@RealNeat great thread.
I think it's worth looking into coils and resonant circuits and how they work, since most of these devices implement one.
Copper coils are cheap and easy to DIY and experiment with. They have a long history of use in the form of jewelry (such as the South African copper bracelets, or the neck rings called idzilas which are believed to have spiritual powers, or the Dacian gold bracelets which are coils with 7-10 turns).
I would also mention Michael Persinger's work and his "God helmet".
Michael Persinger, the Koren Helmet, and Neurotheology - a New Brain Science
How understanding the Brain helps us understand Life after Death, The God Helmet, Visions of God, Love, Enlightenment, Déjà Vu, & even Out Of Body Experiences.www.god-helmet.com
On a Danny Roddy podcast Ray mentioned Persinger's "No More Secrets" talk:
And there is also the copper biocircuit.
View attachment 41231View attachment 41232
And this YouTube channel has some interesting experiments:
Yes I've tried the biocircuit before in a very make shift way, I don't remember my results, I'd like to build a proper one! It'll be my project for the week.