|Other names: helical tube, spring, toroidal helicoid.
|Cartesian parametrization: , where b is the radius of the tube, a the distance from the central helix to the axis, and 2ph the shift of this helix and therefore h/a its slope.
h > 0: right-handed coil, h = 0: torus,
< 0: left-handed coil.
The coils are the tubes the bore of which are circular helices.
They are the helicoids the directrix of which is a circle perpendicular to the speed vector of the helical motion; see at circled helicoid.
© Robert FERRÉOL