Meccano Differential Analyzer

You can still buy buckets/toolboxes of assorted Meccano. They come with plans for things which you can happily ignore.

https://www.amazon.com/Meccano-Junior-100-Piece-Bucket/dp/B0...

https://www.amazon.com/Meccano-20070926-Junior-Bucket-150-Pi...

https://www.amazon.com/Erector-Advanced-Toolbox-Construction...

https://www.amazon.com/Meccano-6024795-Meccano-Erector-Advan...

(Results of a quick Amazon search. I'm sure there are more.)

"The simplest design consists of a pair of contra-rotating drums mounted on collinear input and output shafts. A pair of cords or belts wrapped around the drums connect two arms mounted on the input shaft and output shaft."

From http://www.meccano.us/differential_analyzers/robinson_da/tor....

There's a link to this page http://www.meccano.us/differential_analyzers/robinson_da/tor... with nice close-up photos.

I was just as interested so I looked it up a little. The main design works with a large drum as a power source. The "control rope" is wrapped around a drum and connected to both the input and output. When the input applies torque, it pulls the slack out of the windings and pulls the rope tight against the drum. This frictional force applies torque to the output proportional to the input torque. The wikipedia article is pretty nice: https://en.wikipedia.org/wiki/Torque_amplifier

Once you understand that, the Meccano version makes a little more sense. In this version, he experiments with several arrangements of the same principal. To use it in both directions, you need a drum and windings for each direction.