By Kim Lewis
Updated Mar 24, 2022
A solenoid is an electromagnet made by winding a conductive wire into a helical coil that carries electric current. The resulting magnetic field can be intensified by inserting a ferromagnetic core such as iron. When the solenoid is reshaped into a closed loop, the structure is called a toroid—a doughnut‑shaped coil that concentrates its field inside.
A toroid confines its magnetic flux entirely within the core. The internal field follows concentric circles, while the external field is effectively zero, reducing interference with nearby circuitry. Field strength is proportional to the number of turns and inversely related to the radial distance: the flux is stronger near the inner radius and diminishes toward the outer rim.
Toroids serve as inductors, exploiting Faraday’s Law of Induction discovered by Michael Faraday in 1831. A changing current induces a voltage in adjacent coils, and a toroid’s self‑inductance resists changes in its own current. The magnitude of this self‑inductance depends on the coil turns, core material, and applied AC source.
By winding two or more toroidal coils around a common ferrite or silicon‑steel core, manufacturers create toroidal transformers. These devices excel in RF applications, where they step voltage up or down, isolate sections of a circuit, and perform impedance matching to connect components with differing impedances.
While winding a toroid can be more labor‑intensive than a straight solenoid, the benefits are substantial: fewer turns are required for a given inductance, enabling compact, high‑efficiency designs. The enclosed magnetic field also prevents stray inductive coupling, making toroids ideal for densely packed electronic assemblies.
Toroid coils are ubiquitous across modern technology: from telecommunications and medical imaging to audio amplification and LED ballasts. In fusion research, tokamaks employ toroidal magnetic fields to confine high‑temperature plasma, enabling controlled nuclear fusion experiments.
