Thanks to Youtube users Mark Horstmeier and ramosel I now doubt my own explanation in the video. Please read
http://www.physics.princeton.edu/~mcdonald/examples/motor.pdf for a better operational theory.
Ball Bearing Motor
(None of the dimensions is critical. Material doesn't matter as long as it conducts electricity.)
3 M6 stainless Nuts, 1 M6 stainless Bolt (50mm long) , 2 Ball Bearings (OD: 20mm, ID: 6mm, Width:6mm, seals removed, grease removed with brake cleaner), 2 flexible isolated wires (Diameter 1.5mm, Length 20cm) Wires are soft soldered to outer races of the bearings.
The power supply: Old 12V Battery charger with conventional transformer.
Batteries must not be used (current way too high!!) Modern (eg. computer) power supplies are unlikely to work, as they switch off totally when short circuits are made.
Kids! Don't do this at home! Danger of burns, fire and electric shock.
Be sure you have a residual-current circuit breaker (RCCB) installed. Never use mains electricity directly.
Here's an explanation of the thermal theory (which I doubt after reading Kirk T. McDonald's paper):
Please look at this article which also presents different explanations of how it works: https://en.wikipedia.org/wiki/Ball_bearing_motor