The Ekdahl Moisturizer is a spring reverb desktop unit which has its springs exposed so you can ‘play’ them…. heres a good demo video:
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They cost US$300 and are available here. It would be interesting to hear it played sparsely/gently with the gain cranked up cleanly… As the video shows, apart from the spring the unit has a built in filter (variable LPF, BPF, HPF) & LFO as well as control voltage inputs, heres a block diagram:
Neat gizmo.
FWIW, cheap & dirty fun: an old salvaged reverb tank all by itself can be used like a passive electric instrument.
Those found in old organs (I have one from a trashed Optigan) can be as much fun as the more serious guitar amp tanks. And when hooked up as a passive instrument, both input and output jacks can be used as outputs.
I’ve just ordered mine. I can’t wait to try bowing it.