Editing Electromagnetic coil (section)

===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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