Jan 22, 2014 This answer might prove disappointing, but if the magnet is a dipole, then you would have to reorient the magnet 180 degrees. If it's a monopole, then you would have to get another monopole magnet with the opposite polar orientation to the magnet.
Part of a series of articles about |
Electromagnetism |
---|
|
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.
A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). These include the elements iron, nickel and cobalt, some alloys of rare-earth metals, and some naturally occurring minerals such as lodestone. Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism.
Ferromagnetic materials can be divided into magnetically 'soft' materials like annealediron, which can be magnetized but do not tend to stay magnetized, and magnetically 'hard' materials, which do. Permanent magnets are made from 'hard' ferromagnetic materials such as alnico and ferrite that are subjected to special processing in a strong magnetic field during manufacture to align their internal microcrystalline structure, making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, and this threshold depends on coercivity of the respective material. 'Hard' materials have high coercivity, whereas 'soft' materials have low coercivity. The overall strength of a magnet is measured by its magnetic moment or, alternatively, the total magnetic flux it produces. The local strength of magnetism in a material is measured by its magnetization.
An electromagnet is made from a coil of wire that acts as a magnet when an electric current passes through it but stops being a magnet when the current stops. Often, the coil is wrapped around a core of 'soft' ferromagnetic material such as mild steel, which greatly enhances the magnetic field produced by the coil.
- 2Physics
- 7Types of permanent magnets
- 9Units and calculations
- 9.2Calculating the magnetic force
Discovery and development[edit]
Ancient people learned about magnetism from lodestones (or magnetite) which are naturally magnetized pieces of iron ore. The word magnet was adopted in Middle English from Latinmagnetum 'lodestone', ultimately from Greekμαγνῆτις [λίθος] (magnētis [lithos])[1] meaning '[stone] from Magnesia',[2] a part of ancient Greece where lodestones were found. Lodestones, suspended so they could turn, were the first magnetic compasses. The earliest known surviving descriptions of magnets and their properties are from Greece, India, and China around 2500 years ago.[3][4][5] The properties of lodestones and their affinity for iron were written of by Pliny the Elder in his encyclopedia Naturalis Historia.[6]
By the 12th to 13th centuries AD, magnetic compasses were used in navigation in China, Europe, the Arabian Peninsula and elsewhere.[7]
Physics[edit]
Magnetic field[edit]
The magnetic flux density (also called magnetic B field or just magnetic field, usually denoted B) is a vector field. The magnetic B field vector at a given point in space is specified by two properties:
- Its direction, which is along the orientation of a compass needle.
- Its magnitude (also called strength), which is proportional to how strongly the compass needle orients along that direction.
In SI units, the strength of the magnetic B field is given in teslas.[8]
Magnetic moment[edit]
A magnet's magnetic moment (also called magnetic dipole moment and usually denoted μ) is a vector that characterizes the magnet's overall magnetic properties. For a bar magnet, the direction of the magnetic moment points from the magnet's south pole to its north pole,[9] and the magnitude relates to how strong and how far apart these poles are. In SI units, the magnetic moment is specified in terms of A·m2 (amperes times meters squared).
A magnet both produces its own magnetic field and responds to magnetic fields. The strength of the magnetic field it produces is at any given point proportional to the magnitude of its magnetic moment. In addition, when the magnet is put into an external magnetic field, produced by a different source, it is subject to a torque tending to orient the magnetic moment parallel to the field.[10] The amount of this torque is proportional both to the magnetic moment and the external field. A magnet may also be subject to a force driving it in one direction or another, according to the positions and orientations of the magnet and source. If the field is uniform in space, the magnet is subject to no net force, although it is subject to a torque.[11]
A wire in the shape of a circle with area A and carrying currentI has a magnetic moment of magnitude equal to IA.
Magnetization[edit]
The magnetization of a magnetized material is the local value of its magnetic moment per unit volume, usually denoted M, with units A/m.[12] It is a vector field, rather than just a vector (like the magnetic moment), because different areas in a magnet can be magnetized with different directions and strengths (for example, because of domains, see below). A good bar magnet may have a magnetic moment of magnitude 0.1 A•m2 and a volume of 1 cm3, or 1×10−6 m3, and therefore an average magnetization magnitude is 100,000 A/m. Iron can have a magnetization of around a million amperes per meter. Such a large value explains why iron magnets are so effective at producing magnetic fields.
Modelling magnets[edit]
Two different models exist for magnets: magnetic poles and atomic currents.
Although for many purposes it is convenient to think of a magnet as having distinct north and south magnetic poles, the concept of poles should not be taken literally: it is merely a way of referring to the two different ends of a magnet. The magnet does not have distinct north or south particles on opposing sides. If a bar magnet is broken into two pieces, in an attempt to separate the north and south poles, the result will be two bar magnets, each of which has both a north and south pole. However, a version of the magnetic-pole approach is used by professional magneticians to design permanent magnets.[citation needed]
In this approach, the divergence of the magnetization ∇·M inside a magnet and the surface normal component M·n are treated as a distribution of magnetic monopoles. This is a mathematical convenience and does not imply that there are actually monopoles in the magnet. If the magnetic-pole distribution is known, then the pole model gives the magnetic field H. Outside the magnet, the field B is proportional to H, while inside the magnetization must be added to H. An extension of this method that allows for internal magnetic charges is used in theories of ferromagnetism.
Another model is the Ampère model, where all magnetization is due to the effect of microscopic, or atomic, circular bound currents, also called Ampèrian currents, throughout the material. For a uniformly magnetized cylindrical bar magnet, the net effect of the microscopic bound currents is to make the magnet behave as if there is a macroscopic sheet of electric current flowing around the surface, with local flow direction normal to the cylinder axis.[13] Microscopic currents in atoms inside the material are generally canceled by currents in neighboring atoms, so only the surface makes a net contribution; shaving off the outer layer of a magnet will not destroy its magnetic field, but will leave a new surface of uncancelled currents from the circular currents throughout the material.[14] The right-hand rule tells which direction positively-charged current flows. However, current due to negatively-charged electricity is far more prevalent in practice.[citation needed]
Polarity[edit]
The north pole of a magnet is defined as the pole that, when the magnet is freely suspended, points towards the Earth's North Magnetic Pole in the Arctic (the magnetic and geographic poles do not coincide, see magnetic declination). Since opposite poles (north and south) attract, the North Magnetic Pole is actually the south pole of the Earth's magnetic field.[15][16][17][18] As a practical matter, to tell which pole of a magnet is north and which is south, it is not necessary to use the Earth's magnetic field at all. For example, one method would be to compare it to an electromagnet, whose poles can be identified by the right-hand rule. The magnetic field lines of a magnet are considered by convention to emerge from the magnet's north pole and reenter at the south pole.[18]
Magnetic materials[edit]
The term magnet is typically reserved for objects that produce their own persistent magnetic field even in the absence of an applied magnetic field. Only certain classes of materials can do this. Most materials, however, produce a magnetic field in response to an applied magnetic field – a phenomenon known as magnetism. There are several types of magnetism, and all materials exhibit at least one of them.
