Editing Electromagnetic coil (section)

==Types of coils==
Coils can be classified by the [[frequency]] of the current they are designed to operate with:
:*''Direct current'' or ''DC'' coils or electromagnets operate with a steady [[direct current]] in their windings 
:*''Audio-frequency'' or ''AF'' coils, inductors or transformers operate with [[alternating current]]s in the [[audio frequency]] range, less than 20 kHz
:*''Radio-frequency'' or ''RF'' coils, inductors or transformers operate with alternating currents in the [[radio frequency]] range, above 20 kHz

Coils can be classified by their function:

===Electromagnets===
{{main|Electromagnet}}
[[Image:Stator eines Universalmotor.JPG|thumb|[[Field coil]] electromagnet on the [[stator]] of an AC [[universal motor]]. ]]
[[File:Canon AF-10 Date old camera shutter driver coil..jpg|thumb|Canon AF-10 Date old camera shutter driver coil.]]
Electromagnets are coils that generate a [[magnetic field]] for some external use, often to exert a mechanical force on something.<ref name="google10">{{cite book|title=Newnes Dictionary of Electronics|author1=Amos, S.W.|author2=Amos, R.|date=2002|publisher=Elsevier Science|isbn=9780080524054|url=https://books.google.com/books?id=lROa-MpIrucC&pg=PA113|page=113|access-date=2017-01-07}}</ref> or remove existing background fields.<ref>{{cite journal | last1 = Hobson | first1 = P. J. |display-authors=et al | year = 2022 | title =  Bespoke magnetic field design for a magnetically shielded cold atom interferometer | journal = Sci. Rep. | volume = 12 | issue = 1 | pages = 10520 | doi= 10.1038/s41598-022-13979-4 | pmid = 35732872 | pmc = 9217970 | arxiv = 2110.04498 | bibcode = 2022NatSR..1210520H | s2cid = 238583775 }}</ref> A few specific types:
:*[[Solenoid]] - an electromagnet in the form of a straight hollow helix of wire
:*Motor and generator windings - iron core electromagnets on the [[rotor (electric)|rotor]] or [[stator]] of electric motors and generators which act on each other to either turn the shaft (motor) or generate an electric current (generator)
:**[[Field winding]] - an iron-core coil which generates a steady magnetic field to act on the armature winding.
:**[[Armature (electrical engineering)|Armature]] winding - an iron-core coil which is acted on by the magnetic field of the field winding to either create torque (motor) or induce a voltage to produce power (generator)
:*[[Helmholtz coil]], [[Maxwell coil]] - air-core coils which serve to cancel an external magnetic field
:*[[Degaussing]] coil - a coil used to demagnetize parts
:*[[Voice coil]] - a coil used in a moving-coil [[loudspeaker]], suspended between the poles of a magnet. When the [[audio signal]] is passed through the coil, it vibrates, moving the attached speaker cone to create sound waves. The reverse is used in a dynamic [[microphone]], where sound vibrations intercepted by something like a [[Diaphragm (acoustics)|diaphragm]] physically transfer to a voice coil immersed in a magnetic field, and the coil's terminal ends then provide an electric analog of those vibrations.

===Inductors===
{{main|Inductor}}
[[Inductor]]s or reactors are coils which generate a magnetic field which interacts with the coil itself, to induce a back EMF which opposes changes in current through the coil. Inductors are used as [[circuit element]]s in electrical circuits, to temporarily store energy or resist changes in current. A few types:

:*Tank coil - an inductor used in a [[tuned circuit]]
:*[[Choke (electronics)|Choke]] - an inductor used to block high frequency AC while allowing through low frequency AC or DC.
:*[[Loading coil]] - an inductor used to add inductance to an antenna, to make it resonant, or to a cable to prevent distortion of signals.
:*[[Transformer types#Variometer and variocoupler|Variometer]] - an adjustable inductor consisting of two coils in series, an outer stationary coil and a second one inside it which can be rotated so their magnetic axes are in the same direction or opposed.
:*[[Flyback transformer]] - Although called a transformer, this is actually an inductor which serves to store energy in [[switching power supply|switching power supplies]] and horizontal deflection circuits for [[vathode-ray tube|CRT]] televisions and monitors
:*[[Saturable reactor]] - an iron-core inductor used to control AC power by varying the saturation of the core using a DC control voltage in an auxiliary winding.
:*[[Electrical ballast|Inductive ballast]] - an inductor used in [[gas-discharge lamp]] circuits, such as [[fluorescent lamp]]s, to limit the current through the lamp.
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===Transformers===
[[Image:Trafo-innenleben.jpg|thumb|Transformer]]
{{main|Transformer}}

