What Is a Coil?

A coil is a circuit component designed by winding a conductive wire side-by-side or in multiple stacked layers. This component is also known as an inductor, formed by winding insulated copper wire onto a spool, reel, or bobbin (also referred to as a coil carcass or frame). Each full loop of wire on the reel is called a turn or winding. In coil manufacturing, enameled (varnish-coated), insulated copper wire is commonly preferred to ensure electrical isolation and reliable inductive performance. Coils play a key role in regulating and stabilizing electrical current in both AC and DC circuits. All essential details about coils can be found in the rest of this article.

What Does a Coil Do?
The primary purpose of a coil is to store electrical energy in the form of a magnetic field and transfer it to the circuit in a controlled manner. This process helps regulate current flow, stabilize fluctuations, and optimize energy delivery for ideal circuit operation. Coils can also increase voltage in transformer-based systems and provide filtering and noise suppression in signal lines.

How Does an Inductor Work?
The working principle of an inductor is based on electromagnetic fundamentals. Any conductor carrying current generates a magnetic field. The coil’s core function is to oppose rapid current changes, and this opposition force is called inductance (L), measured in Henry (H), with the coil symbol represented by L.

In AC circuits, the current direction constantly changes, producing an alternating magnetic field around the coil. This magnetic field increases the coil’s impedance (AC resistance), strengthening opposition to AC current flow.

In DC circuits, current direction is constant, so the coil does not generate increasing impedance. The only resistance present is the ohmic resistance caused by the metal conductor and winding material.

How Is a Coil Measured?
Coil measurement requires inductance-specific instruments:

L meter / LCR meter: Used to measure inductance value, capacitance, and resistance of a coil.

Avometer (multimeter): Used to detect wire breaks or discontinuity in coil windings.

Common Coil Types
Coils are categorized based on core material and application:

Air-Core Coils: Preferred for high-frequency circuits. Their inductance values can change due to physical impact, so they are often coated with silicone-like compounds for protection.

Ferrite-Core Coils: The core includes materials such as nickel, iron, cobalt, copper, and aluminum additives. They increase inductance and reduce leakage capacitance.

Iron-Core Coils (Choke Coils): Used in filtering and audio frequency circuits.

Laminated Steel-Core Coils: Reduce eddy current effects in transformers, ballast, AC motors, contactors, and inductive loads.

Movable-Core Coils: Inductance value changes when the core moves.

Winding-Adjusted Coils (Variometer / Varyometre): Coil value can be adjusted using a sliding contact (wiper / tap).

Tapped / Stepped Coils: Used to generate multiple inductance values via a multi-position switch.

Coil Symbol & Unit

Coil unit: Henry (H)

Coil symbol: L

Magnetic field formula: B = µ × n × I / l (Note: The original formula you wrote was partially incomplete; the correct and globally accepted inductor magnetic field expression includes current (I).)

Where:

Symbol    Meaning
B    Magnetic Field
µ    Magnetic Permeability
n    Number of turns per unit length
l    Coil length
I    Current passing through the coil

Coil Pricing
Coil prices vary depending on coil type, supplier, and coil quality. You can find coil models suitable for all budgets at Empa Store, which serves the electronics industry with a broad product range. You can review coil options on Empa Store’s online shop and purchase the most suitable model for your needs and budget.