Sculptor Chris Wallace and I worked together on this Mousetrap-style mechanical chain reaction machine for Edinburgh International Science Festival. It was part of a chemistry exhibition/workshop called Atomise, which ran in April and May 2011.
There's a higher-resolution version of this video here.
The part of the machine that I am most pleased with is the ball feeder that appears between 2:22 and 2:32 in the video. We wanted to pace the feeding of the balls into the pegboard rather than simply tipping them all in at once. The simplest timing device that we could think of was a pendulum; the challenge was to build a ball feeder that (a) didn't sap energy from the pendulum too fast, and (b) was tolerant to the declining amplitude of the pendulum. I filled tens of pages of notebook, and built several prototypes, before I came up with the "multi-storey car park" idea. I felt even happier when I realised how easily it could go together using mainly Corriflute board, thin rods, and elastic cord.
The machine also featured some chemistry word-play, one item of which is the final act of the machine. The other items aren't visible on the video, so I reproduce them below, with some clues. If you don't get them, right-click on the images and look at the information given there.
Vanadium (V) oxide, platinum, iron, amylase…
6 × 1023
You'll need A-level chemistry to work out what this rather isolated example of the genus Pyrus is doing here.