Corkhill Ellipsograph Patent Model

This device is a patent model by H. R. Corkhill Jr. for an ellipsograph. Patent 492,142 was granted on February 21, 1893. “Be it known that I, HENRY CORKHILL, Jr., of Rochester, in the county of Monroe and State of New York, have invented a new and useful improvement in Ellipsographs […] The object of my invention is to produce a device for conveniently and accurately tracing ellipses, which possesses certain advantages over similar devices heretofore used” (U.S. Patent application, March 26, 1892). Corkhill, of Rochester NY, appears to hold several other U.S. patents for various devices such as cigarette boxes, machines for making labels, and machines for folding paper tubes. His father, Henry R. Corkhill Sr., also holds several U.S. patents. Henry Jr. was born in approximately 1840 and lists his profession as machinist in census records.

An oval shape, the ellipse is one of the four conic sections, the others being the circle, the parabola and the hyperbola. Ellipses are important curves used in the mathematical sciences. For example, the planets follow elliptical orbits around the sun. Ellipses are required in engineering, architectural, and machine drawings for two main reasons. First, any circle viewed at an angle will appear to be an ellipse. Second, ellipses were common architectural elements, often used in ceilings, staircases, and windows. For both of these reasons, ellipses needed to be rendered accurately in drawings. Several types of drawing devices that produce ellipses, called ellipsographs or elliptographs, were developed and patented in the late 19th and early 20th centuries.

This brass ellipsograph is T-shaped with slots along the vertical and horizontal arms of the instrument. A crank at the top of the model rotates six gears (the first two are 5 cm in diameter, the next four are 4 cm in diameter) that are connected in line beneath the model. The second gear connects to the slider that moves along the horizontal slot. The fourth and sixth smaller gears connect to sliders on the vertical slot of the T, which are locked together by the rectangular plate shown in the image. All three sliders are allowed to be positioned at various distances along brass bars (radii) rotated by the gears below. By rotating the crank, the whole assembly pivots in such a way that the pencil attached at the bottom of the T traces out a small ellipse. By adjusting the position of each of the three slides along its respective radii of rotation, ellipses of different sizes and eccentricities can be produced. The eccentricity of an ellipse is a number between zero and one that describes how far from circular an ellipse is. A circle has eccentricity zero. As an ellipse becomes longer and thinner, the eccentricity approaches one. (The original patent drawing and description can be viewed at Google Patents.)

This model was transferred to the Smithsonian from the U.S. Patent Office in the early 20th century along with several other patent models.

References:

U.S. Patent records, Google Patents.

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