The overall magnetic behavior of a material can vary widely, depending on the structure of the material, particularly on its electron configuration. Several forms of magnetic behavior have been observed in different materials, including:
- Ferromagnetic and ferrimagnetic materials are the ones normally thought of as magnetic; they are attracted to a magnet strongly enough that the attraction can be felt. These materials are the only ones that can retain magnetization and become magnets; a common example is a traditional refrigerator magnet. Ferrimagnetic materials, which include ferrites and the oldest magnetic materials magnetite and lodestone, are similar to but weaker than ferromagnetics. The difference between ferro- and ferrimagnetic materials is related to their microscopic structure, as explained in Magnetism.
- Paramagnetic substances, such as platinum, aluminum, and oxygen, are weakly attracted to either pole of a magnet. This attraction is hundreds of thousands of times weaker than that of ferromagnetic materials, so it can only be detected by using sensitive instruments or using extremely strong magnets. Magnetic ferrofluids, although they are made of tiny ferromagnetic particles suspended in liquid, are sometimes considered paramagnetic since they cannot be magnetized.
- Diamagnetic means repelled by both poles. Compared to paramagnetic and ferromagnetic substances, diamagnetic substances, such as carbon, copper, water, and plastic, are even more weakly repelled by a magnet. The permeability of diamagnetic materials is less than the permeability of a vacuum. All substances not possessing one of the other types of magnetism are diamagnetic; this includes most substances. Although force on a diamagnetic object from an ordinary magnet is far too weak to be felt, using extremely strong superconducting magnets, diamagnetic objects such as pieces of lead and even mice[19] can be levitated, so they float in mid-air. Superconductors repel magnetic fields from their interior and are strongly diamagnetic.
There are various other types of magnetism, such as spin glass, superparamagnetism, superdiamagnetism, and metamagnetism.
Common uses[edit]
- Magnetic recording media: VHS tapes contain a reel of magnetic tape. The information that makes up the video and sound is encoded on the magnetic coating on the tape. Common audio cassettes also rely on magnetic tape. Similarly, in computers, floppy disks and hard disks record data on a thin magnetic coating.[20]
- Credit, debit, and automatic teller machine cards: All of these cards have a magnetic strip on one side. This strip encodes the information to contact an individual's financial institution and connect with their account(s).[21]
- Older types of televisions (non flat screen) and older large computer monitors: TV and computer screens containing a cathode ray tube employ an electromagnet to guide electrons to the screen.[22]
- Speakers and microphones: Most speakers employ a permanent magnet and a current-carrying coil to convert electric energy (the signal) into mechanical energy (movement that creates the sound). The coil is wrapped around a bobbin attached to the speaker cone and carries the signal as changing current that interacts with the field of the permanent magnet. The voice coil feels a magnetic force and in response, moves the cone and pressurizes the neighboring air, thus generating sound. Dynamic microphones employ the same concept, but in reverse. A microphone has a diaphragm or membrane attached to a coil of wire. The coil rests inside a specially shaped magnet. When sound vibrates the membrane, the coil is vibrated as well. As the coil moves through the magnetic field, a voltage is induced across the coil. This voltage drives a current in the wire that is characteristic of the original sound.
- Electric guitars use magnetic pickups to transduce the vibration of guitar strings into electric current that can then be amplified. This is different from the principle behind the speaker and dynamic microphone because the vibrations are sensed directly by the magnet, and a diaphragm is not employed. The Hammond organ used a similar principle, with rotating tonewheels instead of strings.
- Electric motors and generators: Some electric motors rely upon a combination of an electromagnet and a permanent magnet, and, much like loudspeakers, they convert electric energy into mechanical energy. A generator is the reverse: it converts mechanical energy into electric energy by moving a conductor through a magnetic field.
- Medicine: Hospitals use magnetic resonance imaging to spot problems in a patient's organs without invasive surgery.
- Chemistry: Chemists use nuclear magnetic resonance to characterize synthesized compounds.
- Chucks are used in the metalworking field to hold objects. Magnets are also used in other types of fastening devices, such as the magnetic base, the magnetic clamp and the refrigerator magnet.
- Compasses: A compass (or mariner's compass) is a magnetized pointer free to align itself with a magnetic field, most commonly Earth's magnetic field.
- Art: Vinyl magnet sheets may be attached to paintings, photographs, and other ornamental articles, allowing them to be attached to refrigerators and other metal surfaces. Objects and paint can be applied directly to the magnet surface to create collage pieces of art. Magnetic art is portable, inexpensive and easy to create. Vinyl magnetic art is not for the refrigerator anymore. Colorful metal magnetic boards, strips, doors, microwave ovens, dishwashers, cars, metal I beams, and any metal surface can be receptive of magnetic vinyl art. Being a relatively new media for art, the creative uses for this material is just beginning.
- Science projects: Many topic questions are based on magnets, including the repulsion of current-carrying wires, the effect of temperature, and motors involving magnets.[23]
- Toys: Given their ability to counteract the force of gravity at close range, magnets are often employed in children's toys, such as the Magnet Space Wheel and Levitron, to amusing effect.
- Refrigerator magnets are used to adorn kitchens, as a souvenir, or simply to hold a note or photo to the refrigerator door.
- Magnets can be used to make jewelry. Necklaces and bracelets can have a magnetic clasp, or may be constructed entirely from a linked series of magnets and ferrous beads.
- Magnets can pick up magnetic items (iron nails, staples, tacks, paper clips) that are either too small, too hard to reach, or too thin for fingers to hold. Some screwdrivers are magnetized for this purpose.
- Magnets can be used in scrap and salvage operations to separate magnetic metals (iron, cobalt, and nickel) from non-magnetic metals (aluminum, non-ferrous alloys, etc.). The same idea can be used in the so-called 'magnet test', in which an auto body is inspected with a magnet to detect areas repaired using fiberglass or plastic putty.
