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Old 10-08-2014, 01:12 AM   #1
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Cool Possible inexpensive fraud detector for PMs

I have a good friend in the craft jewelry business, who makes money buying sterling silver at yard sales, smelting it into more useful shapes for jewelry, and selling that. He's another science-y type guy, and came up with a trick so he only gets the real stuff. He says there's quite a lot of stuff out there stamped "silver" or "sterling" that's only plated at most, and he doesn't want to pay even yard sale prices for useless crap.

Well, it turns out pure silver (or even sterling) is a heck of a lot more conductive than most things, as is (pure) gold. Yes, you could get or make an expensive 4 wire ohm-meter designed for measuring that, but what about what's beneath the surface?

Well, it turns out there's a fairly easy way to discover bulk conductivity. All you need is a really strong and small magnet - NeFeB, grade 42 or higher.

Moving a magnet across a conductor creates eddy current in the conductor, which in turn creates a field that resists the motion of the magnet. A common demo is to drop a magnet through a very conductive pipe (example, bored out copper rod or aluminum). It drops very slowly. Good for amazing the rubes. In my demo lashup, it can take over ten seconds for one of these to drop a foot through a thick wall conductive pipe, seems like it'll never finally come out.

My friend uses this trick, letting a magnet slide down the piece, and if it goes really slow, it's really silver or gold, if not - it's plated over a much more resistive base metal, and of course if it sticks - it's really base - iron, nickel, chrome, in that case. It takes some "feel" and getting used to for this to be a really reliable test, but tungsten laden gold will fail, and silver plate will fail and it's obvious. When I get a chance, I'll make a demo movie for all here to see - it's a cool trick. Should work for coins and at least thinner bars. A really thick bar that's mostly a PM might still have a little something you don't want inside at the core and pass, but nothing else will.

Sorry to have been absent so much - it's been really crazy here with "too much attention" to my other main activity, which has been very successful of late. It's still going on....I'd rather fool around in the lab than stand in front of TV cameras any day.
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Old 10-08-2014, 08:03 AM   #2
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Hey DCF, great to see you around again. I was wondering if you were busy doing more interviews for TV shows or something.

There are a few youtube videos out there where people demonstrate this "magnet slide" test. One was posted here:

http://www.pmbug.com/forum/f2/first-....html#post1160
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Old 10-09-2014, 07:41 PM   #3
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Science!

Glad to hear the lab is humming, and only glowing within spec.
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