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here is a more detailed video of the experiment - you need 20 000 volts for it to happen - it is likely that magnetic fields are attracting the balls, although the paths they take are still under investigation (unfortunately Professor Alfred Hübler passed away in 2018).
It *is* possible to try it at home if you can get a 20 kV supply and have the space and knowledge to do the experiment safely 😉
https://www.youtube.com/watch?v=PeHWqr9dz3c