A coil is a series of loops. A coiled coil is a structure where the coil itself is in turn also looping.

General applications

A coil is made up of materials, usually rigid, which can be fashioned into a spiral or helical shape. Flexible materials like wire, rope, hose, cable or paper can also be coiled into empty loops, or wound around a central drum or spindle.

Some common applications of coils include:

• A coil spring is the most common type of spring.
• A spiral staircase, a stairway fashioned in a coil shape.
• A Slinky is a coil-shaped toy.
• Evaporator coils are used in air conditioning and other refrigeration cycles.
• A boiler coil is an element in a water heater.
• An Alpine coil, one of several coil knots, is a method for carrying a rope.
• Quilling coils use shaped paper to create artistic designs.

Electromagnetic coils

Main: Electromagnetic coil

Analysis

The inductance of single-layer coils can be calculated to a reasonable degree of accuracy with the simplified formula

$\backslash mathrm\{\backslash mu\; H\}=\; \backslash frac\{R^2\; N^2\}\{9\; R\; +\; 10\; L\}$ 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 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: $\backslash ,\; Inductance^\{microhenries\}\; =\; (radius^2*number\; of\; turns^2)/(9*radius\; +\; 10*length)$ 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 $\backslash ,\; r\; =\; c\; /2\; \backslash pi$, 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 $c\; =\; \backslash pi\; \backslash cdot\; d$, with d as the diameter of the coil and π as 3.141.

Coil examples

• 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.