alt Hacker NewsThat voltage from the crystal creates an electric field, which exerts a force on free charges in the air. (There's always some free charges floating around, like electrons and ions.) These charges accelerate and collide with air molecules, kicking loose more free electrons that also accelerate and collide, and so on, in an exponential chain reaction. We call this an electron avalanche. And guess what happens? Accelerating charges create a changing electric field—which, yup, creates a changing magnetic field, etc., and that disturbance radiates outward as an electromagnetic wave.
Huh? It has nothing to do with "free charges in the air." That's... kind of the whole idea behind EM theory.
That said, I've never heard of anyone building a detector out of balls of aluminum foil, so that's pretty cool. I'd classify it as a rectifying detector rather than a coherer, though, because nothing is physically moving. A real coherer had to be physically bumped or tapped between received signals.
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Yeah, this write-up seems to be confusing a townsend avalanche (what it describes) with far field electromagnetic wave propagation (an entirely different thing).
This coherer also must be bumped between received sigs.
I suppose what he is saying is that the crystal creates an electric current in some conductor. That conductor then has an electric field around it. If you had some charged particle nearby there could be a force exerted on it. That author then says "charges accelerate and collide with air molecules". Strictly speaking this is true.
But this description sounds more like a particle accelerator and less like anything to do with radio waves. All of this stuff is irrelevant. As you've pointed out, you could just as easily put the crystal in a vacuum and get RF emissions from it.