low-noise receivers or power amplifier/transmitters. Waveguides are used in scientific instruments to measure optical and elastic properties of materials and objects; the waveguide can be put in contact with the specimen, in which case the waveguide ensures that the power of the testing wave is conserved, or the specimen may be put inside the waveguide, so that smaller objects can be tested and the accuracy is better. Transmission lines are a specific type of waveguide commonly used; the first structure for guiding waves was proposed by J. J. Thomson in 1893, was first experimentally tested by Oliver Lodge in 1894; the first mathematical analysis of electromagnetic waves in a metal cylinder was performed by Lord Rayleigh in 1897.
For sound waves, Lord Rayleigh published a full mathematical analysis of propagation modes in his seminal work, “The Theory of Sound”. Jagadish Chandra Bose researched millimetre wavelengths using waveguides, in 1897 described to the Royal Institution in London his research carried out in Kolkata; the study of dielectric waveguides began as early as the 1920s, by several people, most famous of which are Rayleigh and Debye. Optical fiber began to receive special attention in the 1960s due to its importance to the communications industry; the development of radio communication occurred at the lower frequencies because these could be more propagated over large distances. The long wavelengths made these frequencies unsuitable for use in hollow metal waveguides because of the impractically large diameter tubes required. Research into hollow metal waveguides stalled and the work of Lord Rayleigh was forgotten for a time and had to be rediscovered by others. Practical investigations resumed in the 1930s by George C.
Southworth at Bell Labs and Wilmer L. Barrow at MIT. Southworth at first took the theory from papers on waves in dielectric rods because the work of Lord Rayleigh was unknown to him; this misled him somewhat. Serious theoretical work was taken up by Sallie P. Mead; this work led to the discovery that for the TE01 mode in circular waveguide losses go down with frequency and at one time this was a serious contender for the format for long distance telecommunications. The importance of radar in World War II gave a great impetus to waveguide research, at least on the Allied side; the magnetron developed in 1940 by John Randall and Harry Boot at the University of Birmingham in the United Kingdom provided a good power source and made microwave radars feasible. The most important centre of US research was at the Radiation Laboratory at MIT but many others took part in the US, in the UK such as the Telecommunications Research Establishment; the head of the Fundamental Development Group at Rad Lab was Edward Mills Purcell.
His researchers included Julian Schwinger, Nathan Marcuvitz, Carol Gray Montgomery, Robert H. Dicke. Much of the Rad Lab work concentrated on finding lumped element models of waveguide
Deutsches Institut für Normung
Deutsches Institut für Normung e. V. is the German ISO member body. DIN is a German Registered Association headquartered in Berlin. There are around thirty thousand DIN Standards, covering nearly every field of technology. Founded in 1917 as the Normenausschuß der deutschen Industrie, the NADI was renamed Deutscher Normenausschuß in 1926 to reflect that the organization now dealt with standardization issues in many fields. In 1975 it was renamed again to Deutsches Institut für Normung, or'DIN' and is recognized by the German government as the official national-standards body, representing German interests at the international and European levels; the acronym,'DIN' is incorrectly expanded as Deutsche Industrienorm. This is due to the historic origin of the DIN as "NADI"; the NADI indeed published their standards as DI-Norm. For example, the first published standard was'DI-Norm 1' in 1918. Many people still mistakenly associate DIN with the old DI-Norm naming convention. One of the earliest, the best known, is DIN 476 — the standard that introduced the A-series paper sizes in 1922 — adopted in 1975 as International Standard ISO 216.
