Ever since the first patent for this type of instrument was issued to an American, Mr. Hayen (U.S. Pat. No. 3,947,968) in 1874, special publications of this nature have appeared time and again, and a search of international Patent Gazettes issued during the last decade or so reveals that available ellipsograph designs fall into five categories:
(1) The string type utilises the principle that the sum of the distances of any point on the ellipse to two focal points is constant.
(2) The projection image copying type is designed on the principle that all kinds of ellipses can generally be drawn on paper along the edge of the shadow mimicked by a circular board model inclined at various degrees in space. This category may be typified by U.S. Pat. No. 4,194,292.
(3) The obliquely cut cylinder type is prepared by ensleeving another leg on one of the legs of the dividers, enabling it to rotate and slide up and down with the drawing pen, forming an adjustable included angle to it. This is typified by U.S. Pat. No. 4,204,327. Ellipsographs designed according to the above three principles cannot describe ellipses that are very small, nor those that are fairly elongated.
(4) The planetary gear type is typified by the ellipsograph disclosed in U.S. Pat. No. 4,226,022. It can hardly overcome the crawling which causes lines drawn in this way to have a saw-tooth appearance.
(5) The cross groove type includes the majority of ellipsographs designed like those disclosed in DT2743352, SU454139, and BG1754588. This type has the advantage of being able to draw elongated ellipses while its defects include the following: it is difficult to ensure the matching precision of the slide groove and the coincidence of the elliptical line drawn in opposite directions; its grooved frame tends to be clumsy; it requires that the unevenness of the paper surface