Electromagnetic coil

An electromagnetic coil (or simply a "coil") is formed when a conductor (usually a solid copper wire) is wound around a core or form to create an inductor or electromagnet. One loop of wire is usually referred to as a "turn", and a coil consists of one or more turns. For use in an electronic circuit, electrical connection terminals called taps are often connected to a coil. Coils are often coated with varnish and/or wrapped with insulating tape to provide additional insulation and secure them in place. A completed coil assembly with taps etc. is often called a "winding". A transformer is an electromagnetic device that has a "primary winding" and a "secondary winding" that transfers energy from one electrical circuit to another by magnetic coupling without moving parts. The term "tickler coil" usually refers to a third coil placed in relation to a primary coil and secondary coil. A coil tap is a wiring feature found on some electrical transformers, inductors and coil pickups, all of which are sets of wire coils. The coil tap(s) are points in a wire coil where a conductive patch has been exposed (usually on a loop of wire that extends out of the main coil body). As self induction is larger for larger coil diameter the current in a thick wire tries to flow on the inside. The ideal use of copper is achieved by foils. Sometimes this means that a spiral is a better alternative. Multilayer coils have the problem of interlayer capacitance, so when multiple layers are needed the shape needs to be radically changed to a short coil with many layers so that the voltage between consecutive layers is smaller (making them more spiral like).

Analysis

The inductance of single-layer coils can be calculated to a reasonable degree of accuracy with the simplified formula
$mathrm\{mu\; H\}=\; frac\{R^2\; N^2\}\{9\; R\; +\; 10\; L\}$
where µH (microhenries) are units of inductance, "R" is the coil radius (measured in inches to the center of the conductor), "N" is the number of turns, and "L" is the length of the coil in inches. The online [http://www.66pacific.com/calculators/coil_calc.aspx Coil Inductance Calculator] calculates the inductance of any coil using this formula. Higher accuracy estimates of coil inductance require calculations of considerably greater complexity. A layperson's translation is:
$,\; Inductance^\{microhenries\}\; =\; (radius^2*number\; of\; turns^2)/(9*radius\; +\; 10*length)$
In calculating the distances, one centimeter is equal to 0.393700787 inches and one inch is equal to 2.54 centimeters. The inductance formula uses inches. The relationship between the radius and the circumference of a coil is $,\; r\; =\; c\; /2\; pi$, with "r" as the radius, "c" as the circumference, and π (the Greek letter pi) as the constant 3.141. The circumference of a coil can be calculated by $c\; =\; pi\; cdot\; d$, with "d" as the diameter of the coil and π as 3.141.

Coil examples

Some common electromagnetic coils include:
* A "bifilar coil" is a coil that employs two parallel windings.
* A "Barker coil" is used in low field NMR imaging.
* A "Balun" is set of transformer coils for transmission lines.
* A "Braunbeck coil" is used in geomagnetic research.
* A "degaussing coil" is used in the process of removing permanent magnetism (magnetic hysteresis) from an object.
* A "choke coil" (or "choking coil") is low-resistance inductor used to block alternating current while passing direct current.
* A Flat coil is used in thin electric motor.
* A "Garrett coil" is used in metal detectors.
* A "Helmholtz coil" is a device for producing a region of nearly uniform magnetic field.
* A "hybrid coil" (or "bridge transformer") is a single transformer that effectively has three windings.
* An "induction coil" (or "ignition coil") is an electrical device in common use as the ignition system ("ignition coil or spark coil") of internal-combustion engines.
* A "loading coil" is, in electronics, a coil (inductor) inserted in a circuit to increase its inductance. Archaically called "Pupin coils".
* A "multiple coil magnet" is an electromagnet that has several coils of wire connected in parallel.
* A "Maxwell coil" is a device for producing almost a constant magnetic field.
*A Micro coil use in security devices.
* A "Oudin coil" is a disruptive discharge coil.
* The "polyphase coils" are connected together in a polyphase system such as a generator or motor.
* A "relay coil" is the copper winding part of a relay that produces a magnetic field that actuates the mechanism.
* A "Repeating coil" is a voice-frequency transformer.
* A "Rogowski coil" is an electrical device for measuring alternating current.
* A "single coil" is a type of pickup for the electric guitar.
* A "solenoid" is a mechanical device, based around a "coil of wire", that usually converts energy into linear motion, however solenoids also come in a rotary motion (normally up to a turn of 90 degrees).
* A "telephone cord" is usually manufactured in a coiled fashion, as to allow maximum length while taking up minimum space when not in use.
* A "Tesla coil" is category of disruptive discharge coils, usually denoting a resonant transformer that generates very high voltages at radio frequencies.
* A "voice coil" which is mounted to the moving cone of a loudspeaker.

Other applications of coils exist in the field of electromagnetic devices. A coilgun is a type of cannon that uses a series of electromagnetic coils to accelerate a magnetic shell to very high velocities. The filament of an incandescent light bulb has usually the shape of a coiled coil, in order to fit the long filament in a small space.

Further reading

* Querfurth, William, "Coil winding; a description of coil winding procedures, winding machines and associated equipment for the electronic industry" (2d ed.). Chicago, G. Stevens Mfg. Co., 1958.
* Weymouth, F. Marten, "Drum armatures and commutators (theory and practice) : a complete treatise on the theory and construction of drum winding, and of commutators for closed-coil armatures, together with a full résumé of some of the principal points involved in their design; and an exposition of armature reactions and sparking". London, "The Electrician" Printing and Publishing Co., 1893.
* "Coil winding proceedings". International Coil Winding Association.
* Chandler, R. H., "Coil coating review, 1970-76". Braintree, R. H. Chandler Ltd, 1977.

;External articles
* R. Clarke, " [http://www.ee.surrey.ac.uk/Workshop/advice/coils/ Producing wound components] ". Surrey.ac.uk, 2005 October 9.