This online interactive harmonograph was inspired by the excellent book by Anthony Ashton.
The drawing is a visual representation of the interaction between two frequencies. Two oscillations are set at right angles to each other. As the two bobs oscillate the pen makes a drawing that is the result of their combined motion. You can change the ratio between the two frequencies and so change the drawing.
The simplest patterns are drawn when the ratio between the frequencies are in harmony, that is the ratio is a simple fraction such as 1:1, 2:1 and 3:2. More complex, but still pleasing drawings are produced when the there is a small departure from perfect harmony. This is near unison.
Harmonic cinema: The moving scene is also a visual representation of the interaction between two frequencies. A camera swings at the end of a pendulum (Bob's view) and the scene moves at right angles to the pendulum.
<Paul St George>