They are thinking of exactly the correct machine.

A liquid rheostat is just a rheostat https://en.wikipedia.org/wiki/Liquid_rheostat

They were used in the past just like the light dimmer, but I challenge a present day photo in a developing countries workshop.

Heaps of Youtube spam and 'lets try', I'd like to see one in a real workshop in the wild. (That would be cool to see)