Iron Filings Conductivity - Page 1
Forum section: electronics / general

author: nickbild
author_rank: Contributor
timestamp: July 29, 2023, 09:56:46 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: July 29, 2023, 10:05:44 pm
content: 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.

author: nickbild
author_rank: Contributor
timestamp: July 29, 2023, 10:40:39 pm
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: July 29, 2023, 11:03:33 pm
content: 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. Seehttps://en.wikipedia.org/wiki/Coherer

author: floobydust
author_rank: Super Contributor
timestamp: July 29, 2023, 11:29:38 pm
content: 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.

author: nickbild
author_rank: Contributor
timestamp: July 30, 2023, 12:17:43 am
content: 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.

author: rhb
author_rank: Super Contributor
timestamp: July 30, 2023, 01:26:20 am
content: 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

author: nickbild
author_rank: Contributor
timestamp: July 30, 2023, 01:35:46 am
content: 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.

author: Brumby
author_rank: Unknown
timestamp: July 30, 2023, 02:26:48 am
content: Quote from: rhb on July 30, 2023, 01:26:20 amThis 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.Quote from: nickbild on July 30, 2023, 01:35:46 amSounds like a plan. Just keep the humidity under control. You don't want it to become a dew point detector.