🚀 go-pugleaf

In _The_Amateur_Scientist_ section of Scientific American, Feb 1968, page 124,
there's a description of a differential magnetometer capable of detecting a
minimum difference in field strength of 3e-5 oersted.  The magnetic field of
the earth causes protons in water molecules to precess at a rate of about 2025 Hz.
The article describes a device consisting of two bottles of water with coils
wound around them, and a high-gain (2e6) tuned amplifier.  A dc current is passed
through both coils to align the magnetic dipoles, then the current is removed and
the amplifier is connected to the coils.  A beat frequency proportional to the
difference in the magnetic fields at each bottle is heard at the output of the
amplifier.

It might be interesting to build a more modern version of the system described
in the article.  Another installation of _The_Amateur_Scientist_ describes another
NMR setup and suggests adding ferric nitrate to the water.  Why?