Location Location Location
Ocne I assembled the amplifier v2.0 I was keen to see if it worked. There was a promising peak at around 2450 which is not far from the expected Lamour frequency for my location. Sadly that peak persisted with the water sample removed so it was clearly spurious. What followed was a long period of sulking, a side-track with a cheap Ali-express amplifier (less gain and noisier than the one I was using) and I'd pretty much decided the project was beyond me.
Then this year I became interested in the applications of Claude AI to my hobby projects. I'd found it useful for reviewing my software and even my hardware designs (with the kicad-happy skills package in particular). Perhaps Claude could help me systematically debug each section of the magnetometer. With a few AI suggestions and some wider reading I started testing the following areas.
The Coil Polarisation Circuit - Quick enough turn off?
I was pretty sure I was getting a magnetic field generated in the coil. I could see with a clamp meter that I was getting about 8A into the coil which should be plenty. A compass nearby was totally deflected by the coil when current was flowing.
What I hadn't fully appreciated when I began this project was how important it was the current (and the magnetic field) be turned off as quickly as possible. Too slow a turn-off and the protons in the sample just ride the field back to normal conditions. I constructed a shunt from 10x 0.1R 2W resistors, put it in series with the coil and measured the turn off with my oscilloscope.
Turn off was complete within 86us which should be fast enough. So the coil seems to be in good shape.
The Sensor Coil and Amplifier
If the magnetic field was sufficient then perhaps the sensor coil just could not pick it up? The design requires a tuned tank with a switchable bank of capacitors allowing it to be tuned to a resonant frequency near the expected frequency. But this requires knowing the the true inductance of the coil. To measure this I put a known capacitor (0.1uF) across with the sensor coil and fed a relatively slow (100Hz) signal in. The "ringing" was very apparent and appeared to have a frequency of about 7kHz. Using standard formula I could estimate the inductance of about 5.4 mH (a little lower than design spec but I had found the coil winding tricky).
There's also the risk that the amplifier, with such high gain, might self-oscillate. Measurements with the input grounded showed that it was quiet.
Data Collection and Analysis.
So if the sensor coils and capacitor bank and polarisation coil are all working as expected, perhaps it's just the A/D or the analysis software that's a problem. Claude has reviewed the software and made some useful suggestions, including a parameter file to help with consistency and averaging the FFTs over a number of runs as well as the possibility of background removal from a run with the coil not polarised. One way to test the whole thing is to create yet another coil, this one wound around a piece of dowel and inject a very low amplitude signal at the expected frequency into it. With that coil in place of the distilled water I got the following for a 2440 Hz input signal.
Sample and the Orientation
At this point I started casting around for a number of other things. I realised that the book I'm following was written for the Northern Hemisphere (Colorado). So while it suggested the coil should be tilted to the North, for me in the Southern Hemisphere it should be tilted to the South. I had high hopes this was the problem but it didn't produce me a signal. From some reading I realised that the coil in fact could also be aligned East-West and horizontal as that also, by definition, puts it perpendicular to the field as well. I even tried an orientation sweep with several tilt angles but got nothing of interest.
Then, just by chance I noticed that the "distilled water" I had obtained was a super-market "health food" product and had the claim "Oxygen Enriched" on the label. Oxygen, which is paramagnetic, kills the signal. I got some deionised water from an auto supply store and degassed it by boiling it. Still no better. No doubt a problem, just not the actual problem
So, what was I getting?
This was a typical result, whether I had water in place or not:
As well as a bit of noise I was getting this "comb" of peaks, at 50Hz separations. This, it turns out, it typical of mains AC harmonics, in this case they are amplified by the natural resonance of the coil and capacitor bank. Still, to be detecting the 49th harmonic of the mains shows how sensitive the whole thing is. I moved from a 150W mains power supply to a 12V battery to power the polarisation but it was only slightly better. The strength of the peaks and their exact location moved around a bit but the key point was that they tended not to decay like a real precession signal would.
Potentially the AC interference is a problem as the expected frequency for my location is 2435 Hz and the 2450 (49th harmonic) is nearby making it difficult to filter out. But still, it seems there should be a signal in addition to the mains interference (and I would hope considerably stronger).
Could it be the location?
This is a very sensitive instrument. After thinking about it for a while I began to wonder if the location mattered. I'd tried a number of different places in my own property but without much effect, only a slight muting of the AC interference as I got further away from buildings. One thing I began to think about was perhaps the fact I live on a volcanic peninsula, literally on top of an ancient basalt lava flow might be the problem? Such rocks are known in introduce magnetic gradients and there's plenty of them around here. The move to battery power made the whole thing portable so perhaps I could try another location.
A road trip and... success!
I decided to try another location - this one well off the peninsula. I chose the "Groynes", a large regional park well away from houses and on alluvial plain which should be electrically and magnetically quiet. And indeed I did get this with the coil tilted and facing South:
This gave a very clean signal at 2431.3 Hz corresponding to 57.10nT. A similar result was obtained with the coil horizontal and aligned East-West. With the water removed there was just noise. Just for fun I thought I'l put a toolbox full or metal tools next to it and that also killed the signal.
Most importantly the signal showed clear signs of delay, allowing an estimate of relaxation time:
This result was also very close to the average for that day measured at a magnetic observatory about 20km away:
Next steps
This result was a mixed blessing. On one hand it showed that under the right conditions it could work. At the same time it suggests that there's something about how I've been running it so far that it killing the signal. The biggest culprit is likely to be the basalt rocks under my house. This means my hopes of running a magnetometer to look for solar storms may be unlikely to come true.
But all is not lost. In the worst case I might be able to gift it to somebody who lives in a more suitable location. There are, however, some thing to investigate. If there are local gradients it's possible the signal is quenching faster than it would in a magnetically quiet area. My amplifier is "blind" for 40ms after the coil stops because of the induced currents in the sensor. I will investigate if I can reduce that by adding a diode snubber across the coil. I also will try some more careful location studies. Perhaps elevated as high as I can get off the ground, and also be very careful about local metal objects.
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