Torispherical heads are up of parts of two surfaces. The center portion is called the crown or dish and is a spherical segment, the transition portion between the dish and the cylinder is called the knuckle and is a portion of a torus or doughnut. They are also called "F&D″ or "flanged and dished″.
Toroidal transformers are built around a ring-shaped core, which, depending on operating frequency. It is made from a long strip of silicon steel or permalloy wound into a coil, powdered iron, or ferrite. A strip construction ensures that the grain boundaries are optimally aligned, improving the transformer's efficiency by reducing the core's reluctance. The closed ring shape eliminates air gaps inherent in the construction of an E-I core. The cross-section of the ring is usually square or rectangular. The primary and secondary coils are often wound concentrically to cover the entire surface of the core. This minimises the length of wire needed, and also provides screening to minimize the core's magnetic field from generating electromagnetic interference.