Description and Notes:
This is Model C-2 of the "Elliptical Compass" made by A. E. Randles Co., with US patent No. 2670540, issued on March 2, 1954. It comes with the original cardboard box and instruction manual and charts, a pdf file of which can be downloaded here. A stamp on the instruction sheet indicates that this instrument was sold by Brewer Engineering Co. on April 6, 1968, and the original sale price was $44.50.
The complicated-looking drawing mechanism of this instrument is actually simply a four-bar linkage with two perpendicular sliding pairs on the base frame. The length of the upper and lower portions of the drawing arm can be adjusted according to the size of the ellipse to be drawn, the lower scale specifies the length of the semi-minor axis and the upper scale gives the differences between the two semi axes. So if the upper scale is set to zero then it would draw a circle; whereas a straight line can be drawn by setting the lower scale to zero. The pencil tip is spring loaded for maintaining contact pressure with the drawing paper. The kinematics principle behind which is exactly the same as an elliptic trammel. However in use this device is more user friendly and more stable, thanks to the added crank handle and the sturdy die cast iron frame.