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, roe, 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.
• Coils are also used electrically.

Electromagnetic coils

Main: Electromagnetic coil

Analysis

The inductance of single-layer air-cored 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: $\backslash ,\; Inductance^\{microhenries\}\; =\; (radius^2*number\; of\; turns^2)/(9*radius\; +\; 10*length)$ Note that if the coil has a ferrite core, or one made of another metallic material, it's inductance cannot be calculated with this formula.
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