I wanted to have a way of experimenting with various rock crystals
used as radio signal detectors. The usual crystal set detector was
insufficient for my purposes, so I set out to design and make one that
could be precision guided to any spot on a crystal with microscopic
accuracy. This picture shows the final result.
I 
This detector uses a "microscope stage" that I purchased from Edmund
Scientifics. A brass plate serves as an electrical contact under the
crystal. The "needle" is just a copper wire soldered to a brass ball (a
lamp part found at Lowe's). Other materials include a varnished padouk
wood base and small fashioned curved piece to hold the rock in place
(epoxied to microscope retaining lever), an alumininum rod, a fashioned
lucite extension, and a small fluted wood dowel. The microscope stage
does most of the work. When a crystal or rock is placed on the brass
plate it must make electrical contact -- if this does not occur
automatically, an alligator clip probe can be attached to the crystal
and base. The brass plate has a wire lead hidden under the board
for connections. The two black knobs move the crystal under the needle
in precision, measured increments along the two axes (up-down,
left-right). The needle acts as a gravity activated detector. A hole is
drilled through the wood dowel so that the detector wire slips easliy
through it. It falls on the crystal and maintains contact with it when
the crystal is moved. A small alligator lead may be clipped just below
the brass ball for connection. A more elegant solution would be to
create a small mercury well inside the dowel through which the detector
wire travels and an outlet wire connects through the lucite arm.
Here is a picture of the detector in operation with a molybdenite crystal.
The wingnut allows the raising and lowering of the arm to accomodate
most heights needed. One can see from this that most shapes and sizes
of crystals can be accomodated, and the detector wire moves smoothly in
contact with the crystal. I use pieces of a standard "soft vinyl
eraser" to take up slack, and this helps hold the crystal in
position. Any small pieces of rubber would work as well.
The performance of this crystal detector is excellent, and one can return to any spot found on the crystal with some precision.