Common examples in modern technology include DIN and mini-DIN connectors for electronics, the DIN rail. The designation of a DIN standard shows its origin: DIN # is used for German standards with domestic significance or designed as a first step toward international status. E DIN # is a draft standard and DIN V # is a preliminary standard. DIN EN # is used for the German edition of European standards. DIN ISO # is used for the German edition of ISO standards. DIN EN ISO # is used if the standard has been adopted as a European standard. DIN 476: international paper sizes DIN 1451: typeface used by German railways and on traffic signs DIN 31635: transliteration of the Arabic language DIN 72552: electric terminal numbers in automobiles Austrian Standards Institute Swiss Association for Standardization Die Brücke, an earlier German institute aiming to set standard paper sizes DIN film speed DIN connector DQS - Deutsche Gesellschaft zur Zertifizierung von Managementsystemen, a subsidiary of DIN DGQ - Deutsche Gesellschaft für Qualität, founded DQS in 1985 together with DIN DIN home page DIN home page DIN online dictionary of classes and units of measure DQS Holding GmbH DQS HK
Iron is a chemical element with symbol Fe and atomic number 26. It is a metal, that belongs to group 8 of the periodic table, it is by mass the most common element on Earth, forming much of Earth's inner core. It is the fourth most common element in the Earth's crust. Pure iron is rare on the Earth's crust being limited to meteorites. Iron ores are quite abundant, but extracting usable metal from them requires kilns or furnaces capable of reaching 1500 °C or higher, about 500 °C higher than what is enough to smelt copper. Humans started to dominate that process in Eurasia only about 2000 BCE, iron began to displace copper alloys for tools and weapons, in some regions, only around 1200 BCE; that event is considered the transition from the Bronze Age to the Iron Age. Iron alloys, such as steel and special steels are now by far the most common industrial metals, because of their mechanical properties and their low cost. Pristine and smooth pure iron surfaces are mirror-like silvery-gray. However, iron reacts with oxygen and water to give brown to black hydrated iron oxides known as rust.
Unlike the oxides of some other metals, that form passivating layers, rust occupies more volume than the metal and thus flakes off, exposing fresh surfaces for corrosion. The body of an adult human contains about 3 to 5 grams of elemental iron in hemoglobin and myoglobin; these two proteins play essential roles in vertebrate metabolism oxygen transport by blood and oxygen storage in muscles. To maintain the necessary levels, human iron metabolism requires a minimum of iron in the diet. Iron is the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals. Chemically, the most common oxidation states of iron are +2 and +3. Iron shares many properties of other transition metals, including the other group 8 elements and osmium. Iron forms compounds in a wide range of oxidation states, −2 to +7. Iron forms many coordination compounds. At least four allotropes of iron are known, conventionally denoted α, γ, δ, ε; the first three forms are observed at ordinary pressures.
As molten iron cools past its freezing point of 1538 °C, it crystallizes into its δ allotrope, which has a body-centered cubic crystal structure. As it cools further to 1394 °C, it changes to its γ-iron allotrope, a face-centered cubic crystal structure, or austenite. At 912 °C and below, the crystal structure again becomes the bcc α-iron allotrope; the physical properties of iron at high pressures and temperatures have been studied extensively, because of their relevance to theories about the cores of the Earth and other planets. Above 10 GPa and temperatures of a few hundred kelvin or less, α-iron changes into another hexagonal close-packed structure, known as ε-iron; the higher-temperature γ-phase changes into ε-iron, but does so at higher pressure. Some controversial experimental evidence exists for a stable β phase at pressures above 50 GPa and temperatures of at least 1500 K, it is supposed to have a double hcp structure. The inner core of the Earth is presumed to consist of an iron-nickel alloy with ε structure.
The melting and boiling points of iron, along with its enthalpy of atomization, are lower than those of the earlier 3d elements from scandium to chromium, showing the lessened contribution of the 3d electrons to metallic bonding as they are attracted more and more into the inert core by the nucleus. This same trend appears for ruthenium but not osmium; the melting point of iron is experimentally well defined for pressures less than 50 GPa. For greater pressures, published data still varies by tens of gigapascals and over a thousand kelvin. Below its Curie point of 770 °C, α-iron changes from paramagnetic to ferromagnetic: the spins of the two unpaired electrons in each atom align with the spins of its neighbors, creating an overall magnetic field; this happens because the orbitals of those two electrons do not point toward neighboring atoms in the lattice, therefore are not involved in metallic bonding. In the absence of an external source of magnetic field, the atoms get spontaneously partitioned into magnetic domains, about 10 micrometres across, such that the atoms in each domain have parallel spins, but different domains have other orientations.
Thus a macroscopic piece of iron will have a nearly zero overall magnetic field. Application of an external magnetic field causes the domains that are magnetized in the same general direction to grow at the expense of adjacent ones that point in other directions, reinforcing the external field; this effect is exploited in devices that needs to channel magnetic fields, such as electrical transformers, magnetic recording heads, electric motors. Impurities, lattice defects, or grain and particle boundaries can "pin" the domains in the new positions, so that the effect persists after the external field is removed -- thus turning the iron object into a magnet. Similar behavior is exhibited by some iron compounds, such as the fer