In geometry a conoid (from Greek κωνος  'cone' and -ειδης  'similar') is a ruled surface, whose rulings (lines) fulfill the additional conditions:

Right circular conoid:
  Directrix is a circle
  Axis is perpendicular to the   directrix plane
(1) All rulings are parallel to a plane, the directrix plane.
(2) All rulings intersect a fixed line, the axis.

The conoid is a right conoid if its axis is perpendicular to its directrix plane. Hence all rulings are perpendicular to the axis.

Because of (1) any conoid is a Catalan surface and can be represented parametrically by

Any curve x(u0,v) with fixed parameter u = u0 is a ruling, c(u) describes the directrix and the vectors r(u) are all parallel to the directrix plane. The planarity of the vectors r(u) can be represented by

.

If the directrix is a circle, the conoid is called a circular conoid.

The term conoid was already used by Archimedes in his treatise On Conoids and Spheroides.

Examples

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Right circular conoid

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The parametric representation

 
describes a right circular conoid with the unit circle of the x-y-plane as directrix and a directrix plane, which is parallel to the y--z-plane. Its axis is the line  

Special features:

  1. The intersection with a horizontal plane is an ellipse.
  2.   is an implicit representation. Hence the right circular conoid is a surface of degree 4.
  3. Kepler's rule gives for a right circular conoid with radius   and height   the exact volume:  .

The implicit representation is fulfilled by the points of the line  , too. For these points there exist no tangent planes. Such points are called singular.

Parabolic conoid

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parabolic conoid: directrix is a parabola

The parametric representation

 
 

describes a parabolic conoid with the equation  . The conoid has a parabola as directrix, the y-axis as axis and a plane parallel to the x-z-plane as directrix plane. It is used by architects as roof surface (s. below).

The parabolic conoid has no singular points.

Further examples

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  1. hyperbolic paraboloid
  2. Plücker conoid
  3. Whitney Umbrella
  4. helicoid

Applications

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conoid in architecture
 
conoids in architecture

Mathematics

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There are a lot of conoids with singular points, which are investigated in algebraic geometry.

Architecture

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Like other ruled surfaces conoids are of high interest with architects, because they can be built using beams or bars. Right conoids can be manufactured easily: one threads bars onto an axis such that they can be rotated around this axis, only. Afterwards one deflects the bars by a directrix and generates a conoid (s. parabolic conoid).

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  • mathworld: Plücker conoid
  • "Conoid", Encyclopedia of Mathematics, EMS Press, 2001 [1994]

References

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  • A. Gray, E. Abbena, S. Salamon, Modern differential geometry of curves and surfaces with Mathematica, 3rd ed. Boca Raton, FL:CRC Press, 2006. [1] (ISBN 978-1-58488-448-4)
  • Vladimir Y. Rovenskii, Geometry of curves and surfaces with MAPLE [2] (ISBN 978-0-8176-4074-3)