Author Topic: Iron Filings Conductivity  (Read 1324 times)

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Offline nickbildTopic starter

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Iron Filings Conductivity
« on: July 29, 2023, 09:56:46 pm »
I've got a jar of fine iron filings that are not conducting electricity as expected.  They respond to magnetism as they should, so it seems to be iron, and was bought from a reputable brand.  Any ideas on why this might be?  Corrosion?  Thanks in advance.
 

Offline TimFox

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Re: Iron Filings Conductivity
« Reply #1 on: July 29, 2023, 10:05:44 pm »
For some applications of iron filings, such as inductor cores, the particles may have been coated to eliminate conduction.  This would avoid eddy current loss in that application.
 

Offline nickbildTopic starter

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Re: Iron Filings Conductivity
« Reply #2 on: July 29, 2023, 10:40:39 pm »
Good info.  This is just a jar of raw filings for hobbyist/educational use, so I doubt there is a special coating in this case though.
 

Offline Ian.M

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Re: Iron Filings Conductivity
« Reply #3 on: July 29, 2023, 11:03:33 pm »
Cutting or grinding iron to dust is a fairly high energy operation, and the particles typically come off hot enough to form an oxide layer.  That oxide layer may insulate if thick enough or when thinner can even form a point contact Schottky type diode junction with an adjacent particle.  This has interesting effects on the electrical properties of bulk iron (and other metal) filings.  See https://en.wikipedia.org/wiki/Coherer
« Last Edit: July 29, 2023, 11:06:00 pm by Ian.M »
 
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Offline floobydust

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Re: Iron Filings Conductivity
« Reply #4 on: July 29, 2023, 11:29:38 pm »
OP it does depend on how much current and voltage you are attempting to pass through the iron filings.
Most multimeters do not push enough current to punch through any passivation layer. Try using a bench power supply and more current, like a light bulb.

The early radio detectors circa 1890, before galena crystals, the "coherer" was iron filings in a glass tube and I think it was the oxide layer doing rectification.


 

Offline nickbildTopic starter

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Re: Iron Filings Conductivity
« Reply #5 on: July 30, 2023, 12:17:43 am »
Thanks for all the responses, lots of knowledge here.  I've not been able to conduct any current with my benchtop supply either.

Maybe just too heavily corroded.  I might just be better off trying a new medium.  Suggestions on readily available fine conductive powders that won't have this problem?  Looking for inexpensive options, so silver/gold is out.
 

Offline rhb

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Re: Iron Filings Conductivity
« Reply #6 on: July 30, 2023, 01:26:20 am »
Particle contact area between particles is dependent upon the particle shape.  The more angular they are, the lower the contact area.  Filings and saw swarf are very angular.

Have you attempted to measure the resistance of the filings?

If not, place some filings between in an insulating tube with electrodes at the ends and measure the resistance as you compress the particles.

Reg
 

Offline nickbildTopic starter

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Re: Iron Filings Conductivity
« Reply #7 on: July 30, 2023, 01:35:46 am »
The resistance is infinite (without the compression you suggest).  I'm going to bail on the iron and try powdered graphite.  Conductive enough for my purposes, cheap, corrosion-resistant, and non-toxic.  Fingers crossed.
 

Offline Brumby

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Re: Iron Filings Conductivity
« Reply #8 on: July 30, 2023, 02:26:48 am »
... as you compress the particles.

This was going to be one of my suggestions.  The other was to shake/tumble to filings to knock off a bit of surface rust - or to use some chemical means to the same end.

This might get you a finite resistance for now, but I can't see it being stable over time.


I'm going to bail on the iron and try powdered graphite.  Conductive enough for my purposes, cheap, corrosion-resistant, and non-toxic.  Fingers crossed.

Sounds like a plan.  Just keep the humidity under control.  You don't want it to become a dew point detector.   ;D
 


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