- Magnets are found in process industries, food manufacturing especially, in order to remove metal foreign bodies from materials entering the process (raw materials) or to detect a possible contamination at the end of the process and prior to packaging. They constitute an important layer of protection for the process equipment and for the final consumer.[24]
- Magnetic levitation transport, or maglev, is a form of transportation that suspends, guides and propels vehicles (especially trains) through electromagnetic force. Eliminating rolling resistance increases efficiency. The maximum recorded speed of a maglev train is 581 kilometers per hour (361 mph).
- Magnets may be used to serve as a fail-safe device for some cable connections. For example, the power cords of some laptops are magnetic to prevent accidental damage to the port when tripped over. The MagSafe power connection to the Apple MacBook is one such example.
Medical issues and safety[edit]
Because human tissues have a very low level of susceptibility to static magnetic fields, there is little mainstream scientific evidence showing a health effect associated with exposure to static fields. Dynamic magnetic fields may be a different issue, however; correlations between electromagnetic radiation and cancer rates have been postulated due to demographic correlations (see Electromagnetic radiation and health).
If a ferromagnetic foreign body is present in human tissue, an external magnetic field interacting with it can pose a serious safety risk.[25]
A different type of indirect magnetic health risk exists involving pacemakers. If a pacemaker has been embedded in a patient's chest (usually for the purpose of monitoring and regulating the heart for steady electrically induced beats), care should be taken to keep it away from magnetic fields. It is for this reason that a patient with the device installed cannot be tested with the use of a magnetic resonance imaging device.
Children sometimes swallow small magnets from toys, and this can be hazardous if two or more magnets are swallowed, as the magnets can pinch or puncture internal tissues.[26]
Magnetic imaging devices (e.g. MRIs) generate enormous magnetic fields, and therefore rooms intended to hold them exclude ferrous metals. Bringing objects made of ferrous metals (such as oxygen canisters) into such a room creates a severe safety risk, as those objects may be powerfully thrown about by the intense magnetic fields.
Magnetizing ferromagnets[edit]
Ferromagnetic materials can be magnetized in the following ways:
- Heating the object higher than its Curie temperature, allowing it to cool in a magnetic field and hammering it as it cools. This is the most effective method and is similar to the industrial processes used to create permanent magnets.
- Placing the item in an external magnetic field will result in the item retaining some of the magnetism on removal. Vibration has been shown to increase the effect. Ferrous materials aligned with the Earth's magnetic field that are subject to vibration (e.g., frame of a conveyor) have been shown to acquire significant residual magnetism. Likewise, striking a steel nail held by fingers in a N-S direction with a hammer will temporarily magnetize the nail.
- Stroking: An existing magnet is moved from one end of the item to the other repeatedly in the same direction (single touch method) or two magnets are moved outwards from the center of a third (double touch method).[27]
- Electric Current: The magnetic field produced by passing an electric current through a coil can get domains to line up. Once all of the domains are lined up, increasing the current will not increase the magnetization.[28]
Demagnetizing ferromagnets[edit]
Magnetized ferromagnetic materials can be demagnetized (or degaussed) in the following ways:
- Heating a magnet past its Curie temperature; the molecular motion destroys the alignment of the magnetic domains. This always removes all magnetization.
- Placing the magnet in an alternating magnetic field with intensity above the material's coercivity and then either slowly drawing the magnet out or slowly decreasing the magnetic field to zero. This is the principle used in commercial demagnetizers to demagnetize tools, erase credit cards, hard disks, and degaussing coils used to demagnetize CRTs.
- Some demagnetization or reverse magnetization will occur if any part of the magnet is subjected to a reverse field above the magnetic material's coercivity.
- Demagnetization progressively occurs if the magnet is subjected to cyclic fields sufficient to move the magnet away from the linear part on the second quadrant of the B-H curve of the magnetic material (the demagnetization curve).
- Hammering or jarring: mechanical disturbance tends to randomize the magnetic domains and reduce magnetization of an object, but may cause unacceptable damage.
Types of permanent magnets[edit]
Magnetic metallic elements[edit]
Many materials have unpaired electron spins, and the majority of these materials are paramagnetic. When the spins interact with each other in such a way that the spins align spontaneously, the materials are called ferromagnetic (what is often loosely termed as magnetic). Because of the way their regular crystallineatomic structure causes their spins to interact, some metals are ferromagnetic when found in their natural states, as ores. These include iron ore (magnetite or lodestone), cobalt and nickel, as well as the rare earth metals gadolinium and dysprosium (when at a very low temperature). Such naturally occurring ferromagnets were used in the first experiments with magnetism. Technology has since expanded the availability of magnetic materials to include various man-made products, all based, however, on naturally magnetic elements.
Composites[edit]
Ceramic, or ferrite, magnets are made of a sinteredcomposite of powdered iron oxide and barium/strontium carbonate ceramic. Given the low cost of the materials and manufacturing methods, inexpensive magnets (or non-magnetized ferromagnetic cores, for use in electronic components such as portable AM radio antennas) of various shapes can be easily mass-produced. The resulting magnets are non-corroding but brittle and must be treated like other ceramics.
Alnico magnets are made by casting or sintering a combination of aluminium, nickel and cobalt with iron and small amounts of other elements added to enhance the properties of the magnet. Sintering offers superior mechanical characteristics, whereas casting delivers higher magnetic fields and allows for the design of intricate shapes. Alnico magnets resist corrosion and have physical properties more forgiving than ferrite, but not quite as desirable as a metal. Trade names for alloys in this family include: Alni, Alcomax, Hycomax, Columax, and Ticonal.[29]
Injection-molded magnets are a composite of various types of resin and magnetic powders, allowing parts of complex shapes to be manufactured by injection molding. The physical and magnetic properties of the product depend on the raw materials, but are generally lower in magnetic strength and resemble plastics in their physical properties.
Flexible magnets are composed of a high-coercivityferromagnetic compound (usually ferric oxide) mixed with a plastic binder. This is extruded as a sheet and passed over a line of powerful cylindrical permanent magnets. These magnets are arranged in a stack with alternating magnetic poles facing up (N, S, N, S...) on a rotating shaft. This impresses the plastic sheet with the magnetic poles in an alternating line format. No electromagnetism is used to generate the magnets. The pole-to-pole distance is on the order of 5 mm, but varies with manufacturer. These magnets are lower in magnetic strength but can be very flexible, depending on the binder used.[30]
Rare-earth magnets[edit]
Rare earth (lanthanoid) elements have a partially occupied felectron shell (which can accommodate up to 14 electrons). The spin of these electrons can be aligned, resulting in very strong magnetic fields, and therefore, these elements are used in compact high-strength magnets where their higher price is not a concern. The most common types of rare-earth magnets are samarium-cobalt and neodymium-iron-boron (NIB) magnets.
