
torus (anchor ring)


A closed curved surface that is donutshaped with a hole in the middle. It can be formed by rotating a circle about an axis that lies in the same plane as the circle but does not cut it. A crosssection of the torus in a plane perpendicular to the axis is two concentric circles. A crosssection in any plane that contains the axis is a pair of congruent circles at equal distances on both sides of the axis.
The volume of the torus is 2^{2}r^{2}d, and its surface area is 4^{2}rd, where r is the radius of the generating circle, and d is the distance of its center from the axis.

