The trolley carries a magnet with its north pole facing up. It falls under its own weight from the gate and cannot be touched: the only thing you can do is screw magnets into the brass sockets and let the field steer it.
You have two magnets. Tap a socket to place one; tap it again to flip it over; a third tap removes it.
A tilted table with felt, two walnut crossbars, a tower in the middle and, at the far end, the brass cup. Everything that falls and does not land in the cup goes down the grate. The trolley is never touched: you place a field in front of it and let it fall. Two magnets, and each one can face the side that attracts or the side that repels.
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