Single-molecule magnets (SMMs) and single-chain magnets (SCMs)[edit]
In the 1990s, it was discovered that certain molecules containing paramagnetic metal ions are capable of storing a magnetic moment at very low temperatures. These are very different from conventional magnets that store information at a magnetic domain level and theoretically could provide a far denser storage medium than conventional magnets. In this direction, research on monolayers of SMMs is currently under way. Very briefly, the two main attributes of an SMM are:
- a large ground state spin value (S), which is provided by ferromagnetic or ferrimagnetic coupling between the paramagnetic metal centres
- a negative value of the anisotropy of the zero field splitting (D)
Most SMMs contain manganese but can also be found with vanadium, iron, nickel and cobalt clusters. More recently, it has been found that some chain systems can also display a magnetization that persists for long times at higher temperatures. These systems have been called single-chain magnets.
Nano-structured magnets[edit]
Some nano-structured materials exhibit energy waves, called magnons, that coalesce into a common ground state in the manner of a Bose–Einstein condensate.[31][32]
Rare-earth-free permanent magnets[edit]
The United States Department of Energy has identified a need to find substitutes for rare-earth metals in permanent-magnet technology, and has begun funding such research. The Advanced Research Projects Agency-Energy (ARPA-E) has sponsored a Rare Earth Alternatives in Critical Technologies (REACT) program to develop alternative materials. In 2011, ARPA-E awarded 31.6 million dollars to fund Rare-Earth Substitute projects.[33]
Costs[edit]
The current cheapest permanent magnets, allowing for field strengths, are flexible and ceramic magnets, but these are also among the weakest types. The ferrite magnets are mainly low-cost magnets since they are made from cheap raw materials: iron oxide and Ba- or Sr-carbonate. However, a new low cost magnet, Mn-Al alloy,[34] has been developed and is now dominating the low-cost magnets field. It has a higher saturation magnetization than the ferrite magnets. It also has more favorable temperature coefficients, although it can be thermally unstable.Neodymium-iron-boron (NIB) magnets are among the strongest. These cost more per kilogram than most other magnetic materials but, owing to their intense field, are smaller and cheaper in many applications.[35]
Temperature[edit]
Temperature sensitivity varies, but when a magnet is heated to a temperature known as the Curie point, it loses all of its magnetism, even after cooling below that temperature. The magnets can often be remagnetized, however.
Additionally, some magnets are brittle and can fracture at high temperatures.
The maximum usable temperature is highest for alnico magnets at over 540 °C (1,000 °F), around 300 °C (570 °F) for ferrite and SmCo, about 140 °C (280 °F) for NIB and lower for flexible ceramics, but the exact numbers depend on the grade of material.
Electromagnets[edit]
An electromagnet, in its simplest form, is a wire that has been coiled into one or more loops, known as a solenoid. When electric current flows through the wire, a magnetic field is generated. It is concentrated near (and especially inside) the coil, and its field lines are very similar to those of a magnet. The orientation of this effective magnet is determined by the right hand rule. The magnetic moment and the magnetic field of the electromagnet are proportional to the number of loops of wire, to the cross-section of each loop, and to the current passing through the wire.[36]
If the coil of wire is wrapped around a material with no special magnetic properties (e.g., cardboard), it will tend to generate a very weak field. However, if it is wrapped around a soft ferromagnetic material, such as an iron nail, then the net field produced can result in a several hundred- to thousandfold increase of field strength.
Uses for electromagnets include particle accelerators, electric motors, junkyard cranes, and magnetic resonance imaging machines. Some applications involve configurations more than a simple magnetic dipole; for example, quadrupole and sextupole magnets are used to focusparticle beams.
Units and calculations[edit]
For most engineering applications, MKS (rationalized) or SI (Système International) units are commonly used. Two other sets of units, Gaussian and CGS-EMU, are the same for magnetic properties and are commonly used in physics.[citation needed]
In all units, it is convenient to employ two types of magnetic field, B and H, as well as the magnetizationM, defined as the magnetic moment per unit volume.
- The magnetic induction field B is given in SI units of teslas (T). B is the magnetic field whose time variation produces, by Faraday's Law, circulating electric fields (which the power companies sell). B also produces a deflection force on moving charged particles (as in TV tubes). The tesla is equivalent to the magnetic flux (in webers) per unit area (in meters squared), thus giving B the unit of a flux density. In CGS, the unit of B is the gauss (G). One tesla equals 104 G.
- The magnetic field H is given in SI units of ampere-turns per meter (A-turn/m). The turns appear because when H is produced by a current-carrying wire, its value is proportional to the number of turns of that wire. In CGS, the unit of H is the oersted (Oe). One A-turn/m equals 4π×10−3 Oe.
- The magnetization M is given in SI units of amperes per meter (A/m). In CGS, the unit of M is the oersted (Oe). One A/m equals 10−3 emu/cm3. A good permanent magnet can have a magnetization as large as a million amperes per meter.
- In SI units, the relation B = μ0(H + M) holds, where μ0 is the permeability of space, which equals 4π×10−7 T•m/A. In CGS, it is written as B = H + 4πM. (The pole approach gives μ0H in SI units. A μ0M term in SI must then supplement this μ0H to give the correct field within B, the magnet. It will agree with the field B calculated using Ampèrian currents).
Materials that are not permanent magnets usually satisfy the relation M = χH in SI, where χ is the (dimensionless) magnetic susceptibility. Most non-magnetic materials have a relatively small χ (on the order of a millionth), but soft magnets can have χ on the order of hundreds or thousands. For materials satisfying M = χH, we can also write B = μ0(1 + χ)H = μ0μrH = μH, where μr = 1 + χ is the (dimensionless) relative permeability and μ =μ0μr is the magnetic permeability. Both hard and soft magnets have a more complex, history-dependent, behavior described by what are called hysteresis loops, which give either B vs. H or M vs. H. In CGS, M = χH, but χSI = 4πχCGS, and μ = μr.
Caution: in part because there are not enough Roman and Greek symbols, there is no commonly agreed-upon symbol for magnetic pole strength and magnetic moment. The symbol m has been used for both pole strength (unit A•m, where here the upright m is for meter) and for magnetic moment (unit A•m2). The symbol μ has been used in some texts for magnetic permeability and in other texts for magnetic moment. We will use μ for magnetic permeability and m for magnetic moment. For pole strength, we will employ qm. For a bar magnet of cross-section A with uniform magnetization M along its axis, the pole strength is given by qm = MA, so that M can be thought of as a pole strength per unit area.
