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PAGE 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 in a doughnut shape (usually with a core
of iron or similar metal) 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 windings. The magnetic flux in a toroid is confined to the core, preventing its energy from being absorbed by nearby objects.
An example of a mathematical toroid