History
The history of coils is inseparable from the discovery of electromagnetism. Once experimenters learned that current in a wire produces a magnetic field, winding the wire into many turns became an obvious way to strengthen and shape that field.
- 1820 — Oersted links current and magnetism. Hans Christian Oersted observed that current in a wire deflected a nearby compass needle, showing that electricity and magnetism were connected.
- 1820s — Early electromagnets. William Sturgeon demonstrated an iron-core electromagnet. Coiling current around iron concentrated the magnetic field and produced a useful lifting force.
- 1827–1831 — Joseph Henry improves coil construction. Henry insulated the conducting wire so many close turns and layers could be wound on an iron core. This greatly increased magnetic strength and allowed experimentation with long series-connected and multiple parallel windings.
- 1831 — Electromagnetic induction. Michael Faraday and Joseph Henry independently discovered that a changing magnetic field can induce electrical current. Faraday published first. This principle became the basis of transformers, generators, inductors, and much of electrical engineering.
- Telegraph and relay era. Electromagnetic coils became central to telegraph sounders, relays, bells, signaling equipment, and early electrical machinery.
- Radio era. Air-core and iron-core coils were used for tuning, coupling, audio transformers, RF chokes, and filters. Adjustable coils became familiar alignment components in receivers and transmitters.
- Ferrites and higher frequencies. Ferrite magnetic materials made practical cores with low eddy-current loss at frequencies far above those suitable for ordinary laminated iron. Ferrites became important in radio, television, EMI suppression, and switching power supplies.
- Switch-mode power conversion. As transistors and integrated circuits allowed power supplies to switch at tens of kilohertz and later into the megahertz region, magnetic components became much smaller. Power-inductor design increasingly emphasized core saturation, AC copper loss, shielding, and thermal behavior.
- Modern surface-mount magnetics. Today inductors may be wirewound, multilayer, molded, thin-film, or embedded in modules. Despite the packaging changes, they still rely on the same relationship between current and magnetic field discovered two centuries ago.
The henry
The SI unit of inductance is named for Joseph Henry. The unit recognizes his important work on electromagnets and electromagnetic induction.
From large coils to tiny components
Early coils were hand-wound and often physically large because insulation, magnetic materials, and operating frequencies limited what was possible. Modern magnetic powders, ferrites, enamel wire, multilayer fabrication, precision winding, and automated assembly allow inductors with microhenry and nanohenry values to occupy only a few square millimeters.