A Proton Precession Magnetometer - Part 1: Overview and Polarisation Coil

For some time now I've wanted to add a magnetometer to my weather station. Mostly because it seemed like a cool project, tracking the earth's electromagnetic fields, but a trip to Iceland also made me aware of the relationship between that and the Aurora. We are a little too far North to routinely see any but the strongest Auroral Australis events.  Of course there are plenty of ways online to view "Space Weather" and get predictions of when Aurora viewing is feasible, but I like the idea of a DIY version. Also a nice adjunct to my TC1 seismometer which has been running for several years now and collected some fascinating data.

There are several ways to measure changes in the earth's magnetic field. The simplest can just be a suspended magnet in a container which serves as a type of moving compass. More sophisticated versions of the so-called "Torsion" magnetometer involve lasers and photocell detectors. I came close to starting on one of these when I became aware of another way to do it. 

A Proton Precession Magnetometer relies on the fact that when placed in a strong magnetic field the tiny magnetic field protons in hydrogen atoms have will rotate (or "precess") around that field. By suddenly removing the external field the protons will decay to their regular state and release a very weak signal with a frequency proportional to the earth's magnetic field.  It's rather like bringing a strong magnet up to a compass and then removing it - the needle will oscilatte slightly as it returns to the influence of Earth's magnetic field. 

This process is related in many ways to Nuclear Magnetic Resonance (NMR). As a graduate student in Chemistry many years ago I did a lot with NMR, including some late night experiments which collected data for hours. Of course that was an elaborate machine with superconducting magnets cooled with liquid helium:

A PPM is a bit simpler, although it still contains some interesting challenges for a DIY constructor in terms of materials, sensitivity to noise and analysis of data.

Turns out I'm not the only Chemist who's taken an interest in building a PPM. There's some great details on this project here. 

But I'm using as the basis this book 

It's got a lot to really great details, full schematics and lots of photos. I'm going to use slightly different technology in places (since it was written in 2007) but the basic idea is the same.

The first part which I've attempted is the polarisation coil. Immediately I began to relaise this may be quite an expensive project. I needed to buy a metre of acrylic tube even though I only needed 1/10 of that and the only place I could find suitable wire (solid core) was on Amazon. The rest of the coil and mountings were laser cut out of acrylic.  All other fittings are brass as you can't use magnetic metals in the vicinity. Becasue it needs to be aligned with the local magnetic field the coil is places on a hinged mount - the hinges are model aircraft control surfaces as they have brass pins. I'm yet to actually test it but it looks OK:

The next part will be the powersupply to engergise the coid. Given the large amount of current required and the ability to switch it off very rapidly there are some interesting challenges.