[tscumkru] Mechanical pi

Turn one knob one revolution and another dial turns exactly pi times.  Build this as a mechanical device so that is it self-evident and understandable that the multiplier really is pi.  Plain gears won't work because only ratios of integers are possible with them, and pi is irrational.

Easier might be to turn a knob of diameter D and cause a linear slider to move the distance pi*D.