Toroid

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An example of a mathematical toroid

An example of a toroidal RF suppression inductor

A toroid is a doughnut-shaped object whose surface is a torus. Its annular shape is generated by revolving a circle around an axis external to the circle.

A coil of insulated wire (usually around a core of iron or similar metal) in a doughnut or pretzel shape is an example of a toroidal object. These are used as inductors in circuits such as low frequency transmitters and receivers because they possess higher inductance and carry greater current than similarly constructed solenoids. They are also used as transformers in main power supplies. Toroidal coils reduce resistance, due to the larger diameter and smaller number of turns in the winding. The magnetic flux in a toroid is confined to the core, preventing its energy from being absorbed by nearby objects.

In the geometry of torus-shaped magnetic fields, the poloidal flux direction threads the "donut hole" in the center of the torus, while the toroidal flux direction is parallel the core of the torus.

In fluid mechanics, a toroid-shaped flow is called a vortex ring.