Coils

Coils are structures formed by winding a flexible conductor or a length of metal into a helical shape. In electrical engineering, a coil refers to a wound wire that can function as an inductor or transformer winding. In mechanical engineering, the term commonly denotes a coil spring, a helical element used to store and release energy or absorb shocks.

Electrical coils produce magnetic fields and store energy. The inductance, L, depends on the number of turns N, the cross-sectional area A, the length of the coil l, and the magnetic permeability μ of the core. For an ideal long solenoid with a nonmagnetic core, L ≈ μ0 N^2 A / l. Coils may be air‑cored or mounted on ferromagnetic cores or ferrite materials to increase inductance. Energy stored is E = 1/2 L I^2. Coils are essential in transformers, inductors, chokes, electromagnets, and RF components; they can be arranged as solenoids, toroids, or laminated cores to reduce eddy currents.

Fabrication and performance depend on wire gauge, insulation, turn count, and core material. Copper resistance causes power loss and heating; core material causes hysteresis and eddy losses at high frequencies. Care is taken to avoid overheating and to manage parasitic capacitance in high-frequency applications.

Mechanical coils, or coil springs, come in compression, tension, and torsion varieties. Made from steel alloys or other metals, they are shaped as helices to provide stiffness and energy storage, used in suspension systems, cushions, and mechanical transmitters. Their stiffness is described by the spring constant, k, and design must consider fatigue, load limits, and operating environment.

Coils are foundational components across electronics and mechanics, with a long history dating to early electromagnets and spring technologies.