What's a harmonograph, you ask? Well, it's something like a Spirograph for grown-ups. A harmonograph uses a construction with pendulums to draw pretty patterns.
One particular incarnation uses two pendulums of nearly equal length, each about two metres. To the right pendulum, a slowly revolving disk is attached, which is driven by a small electrical motor. The drawing paper is taped onto the disk. To the left pendulum, an arm is attached with the pen at the end. When the pressure and thus the friction of the pen is sufficiently low, the pendulums can keep swinging for tens of minutes.
With the link at the top, you can save and bookmark your creations or send them to your friends. Enjoy!