Toroidal transformers (toroids) are built around a circular core. A primary and secondary coil is wound around the ring shaped core, which is typically made out of silicon steel, perm alloy, or a sintered ferrite. The coils are wound concentrically to cover the surface of the core.
Toroidal Transformers are more efficient than E-type transformers for similar power levels. Toroid transformers are also typically smaller and weigh less than laminated E types. Some of the other advantages include a low exterior magnetic field, low off-load losses, and single-bolt mounting options.