Jim MacArthur built this incredible Turing machine using wood and scrap metal, which he demonstrated at MakerFaire UK in Newcastle. It is entirely mechanical, except for the electric motor used to drive it. He writes:

This is a mechanical universal Turing machine (given an infinite track). It uses ball bearings as its memory and has no electrical components, other than a small motor used to drive it. This is a quick overview video filmed at Maker Faire UK 2011.

more from New Scientist:

The machine is a close physical model of the theoretical Turing machine – a device first described by Alan Turing in 1937 as a thought experiment to understand the limits of mechanical computation. According to the theory, the machine performs calculations using a set of rules to manipulate symbols on an infinite strip of tape.

Instead of using tape, this machine’s memory uses ball bearings placed on a steel grid. A ball can represent one of five different symbols based on its position on the grid. The machine reads and writes data by repositioning the balls into different cells. It does this by moving along the grid, lifting ball bearings with magnets and then depositing them into a new position based on a set of rules.

A true Turing machine requires an infinite track or tape to run on but according to MacArthur, his machine is as close as you can get to a physical replica. It has no practical computing applications and would take months to add a few numbers together but MacArthur says it was fun to build. “Since you can see this computer working, it could be useful for educational purposes,” he says.

Beautiful work, Jim!