A Strange Change of Rotation

Alex Bellos discusses how the Incans used knots in string (Quipu) to record numbers.Check out Brilliant (get 20% off their premium service): https://brillian...

Rotating cedar balls.

Here's a playlist of Tadashi Tokieda videos: http://bit.ly/tadashi_vids

Electric slide rule: https://youtu.be/MEyIppEOQTw