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Sadly the premise that it operates on a thermal principle has been pretty much debunked - it operates instead on electromagnetism, due to the induced EMF when the ball bearing rotates through a magnetic field caused by the current through it. Here’s a paper describing the effect (which has been experimentally verified)... https://news.ycombinator.com/item?id=20415755

I built one of these at university as a research project and it works well (albeit drawing a huge amount of current!). Interestingly enough it works just as well in either direction, depending which way you give it an initial push - the polarity of the DC terminals doesn’t matter at all!

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