A transformer is a device with two or more magnetically coupled windings (or sections of a single winding). A time varying current in one coil (called the [[primary winding]]) generates a magnetic field which induces a voltage in the other coil (called the [[secondary winding]]). A few types:
:*[[Distribution transformer]] - A transformer in an [[electric power grid]] which transforms the high voltage from the electric [[power line]] to the lower voltage used by utility customers.
:*[[Autotransformer]] - a transformer with only one winding. Different portions of the winding, accessed with taps, act as primary and secondary windings of the transformer.
:*[[Toroidal transformer]] - the core is in the shape of a [[toroid]]. This is a commonly used shape as it decreases the leakage flux, resulting in less electromagnetic interference.
:*[[Induction coil]] or [[trembler coil]] - an early transformer which uses a vibrating interrupter mechanism to break the primary current so it can operate off of DC current. 
:**[[Ignition coil]] - an induction coil used in [[internal combustion engine]]s to create a pulse of high voltage to fire the [[spark plug]] which initiates the fuel burning.
:*[[Balun]] - a transformer which matches a [[balanced transmission line]] to an unbalanced one.
:*[[Bifilar coil]] - a coil wound with two parallel, closely spaced strands. If AC currents are passed through it in the same direction, the [[magnetic flux]]es will add, but if equal currents in opposite directions pass through the windings the opposite fluxes will cancel, resulting in zero flux in the core. So no voltage will be induced in a third winding on the core. These are used in instruments and in devices like [[Ground Fault Interrupter]]s. They are also used in low inductance wirewound resistors for use at RF frequencies.
:*[[Audio transformer]] - A transformer used with [[audio signal]]s. They are used for [[impedance matching]].
:**[[Hybrid coil]] - a specialized audio transformer with 3 windings used in [[telephony]] circuits to convert between [[two-wire circuit|two-wire]] and [[four-wire circuit]]s
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===Electric machines===
[[Electric machine]]s such as [[electric motor|motors]] and [[electric generator|generators]] have one or more windings which interact with moving magnetic fields to convert electrical energy to mechanical energy.  Often a machine will have one winding through which passes most of the power of the machine (the [[Armature (electrical engineering)|"armature"]]), and a second winding which provides the magnetic field of the rotating element ( the "field winding") which may be connected by brushes or slip rings to an external source of electric current. In an [[induction motor]], the "field" winding of the rotor is energized by the slow relative motion between the rotating winding and the rotating magnetic field produced by the stator winding, which induces the necessary exciting current in the rotor.

===Transducer coils===
[[Image:Metal detecting in progress on a Time Team dig.jpg|thumb|upright=0.7|The sensor coil of a [[metal detector]].]]
These are coils used to translate time-varying magnetic fields to electric signals, and vice versa. A few types:
:*Sensor or pickup coils - these are used to detect external time-varying magnetic fields 
:*[[Inductive sensor]] - a coil which senses when a magnet or iron object passes near it 
:*[[Recording head]] - a coil which is used to create a magnetic field to write data to a [[magnetic storage]] medium, such as [[magnetic tape]], or a [[hard disk]]. Conversely it is also used to read the data in the form of changing magnetic fields in the medium.
:*[[Induction heating|Induction heating coil]] - an AC coil used to heat an object by inducing [[eddy current]]s in it, a process called [[induction heating]].
:*[[Loop antenna]] - a coil which serves as a [[antenna (radio)|radio antenna]], to convert radio waves to electric currents. 
:*[[Rogowski coil]] - a toroidal coil used as an AC measuring device
:*[[Pickup (music technology)|Musical instrument pickup]] - a coil used to produce the output [[audio signal]] in an [[electric guitar]] or [[electric bass]].
:*[[Flux-gate magnetometer|Flux gate]] - a sensor coil used in a [[magnetometer]]
:*[[Magnetic cartridge|Magnetic phonograph cartridge]] - a sensor in a [[record player]] that uses a coil to translate vibration of a needle to an audio signal in playing vinyl [[phonograph record]]s.

There are also types of coil which don't fit into these categories.