A proton precession magnetometer

[Doodlebug]

There's a simple one here in first part of site.
http://ilotresor.com/build-a-proton-prec...netometer/

An advanced one here in this book
Signals from the Subatomic World: How to Build a Proton Precession Magnetometer

Part of book here. Well explained too.
http://www.exstrom.com/magnum/master-Z-H-4.html