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What idea did Tesla sell?

I'm aware of him working as a scientist for most of his life, but not selling an idea according to the criteria stated above.

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I believe that Tesla sold several of his patents.

Doesn't a (non-software) patent require something resembling a semi-working prototype (at least back when Tesla was getting patents)? I say "semi-working" because there are a lot of patents for things like free-energy devices that never worked, but were built.

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