I've wanted for a long time to build a proper rotary harmonograph. One with gimbals and everything, pretty much like this one here (which is pure awesome), but it's a project that requires some time, some tools and some money, so it's been on the back burner for well over a year. I figured I needed to stop waiting and just make something that although not fabulous, would at least amaze the kids and not cost us anything.
I decided to opt for a solution based on this set up here. I didn't have a step ladder to use though, so I hung our dumbells from a beam in the garage. I just made a simple set up that was evenly weighted for the kids to try out. You can get different patterns if you offset the weights.
From one side of the dumbells I taped a paint stirrer with some pvc plumbing bits that would loosely hold a big fat marker pen, and on the other side of the dumbell I taped a metal ruler that had a couple of hacksawed bits of random tubing that would hold a regular sized felt tip pen. I figured this way the kids could experiment with different pens. You have to have something that doesn't grip the pen tightly because it needs to be able to move up and down freely in order to maintain contact with the paper.
This set up stayed in the garage for about a month with various sets of kids having a go at it and coming up with different works of art. The pattern that it formed was quickly referred to as "the watermellon pattern".
I do want to do this again with a better set up, but that's just the geek in me wanting to create a set up where you can fully and easily vary all the parameters, so that the kids can explore what effects those changes have. I'd like to try making one where the pen is static and the table moves before we try a set up where both do. I wonder if I'll ever get around to it. I have the attension span of a goldfish it seems!
The kids really did seem impressed with the set up and seemed fascinated by the rather hypnotic motion of the pendulum as it drew. They also loved the word "harmonograph" rather a lot too. I guess it sounds sciency. I like the idea of introducing "science concepts" in ways that pique their curiosity. So I guess this was an introduction to simple harmonic motion and pendulums without any mathamatics or pressure to do anything other than have a bit of fun.
If you want to involve some splatty paint to make pendulum art then check out Teacher Tom's pendulum painting art attack.