When G. Harry Stine wrote about this gadget in his Mind Machines, he said he was really hesitant to include it because it made no sense to him at all. I'm hesitant because it is so damned simple you might be tempted not to bother with the other gadgets on this site!

Don't let the diagram fool you. All you need is a piece of posterboard folded in half, two squares of foil, some speaker wire and a plug and an earphone plus any audio amplifier, like a small tape recorder.

Glue the pieces of foil to the inside of the folded posterboard so that you will have a sandwich of sorts when it is folded together. Attach the ends of the speaker wire to the foil so one strand is to each sheet. This is then plugged into the microphone input of the amplifier (assuming you are using a tape recorder for your amp.) Plug the earphone into the earphone jack so the earphone can act as the antenna.

To use this, place the witness between the foil plates of the sandwich and then close it, clipping it to hold it together. Now, turn on the amplifier and then just think. That's all you have to do. No complicated dial settings, nothing else.
Like I said, this one is damned simple.