Electromagnetic coil
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 electric motors, generators, 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 the 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. 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. 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. A winding that has a single tap in the center of its length is called center-tapped. 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. A time-varying current through one winding will create a time-varying magnetic field that passes through the other winding, which will induce a time-varying voltage  in the other windings. This is called a transformer. The winding to which current is applied, which creates the magnetic field, is called the "primary winding". The other windings are called "secondary windings". Magnetic core. Many electromagnetic coils have a magnetic core, a piece of ferromagnetic material like iron in the center to increase the magnetic field. The current through the coil magnetizes the iron, and the field of the magnetized material adds to the field produced by the wire. This is called a ferromagnetic-core or iron-core coil. A ferromagnetic core can increase the magnetic field and inductance  of a coil by hundreds or thousands of times over what it would be without the core. A ferrite core coil is a variety of coil with a core made of ferrite, a ferrimagnetic ceramic compound. Ferrite coils have lower core losses at high frequencies. A coil without a ferromagnetic core is called an air-core coil. This includes coils wound on plastic or other nonmagnetic forms, as well as coils which actually have empty air space inside their windings. Types of coils. Coils can be classified by the frequency of the current they are designed to operate with: Coils  can be classified by their function: Electromagnets. Electromagnets are coils that generate a magnetic field for some external use, often to exert a mechanical force on something. A few specific types: Inductors. Inductors 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 elements in electrical circuits, to temporarily store energy or resist changes in current. A few types: Transformers. 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: Electric machines. Electric machines such as motors and 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"), 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. These are coils used to translate time-varying magnetic fields to electric signals, and vice versa. A few types: There are also types of coil which don't fit into these categories.