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An electromagnetic coil is an electrical conductor such as a wire in the shape of a coil, spiral or helix. Electromagnetic coils are used in electrical engineering, in applications where electric currents interact with magnetic fields, in devices such as inductors, electromagnets, transformers, and sensor coils. Either an electric current is passed through the wire of the coil to generate a magnetic field, or conversely an external ''time-varying'' magnetic field through the interior of the coil generates an EMF (voltage) in the conductor. A current through any conductor creates a circular magnetic field around the conductor due to Ampere's law. The advantage of using the coil shape is that it increases the strength of magnetic field produced by a given current. The magnetic fields generated by the separate turns of wire all pass through the center of the coil and add (superpose) to produce a strong field there.〔 The more turns of wire, the stronger the field produced. Conversely, a ''changing'' external magnetic flux induces a voltage in a conductor such as a wire, due to Faraday's law of induction.〔〔( Newnes 2002, p. 129 )〕 The induced voltage can be increased by winding the wire into a coil, because the field lines intersect the circuit multiple times.〔 The direction of the magnetic field produced by a coil can be determined by the right hand grip rule. If the fingers of the right hand are wrapped around the magnetic core of a coil in the direction of conventional current through the wire, the thumb will point in the direction the magnetic field lines pass through the coil. The end of a magnetic core from which the field lines emerge is defined to be the North pole. There are many different types of coils used in electric and electronic equipment. ==Windings and taps== The wire or conductor which constitutes the coil is called the winding.〔(Stauffer 2005, p. 273 )〕 The hole in the center of the coil is called the core area or ''magnetic axis''. Each loop of wire is called a turn.〔 In windings in which the turns touch, the wire must be insulated with a coating of nonconductive insulation such as plastic or enamel to prevent the current from passing between the wire turns. The winding is often wrapped around a ''coil form'' made of plastic or other material to hold it in place.〔 The ends of the wire are brought out and attached to an external circuit. Windings may have additional electrical connections along their length; these are called taps.〔( Laplante 1999, p. 633 )〕 A winding which has a single tap in the center of its length is called center-tapped.〔(Stauffer 2005, p. 29 )〕 Coils can have more than one winding, insulated electrically from each other. When there are two or more windings around a common magnetic axis, the windings are said to be inductively coupled or magnetically coupled.〔( Newnes 2002, p. 167 )〕 A time-varying current through one winding will create a time-varying magnetic field which passes through the other winding, which will induce a time-varying voltage in the other windings. This is called a transformer.〔( Newnes 2002, p. 326 )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「electromagnetic coil」の詳細全文を読む スポンサード リンク
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