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Disclaimer: I'm just computer scientist.

I'm planning a simple experiment that uses a coil to measure a small varying magnetic field. How small ? I don't know yet. I'm exploring the unknown. All I know is that other people claim they made it with a 300 Ohms coil that had a 6mm diameter soft iron core. That is all the information I have. I don't know the wire gauge, the number of turns, or the coil dimension.

In order to try reproducing this experiment I would like to know if I should better use a ferrite core or an soft iron core. The frequency are low, below 20kHz.

Another question is the diameter. Will I gain in sensibility with a larger core diameter ?

It is surprisingly difficult to find soft iron cores. I found some in China and the smallest diameter is 10mm. I found ferrite cores of 8mm diameter.

I know that soft iron is easier to cut at the right dimension. I read that ferrite falls appart when we try to cut it with a conventional saw.

A last question is if the inductance of a coil is a directly linked to the sensibility to varying magnetic field. In other words, may I assume that the higher the inductance, the more sensible to magnetic field my coil will be ?

EDIT The output voltage of the coil is described to be in the microvolt range.

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    \$\begingroup\$ Four inch nail; saw the ends off, cut to length. Heat to cherry red and allow to cool slowly. \$\endgroup\$
    – user16324
    Dec 13, 2020 at 16:11

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if I should better use a ferrite core or an soft iron core. The frequency are low, below 20kHz.

Soft iron will produce eddy currents in the presence of the magnetic field you are trying to sense. Those eddy currents will reduce your signal compared to ferrite so I would prefer to use ferrite.

Will I gain in sensibility with a larger core diameter ?

I assume you mean sensitivity; a larger diameter winding will be more sensitive i.e. it will produce a bigger signal voltage because more magnetic flux will be attracted through the cross sectional area of the bigger winding.

I read that ferrite falls appart when we try to cut it with a conventional saw.

You could pretty much grind ferrite down to small particles and reshape it in any way and get a decent result with glue. It's easily ground down and sand-papered. You could try using a fine grade hacksaw and I wouldn't be that concerned about little chips breaking-off because, the dominant path for magnetism is still via "the air" and, small to medium irregularities in the ferrite won't cause a problem. The ferrite core is just a "concentrator" of flux and it can be any shape and give pretty much the same benefits.

may I assume that the higher the inductance, the more sensible to magnetic field my coil will be ?

Inductance is a by-product - the sensitivity of the coil is related to number of turns and loop diameter. More turns means more inductance.

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  • \$\begingroup\$ Thank you very much. your answer help me a lot. Regarding the word sensibility, I was caught by a faux ami. I’m french and "sensibiilté" translates into sensitivity, not sensibility. Thank you for clarifying this. \$\endgroup\$
    – chmike
    Dec 13, 2020 at 11:20
  • \$\begingroup\$ I than had another question. It may be that the magnetic field that I plan to measure varies a lot in space. I thus intuitively assumed that if I have a ferrite core of a big diameter, it will average the magnetic field at its top in contact with the source of magnetic field. Would I avoid averaging by shaping the ferrite core with a sharp point like a pencil ? Will it reduce the magnetic field measured ? \$\endgroup\$
    – chmike
    Dec 13, 2020 at 11:26
  • \$\begingroup\$ @chmike A bigger coil will average the magnetic field. If you want higher spatial resolution you should use a smaller diameter. Sharpening the ferrite core won't make any difference; it's the general mass of ferrite that attracts (concentrates) magnetic field lines through the coil. \$\endgroup\$
    – Andy aka
    Dec 13, 2020 at 11:30
  • \$\begingroup\$ thank you. I had one last question. I assume I have to position the coil with the ferrite core pointing and close to the thing emitting the magnetic field I want to measure. So the coil turns would be tangent to the surface of the container. How would the length of the coil affect sensitivity ? Is it preferable to have a short coil with a larger external diameter, or a longer coil with smaller external diameter ? By length I mean height of the coil. \$\endgroup\$
    – chmike
    Dec 13, 2020 at 13:20
  • \$\begingroup\$ I do not understand this: So the coil turns would be tangent to the surface of the container. - what container? Coil winding i.e. stacked and short vs unstacked and long largely determine the self resonant frequency of the resulting inductance. Long and unstacked means a higher resonant frequency. \$\endgroup\$
    – Andy aka
    Dec 13, 2020 at 13:45

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