The model consists of a rigid frame representing the focal ellipse and the focal hyperbola of a system of confocal quadric surfaces. The plane of the hyperbola is orthogonal to the plane of the ellipse and contains the major axis of the latter. The hyperbola has the foci of the ellipse as vertices and the vertices of the latter as foci. In one vertex of the ellipse and the non-corresponding focus a loop of string is fixed and placed about the focal conic section in a certain way. It can be shown that all the points one receives by drawing taut the string in any point P are located on an ellipsoid.