Fields of a magnet[edit]
Far away from a magnet, the magnetic field created by that magnet is almost always described (to a good approximation) by a dipole field characterized by its total magnetic moment. This is true regardless of the shape of the magnet, so long as the magnetic moment is non-zero. One characteristic of a dipole field is that the strength of the field falls off inversely with the cube of the distance from the magnet's center.
Closer to the magnet, the magnetic field becomes more complicated and more dependent on the detailed shape and magnetization of the magnet. Formally, the field can be expressed as a multipole expansion: A dipole field, plus a quadrupole field, plus an octupole field, etc.
At close range, many different fields are possible. For example, for a long, skinny bar magnet with its north pole at one end and south pole at the other, the magnetic field near either end falls off inversely with the square of the distance from that pole.
Calculating the magnetic force[edit]
Pull force of a single magnet[edit]
The strength of a given magnet is sometimes given in terms of its pull force— its ability to move (push/ pull) other objects. The pull force exerted by either an electromagnet or a permanent magnet at the 'air gap' (i.e., the point in space where the magnet ends) is given by the Maxwell equation:[37]
- ,
where
- F is force (SI unit: newton)
- A is the cross section of the area of the pole in square meters
- B is the magnetic induction exerted by the magnet
Therefore, if a magnet is acting vertically, it can lift a mass m in kilograms given by the simple equation:
- .
Force between two magnetic poles[edit]
Classically, the force between two magnetic poles is given by:[38]
where
- F is force (SI unit: newton)
- qm1 and qm2 are the magnitudes of magnetic poles (SI unit: ampere-meter)
- μ is the permeability of the intervening medium (SI unit: teslameter per ampere, henry per meter or newton per ampere squared)
- r is the separation (SI unit: meter).
The pole description is useful to the engineers designing real-world magnets, but real magnets have a pole distribution more complex than a single north and south. Therefore, implementation of the pole idea is not simple. In some cases, one of the more complex formulae given below will be more useful.
Force between two nearby magnetized surfaces of area A[edit]
The mechanical force between two nearby magnetized surfaces can be calculated with the following equation. The equation is valid only for cases in which the effect of fringing is negligible and the volume of the air gap is much smaller than that of the magnetized material:[39][40]
where:
- A is the area of each surface, in m2
- H is their magnetizing field, in A/m
- μ0 is the permeability of space, which equals 4π×10−7 T•m/A
- B is the flux density, in T.
Force between two bar magnets[edit]
The force between two identical cylindrical bar magnets placed end to end at large distance is approximately:[dubious],[39]
where:
- B0 is the magnetic flux density very close to each pole, in T,
- A is the area of each pole, in m2,
- L is the length of each magnet, in m,
- R is the radius of each magnet, in m, and
- z is the separation between the two magnets, in m.
- relates the flux density at the pole to the magnetization of the magnet.
Note that all these formulations are based on Gilbert's model, which is usable in relatively great distances. In other models (e.g., Ampère's model), a more complicated formulation is used that sometimes cannot be solved analytically. In these cases, numerical methods must be used.
Force between two cylindrical magnets[edit]
For two cylindrical magnets with radius and length , with their magnetic dipole aligned, the force can be asymptotically approximated at large distance by,[41]
where is the magnetization of the magnets and is the gap between the magnets.A measurement of the magnetic flux density very close to the magnet is related to approximately by the formula
The effective magnetic dipole can be written as
Where is the volume of the magnet. For a cylinder, this is .
When , the point dipole approximation is obtained,
which matches the expression of the force between two magnetic dipoles.
See also[edit]
Notes[edit]
- ^Platonis OperaArchived 2018-01-14 at the Wayback Machine, Meyer and Zeller, 1839, p. 989.
- ^The location of Magnesia is debated; it could be the region in mainland Greece or Magnesia ad Sipylum. See, for example, 'Magnet'. Language Hat blog. 28 May 2005. Archived from the original on 19 May 2012. Retrieved 22 March 2013.
- ^Fowler, Michael (1997). 'Historical Beginnings of Theories of Electricity and Magnetism'. Archived from the original on 2008-03-15. Retrieved 2008-04-02.
- ^Vowles, Hugh P. (1932). 'Early Evolution of Power Engineering'. Isis. 17 (2): 412–420 [419–20]. doi:10.1086/346662.
- ^Li Shu-hua (1954). 'Origine de la Boussole II. Aimant et Boussole'. Isis. 45 (2): 175. doi:10.1086/348315. JSTOR227361.
- ^Pliny the Elder, The Natural History, BOOK XXXIV. THE NATURAL HISTORY OF METALS., CHAP. 42.—THE METAL CALLED LIVE IRONArchived 2011-06-29 at the Wayback Machine. Perseus.tufts.edu. Retrieved on 2011-05-17.
- ^Schmidl, Petra G. (1996–1997). 'Two Early Arabic Sources On The Magnetic Compass'(PDF). Journal of Arabic and Islamic Studies. 1: 81–132. Archived(PDF) from the original on 2012-05-24.
- ^Griffiths, David J. (1999). Introduction to Electrodynamics (3rd ed.). Prentice Hall. pp. 255–8. ISBN0-13-805326-X. OCLC40251748.
- ^Knight, Jones, & Field, 'College Physics' (2007) p. 815.
- ^Cullity, B. D. & Graham, C. D. (2008). Introduction to Magnetic Materials (2 ed.). Wiley-IEEE Press. p. 103. ISBN0-471-47741-9.
- ^Boyer, Timothy H. (1988). 'The Force on a Magnetic Dipole'. American Journal of Physics. 56 (8): 688–692. Bibcode:1988AmJPh..56..688B. doi:10.1119/1.15501.
- ^'Units for Magnetic Properties'(PDF). Lake Shore Cryotronics, Inc. Archived from the original(PDF) on 2011-07-14. Retrieved 2012-11-05.
- ^Allen, Zachariah (1852). Philosophy of the Mechanics of Nature, and the Source and Modes of Action of Natural Motive-Power. D. Appleton and Company. p. 252. Archived from the original on 2014-06-27.
- ^Saslow, Wayne M. (2002). Electricity, Magnetism, and Light (3rd ed.). Academic Press. p. 426. ISBN978-0-12-619455-5. Archived from the original on 2014-06-27.
- ^Serway, Raymond A.; Chris Vuille (2006). Essentials of college physics. USA: Cengage Learning. p. 493. ISBN0-495-10619-4. Archived from the original on 2013-06-04.
