Great question — answering as the UphillTech team:
In the case of a Class II power supply (wall wart) -- there is no power switch, and the equipment frankly doesn't care which way it's plugged in as it treats the mains as if it were floating and all the user-touchable parts are well-insulated from the mains. For other equipment, a polarized plug is used that can only go into its mating mains receptacle one way around. This means that the appliance designer can safely assume that the hot wire is hot and the neutral wire is neutral, instead of having to design defensively against being plugged in the other way around. Since your wall sockets are old, crusty, and not polarized, though -- you'll have to have them replaced by polarized sockets...
Full answer by ThreePhaseEel: https://diy.stackexchange.com/a/108013 (CC BY-SA 3.0).
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