Pendulum Toy; E. J. Lias, 1968
This is a reconfiguration of the venerable Twin-elliptic Pendulum Harmonograph which was popular during the Victorian Era (1850-1890). Arranging the drawing surface in the middle of the pendulum apparatus is not a new idea and this design allows for longer pendulums while keeping the pen and paper easily accessible for the user.
This design creates a few complications that must be overcome which the classic “Top Hamper” Harmonograph (which I prefer) doesn’t encounter. Care must be taken so that the pendulum rod and the pen arm & swivel design is robust enough to withstand the torque imparted from the gyrations of the Harmonograph. Starting the device into motion and then lowering the pen onto the paper is problematic. (Goold's Twin-elliptic Pendulum Harmonograph from the early 1900's is probably a superior design; it has the paper tray on the pendulum rod in a stirrup arrangement and the pen arm on a separate stand - the reverse of this idea...)
In Fig. 4 there is a clever bob placed at the top of the Harmonograph which is designed to impart additional motions to the pendulums. I don’t recommend going there unless you are a masochist… (Remember K.I.S.S.; Keep it Simple, Stupid!). Getting this bob adjusted correctly to move in "tune" with the rest of the apparatus would be very difficult.
Placing the drawing surface in the center (rather than at the top or bottom of the apparatus) is refined in subsequent patents. An adaptation of this design is recommended for the hobbyist or science fair aspirant. The basic Twin-elliptic Pendulum is one of the simpler Harmonograph designs.Click on an image to view full size -