- ^Emiliani, Cesare (1992). Planet Earth: Cosmology, Geology, and the Evolution of Life and Environment. UK: Cambridge University Press. p. 228. ISBN0-521-40949-7. Archived from the original on 2016-12-24.
- ^Manners, Joy (2000). Static Fields and Potentials. USA: CRC Press. p. 148. ISBN0-7503-0718-8. Archived from the original on 2016-12-24.
- ^ abNave, Carl R. (2010). 'Bar Magnet'. Hyperphysics. Dept. of Physics and Astronomy, Georgia State Univ. Archived from the original on 2011-04-08. Retrieved 2011-04-10.
- ^Mice levitated in NASA labArchived 2011-02-09 at the Wayback Machine. Livescience.com (2009-09-09). Retrieved on 2011-10-08.
- ^Mallinson, John C. (1987). The foundations of magnetic recording (2nd ed.). Academic Press. ISBN0-12-466626-4.
- ^'The stripe on a credit card'. How Stuff Works. Archived from the original on 2011-06-24. Retrieved 19 July 2011.
- ^'Electromagnetic deflection in a cathode ray tube, I'. National High Magnetic Field Laboratory. Archived from the original on 2012-04-03. Retrieved 20 July 2011.
- ^'Snacks about magnetism'. The Exploratorium Science Snacks. Exploratorium. Archived from the original on 7 April 2013. Retrieved 17 April 2013.
- ^'Archived copy'. Archived from the original on 2017-05-10. Retrieved 2016-12-05.CS1 maint: Archived copy as title (link) Source on magnets in process industries
- ^Schenck JF (2000). 'Safety of strong, static magnetic fields'. J Magn Reson Imaging. 12 (1): 2–19. doi:10.1002/1522-2586(200007)12:1<2::AID-JMRI2>3.0.CO;2-V. PMID10931560.
- ^Oestreich AE (2008). 'Worldwide survey of damage from swallowing multiple magnets'. Pediatr Radiol. 39 (2): 142–7. doi:10.1007/s00247-008-1059-7. PMID19020871.
- ^McKenzie, A. E. E. (1961). Magnetism and electricity. Cambridge. pp. 3–4.
- ^'Ferromagnetic Materials'. Phares Electronics. Archived from the original on 27 June 2015. Retrieved 26 June 2015.
- ^Brady, George Stuart; Henry R. Clauser; John A. Vaccari (2002). Materials Handbook: An Encyclopedia for Managers. McGraw-Hill Professional. p. 577. ISBN0-07-136076-X. Archived from the original on 2016-12-24.
- ^'Press release: Fridge magnet transformed'. Riken. March 11, 2011. Archived from the original on August 7, 2017.
- ^'Nanomagnets Bend The Rules'. Archived from the original on December 7, 2005. Retrieved November 14, 2005.
- ^Della Torre, E.; Bennett, L.; Watson, R. (2005). 'Extension of the Bloch T3/2 Law to Magnetic Nanostructures: Bose-Einstein Condensation'. Physical Review Letters. 94 (14): 147210. Bibcode:2005PhRvL..94n7210D. doi:10.1103/PhysRevLett.94.147210.
- ^'Research Funding for Rare Earth Free Permanent Magnets'. ARPA-E. Archived from the original on 10 October 2013. Retrieved 23 April 2013.
- ^'Nanostructured Mn-Al Permanent Magnets'. Retrieved 18 Feb 2017.
- ^Frequently Asked QuestionsArchived 2008-03-12 at the Wayback Machine. Magnet sales. Retrieved on 2011-10-08.
- ^Ruskell, Todd; Tipler, Paul A.; Mosca, Gene (2007). Physics for Scientists and Engineers (6 ed.). Palgrave Macmillan. ISBN1-4292-0410-9.
- ^Cardarelli, François (2008). Materials Handbook: A Concise Desktop Reference (Second ed.). Springer. p. 493. ISBN9781846286681. Archived from the original on 2016-12-24.
- ^'Basic Relationships'. Geophysics.ou.edu. Archived from the original on 2010-07-09. Retrieved 2009-10-19.
- ^ ab'Magnetic Fields and Forces'. Archived from the original on 2012-02-20. Retrieved 2009-12-24.
- ^'The force produced by a magnetic field'. Archived from the original on 2010-03-17. Retrieved 2010-03-09.
- ^David Vokoun; Marco Beleggia; Ludek Heller; Petr Sittner (2009). 'Magnetostatic interactions and forces between cylindrical permanent magnets'. Journal of Magnetism and Magnetic Materials. 321 (22): 3758–3763. Bibcode:2009JMMM..321.3758V. doi:10.1016/j.jmmm.2009.07.030.
References[edit]
- 'The Early History of the Permanent Magnet'. Edward Neville Da Costa Andrade, Endeavour, Volume 17, Number 65, January 1958. Contains an excellent description of early methods of producing permanent magnets.
- 'positive pole n'. The Concise Oxford English Dictionary. Catherine Soanes and Angus Stevenson. Oxford University Press, 2004. Oxford Reference Online. Oxford University Press.
- Wayne M. Saslow, Electricity, Magnetism, and Light, Academic (2002). ISBN0-12-619455-6. Chapter 9 discusses magnets and their magnetic fields using the concept of magnetic poles, but it also gives evidence that magnetic poles do not really exist in ordinary matter. Chapters 10 and 11, following what appears to be a 19th-century approach, use the pole concept to obtain the laws describing the magnetism of electric currents.
- Edward P. Furlani, Permanent Magnet and Electromechanical Devices:Materials, Analysis and Applications, Academic Press Series in Electromagnetism (2001). ISBN0-12-269951-3.
External links[edit]
Look up magnet in Wiktionary, the free dictionary. |
Wikimedia Commons has media related to Magnets. |
- How magnets are made (video)
- Floating Ring Magnets, Bulletin of the IAPT, Volume 4, No. 6, 145 (June 2012). (Publication of the Indian Association of Physics Teachers).
A humbucking pickup, humbucker, or double coil, is a type of electric guitarpickup that uses two coils to 'buck the hum' (or cancel out the interference) picked up by coil pickups caused by electromagnetic interference, particularly mains hum. Most pickups use magnets to produce a magnetic field around the strings, and induce an electrical current in the surrounding coils as the strings vibrate (a notable exception is the piezoelectric pickup). Humbuckers work by pairing a coil with the north poles of its magnets oriented 'up', (toward the strings) with another coil right next to it, which has the south pole of its magnets oriented up. By connecting the coils together out of phase, the interference is significantly reduced via phase cancellation: the string signals from both coils add up instead of canceling, because the magnets are placed in opposite polarity. The coils can be connected in series or in parallel in order to achieve this hum-cancellation effect, although it's much more common for the coils of a humbucker pickup to be connected in series (see 'series/parallel' below). In addition to electric guitar pickups, humbucking coils are sometimes used in dynamic microphones to cancel electromagnetic hum.
