Been nearly ten years since I last looked at this. Most people’s interest in paramagnetism seemed to be soil testing with the Phil Callahan Soil Meter (PCSM), mine was investigating megalithic sites. I have Fabrice to thank for this and much useful information about the PCSM. I had unsatisfactory results in the field, lacking sensitivity. This was probably due to a bad choice of coil design. Looking at Bartington’s MS2D surface scanning probe, it’s clear I should have designed this more like a metal detector search coil.
Which explains why I shelved this project after dismal results at Gors-Fawr I recall.
Sadly Philip Callahan passed in November 2017, it seems interest has dropped away without such an eloquent protagonist. For the first time i though I would look at soil, which you can enclose in a solenoid, which takes away a lot of the handwaving. I bought some rock dust from Allgood Farm on ebay and thought I’d investigate. Before we go on I should emphasise that Allgood don’t say anything about paramagnetism in their description –
Volcanic rock dust is rich in essential minerals and trace elements like magnesium, iron, and calcium that plants need to grow.
Slow Release: These minerals are released slowly over time, providing a long-term nutrient supply.
so this is about mineralisation, not paramagnetism. The last volcanic activity in Britain was 50 million years ago, so volcanic rock from the UK won’t be highly paramagnetic, because much will have eroded over 50mn years 😉
I used a discrete component Franklin oscillator. I am also testing a LM358 oscillator, on the same breadboard
It’s tasteless as hell to make these oscillators on a breadboard, but the discrete component oscillator had a tendency to squegging at these low frequencies so it helped debug that. I have this on DL4YHF’s Spectrum Lab
and you can see there’s some scintillation, whether this can be improved by building this on Veroboard remains to be seen. Using Spectrum Lab I measured the frequency f2 at 5.4886 kHz and f1 with sample at 5.4861 kHz.
Using χ = (f1²-f2²)/(f1² × 4 π) from last time1 I infer the susceptibility is
(5488.6² – 5486.1²)/(5488.6² × 4 Ï€) = 7.25E-05 = 72 µCGS. Probably +/- 5µCGS given the scintillation in frequency. Definitely paramagnetic, probably not stupendously so. I need to find some powdered materials to calibrate this with. And test the Franklin discrete component oscillator and the LM358 version for stability when soldered, rather than bodged on a breadboard 😉
My coil was ~1.2mH and resonated at ~5.5 kHz with a 1 µF mylar capacitor and the stray coupling capacitors of the Franklin oscillator. To be continued