Hum is caused by the alternating magnetic fields created by transformers and power supplies inside electrical equipment using alternating current. While playing a guitar without humbuckers, a musician would hear a hum through the pickups during quiet sections of music. Sources of studio and stage hum include high-power amps, processors, mixers, motors, power lines, and other equipment. Compared to single coil pickups, especially unshielded ones, humbuckers dramatically reduce hum, and (especially when the coils are connected in series) produce a louder signal with more mid-range presence.
- 3Alternative humbucker designs
- 5Other noise-reducing pickup designs
History[edit]
The 'humbucking coil' was invented in 1934 by Electro-Voice, an American professional audio company based in South Bend, Indiana that Al Kahn and Lou Burroughs incorporated in 1930 for the purpose of manufacturing portable public address equipment, including microphones and loudspeakers.[1]
The twin coiled guitar pickup invented by Arnold Lesti in 1935 is arranged as a humbucker, and the patent USRE20070[2] describes the noise cancelation and current summation principles of such a design. This 'Electric Translating Device' employed the solenoid windings of the pickup to magnetize the steel strings by means of switching on a short D.C. charge before switching over to amplification.
In 1938 A.F. Knoblaugh invented a pickup for stringed instruments involving 2 stacked coils (US Pat. 2,119,584). This pickup was to be used in pianos, since he was working for Baldwin Piano at the time.
The 1939 April copy of 'Radio Craft Magazine'[3] shows how to construct a guitar pickup made with two identical coils wrapped around self-magnetized iron cores, where one is then flipped over to create a reverse wound, reverse polarity, humbucking orientation. The iron cores of these pickups were magnetized to have their north-south poles at the opposite ends of the core, rather than the now more common top-bottom orientation.
To overcome the hum problem for guitars, a humbucking pickup was invented by Seth Lover of Gibson under instruction of then-president Ted McCarty. About the same time, Ray Butts developed a similar pickup that was taken up by Gretsch guitars. Although Gibson's patent was filed almost 2 years before Gretsch's, Gibson's patent was issued 4 weeks after Gretsch's. Both patents describe a reverse wound and reverse polarity pair of coils.[4][5]
A successful early humbucking pickup was the so-called PAF (literally 'Patent Applied For') invented by Seth Lover in 1955.[6] Because of this, and because of its use on the Gibson Les Paul guitar, popularization of the humbucker is strongly associated with Gibson, although humbuckers had been used in many different guitar designs by many different manufacturers before. Humbuckers are also known as dual-coil, double-coil, or hum-canceling pickups. Rickenbacker offered dual coil pickups arranged in a humbucking pattern beginning in late 1953 but dropped the design in 1954 due to the perceived distorted sound, which had stronger mid-range presence. The Gibson Les Paul was the first guitar to use humbuckers in substantial production, but since then, even some models of Fender Stratocasters and Telecasters, traditionally fitted with single-coil pickups, are factory-equipped with humbuckers. Stratocasters fitted with one humbucker in the bridge position, resulting in a pickup configuration noted as H-S-S (starting at bridge pickup: H for humbucker, S for single coil) are referred to as 'Fat Strats', because of the 'fatter', 'rounder' tone offered by the humbucking pickup.
How humbuckers work[edit]
In any magnetic pickup, a vibrating guitar string, magnetized by a fixed magnet within the pickup, induces an alternating voltage across its coil(s). However, wire coils also make excellent antennae and are therefore sensitive to electromagnetic interference caused by alternating magnetic fields from mains wiring (mains hum) and electrical appliances like transformers, motors, and computer screens, especially the older CRT monitors. Guitar pickups reproduce this noise, which can be quite audible, sounding like a constant hum or buzz. This is most noticeable when using distortion, fuzz, compressors, or other effects which reduce the signal-to-noise ratio and therefore amplify the unwanted interference relative to the signal from the strings.
The direction of voltage induced across a coil by the moving string depends on both the coil winding direction and the polarity of the fixed magnet. On the other hand, the direction of current induced by external fields is only dependent on the direction of winding. Therefore, a humbucker has two coils wound in such a way to cancel hum and maximize signal when they are connected. By convention humbucker coils are both wound counterclockwise; however, typically the outside ends of the coils are connected together so that the coil starts are out of phase. The magnets in the two coils are arranged with opposite polarity so that the string motion induces voltages across both coils that become additive (exactly the opposite of phase cancellation) when the coils are wired in this manner. Electromagnetic interference induces identical, or almost identical voltage across both coils, because they are directly adjacent, and therefore pick up the same interference. When the signals from both coils are summed together, the two noise voltages cancel each other out, while the signal voltages add.[7] This dramatically improves the signal-to-noise ratio. The technique has something in common with what electrical engineers call 'common-mode rejection,' and is also found in the balanced lines used in audio equipment.
The arrangement of the humbucker coils to achieve reverse current flow with opposite magnetic polarity in each coil is known as Reverse Wound, Reverse Polarity, or RWRP for short. This nomenclature does not indicate that the coils are wound in opposition, but that the current flows in opposition.
Alternative humbucker designs[edit]
Mini-humbuckers[edit]
Many solid-body guitars feature cavities only for single-coil pickups. Installing full-sized humbuckers in this type of guitar requires additional routing of the woodwork, and/or cutting of the pickguard if the instrument has one. If the process is not carefully done, the instrument's body and pickguard may be damaged. For most guitarists, this is unacceptable, especially for expensive vintage guitars where it is vital to preserve cosmetic appearance. As a result, many pickup manufacturers now produce humbucking pickups compacted into the size of a single coil. Many different kinds of mini-humbuckers are available from numerous manufacturers, and they produce a wide range of different tones. The most common design is very similar to a traditional humbucker.
Stacked coils[edit]
As a concept similar to mini-humbuckers, a stacked pickup offers the more subtle and delicate sound of a single-coil, while still retaining the hum-cancellation properties of a humbucker. One of the coils simply has no magnets, so the inverted signal of this coil only serves to cancel out the hum picked up by the other coil, with the actual string signal remaining unaffected. This is often used on bass guitars, where the type of pickups used has a more substantial effect on the instrument's overall sound, and the lower range of note fundamental frequencies can match frequencies typically more heavily affected by hum. This is often called a 'stacked' pickup, because the coils are most often 'stacked' vertically, with the coil containing magnets placed closer to the strings.
Rail humbuckers[edit]
Another design known as the rail humbucker features a single magnetic component spanning across the entire string width of the instrument. These pickups look like a normal mini-humbucker, except without the typical dots corresponding to the magnetic pole-pieces. This is sometimes expanded into a normal-size 'quadrail', or double humbucker, effectively combining 4 coils connected in series to produce an extremely high-output pickup. The Kent Armstrong 'Motherbucker' is an example of such an overpowered pickup.
The same type of rails can also be found in a normal-size humbucker, however. Heavy metal guitarist Dimebag Darrell made heavy use of this type of pickup wired in the bridge position. These tend to also sound fuller and have a higher output and bass response than the single coil-size version. Dimarzio has designed and sold many such pickups.
Coil splits[edit]
Some guitars which have humbucking pickups feature 'coil splits', which allow the pickups to act as 'pseudo-single' coils by either short-circuiting or bypassing one coil. The electrical circuit of the pickup is reduced to that of a true single coil so there is no hum canceling effect. Usually, this feature is activated using a miniature toggle switch or a DPDT push-pull switch mounted on a potentiometer. Some guitars (e.g., the Peavey T-60 and the Fender Classic Player Jaguar HH) make use of a variable coil split circuit that allows the guitarist to dial a variable amount of signal from the second coil, from pure single-coil to full humbucker and everything in-between.
Another similar option is a series/parallel switch, which in one position causes the coils to be connected in parallel rather than in series. This retains the humbucker's noise-cancellation properties, and gives a sound closer to that of two single-coil pickups used together.
Coil splits are often wrongly referred to as a 'coil tap'. Coil taps are most commonly found on single coil pickups, and involve an extra hook-up wire being included during the manufacture of the pickup so the guitarist can choose to have all the windings of the pickup included in the circuit, for a fatter, higher output sound with more midrange; or switch the output to 'Tap' into the windings at a point that is less than the full coil for a brighter, lower output, cleaner sound. For example: a full pickup coil may be 10,000 turns of wire and the 'Tap' may be at 8000 turns. Because of the confusion between coil splits and coil taps—and the rareness of coil taps in general—it is difficult to find tappable single coil pickups for sale. However, pickup manufacturer Seymour Duncan offers tapped versions of many of their Telecaster and Stratocaster pickups on their website at an increased cost.
Notable humbucker designs[edit]
- Gibson 'PAF' - Seth Lover's humbucker design
- Gretsch Filter'Tron Prototype – Ray Butts' first humbucker design[8][9]
- Fender Wide Range – Fender's first humbucker design, also by Seth Lover
- Epiphone (and later Gibson) mini-humbucker – a smaller humbucker design with adjustable pole pieces. A Gibson design which reduced their standard humbucker to fit in Epiphones routed for the 1950s Epi 'New York' pickup, they were later used most famously in the Gibson Les Paul Deluxe.
- Gibson Firebird pickup – inspired by the Epiphone pickup, and shares its basic dimensions, but is different in terms of design, appearance, and tone, using single blade pole pieces.
- Q-tuner – neodymium magnet humbuckers
- EMG Pickups – active pickups since 1976
- Seymour Duncan - by Seymour W. Duncan
Other noise-reducing pickup designs[edit]
While the original humbucker remains the most common noise-reducing pickup design, inventors have tried many other approaches to reducing noise in guitar pickups.
Combining two single-coil pickups[edit]
Many instruments combine separate single coil pickups in a hum reducing configuration by reversing the electrical phase of one of the pickups. This arrangement is similar to that of a humbucking pickup and effectively reduces noise. Examples of this include the Fender Jazz Bass, introduced in 1960, which has used a pair of single coil pickups, one near the bridge and another one about halfway between the bridge and the neck—and many Stratocaster style guitars, which often have 3 pickups with the middle one reversed electrically and magnetically. The (usually) five-way selector switch provides two humbucking settings, using the reversed middle pickup in parallel with either the bridge or neck pickup.
If the pickups are wired in series instead of parallel, the resulting sound is close to that of a normal humbucker.
Proprietary designs[edit]
In 1957, Fender introduced a split pickup to its Precision Bass, which was wired in humbucking fashion, with one coil serving the E and A strings, the other the D and G strings. Both coils pick up the same hum, but since each string is only served by one coil, a single-coil sound is provided. The concept of this later expanded to G&L's Z-coil pickup, which is used for standard guitars such as their Comanche model.
In 1985, Lace Music Products introduced the Lace Sensor pickups, which use proprietary hum-screening bobbins to reduce hum while preserving single-coil tone.[10]
In 1996, Kinman Guitar Electrix introduced replacement pickups for Stratocaster and Telecaster guitars. These were of the stacked humbucker design, where the lower pickup coil functions solely to cancel hum.[11]
See also[edit]
References[edit]
![Polarity Polarity](https://www.fralinpickups.com/wp-content/uploads/2017/01/Magnetic-Orientation-1-768x357.png)
- ^Mix, June 17, 2005. 'Al Kahn (1906–2005)'. Retrieved on August 24, 2009.
- ^https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/USRE20070.pdf
- ^Kendall Ford 'A Home-Made String-Music Pickup' Radio Craft Magazine, April 1939 p.601,624-5.
- ^'Patent US2896491 - Magnetic pickup for stringed musical instrument'. google.com.
- ^'Patent US2892371 - Pickup'. google.com.
- ^'Seth Lover interview 1978 vintage Gibson PAF humbucking humbucker pickups guitars'. Provide.net. Retrieved 2013-05-02.
- ^Lawrence, Robb. The Early Years of the Les Paul Legacy 1915-1963. Hal Leonrd Corp. p. 107.
- ^By Rich Kienzie, 'Riffs, Amps, and Butts', Guitar Player magazine, March 1990, P.14
- ^Joseph Raymond Butts, U.S. Patent 2892371, Issued 6-22-1959
- ^'Archived copy'. Archived from the original on 2013-11-24. Retrieved 2013-12-02.CS1 maint: Archived copy as title (link)
- ^'Humbucker Pickups'. Kinman. Archived from the original on 2013-07-05. Retrieved 2013-05-02.
- US patent 2896491, Seth Lover, 'Magnetic pickup for stringed musical instrument', issued 1959-07-28
External links[edit]
Wikimedia Commons has media related to Humbucker. |