ohm meter to test for real from fake siler

some friend told me i can use an ohm meter to test for fake silver dollars because silver do not resist current like copper or other metal any one heard of that before or it is a myth

 

Log in or Sign up to hide this ad.

Myth. All of the coins will read the same. Don't stick any coins in 120vac outlets. Leave the electrical stuff to professionals.

 

thanks million i will notget close with a meter any thing

 

It's true that silver has lower resistance, but you won't have any luck trying to measure it with a regular meter. All you'll really measure is how hard you're pressing the meter's probes into the coin, and doing that is likely to damage the coin.

 

what do u mean damaging the coin how that happen shed more knowledge on this

 

I just meant that poking the coin with metal probes is likely to scratch it. You generally must push quite hard to get good electrical contact for measuring resistance. I'm not sure how to do that without leaving a mark on the coin's surface.

The basic problem is that the resistance of the connection between each probe and the coin will usually be much, much larger than the resistance of the coin itself. And the connection resistance is highly variable based on how hard you're pressing on the probes.

 

Here's a table of resistivity of different metals. I think the only way you could measure the difference between a silver dollar made from silver and copper using an ohmmeter would be by first drawing them out into identical, really long, thin wires. Sort of a moot point once you've done that.

 

deleted

 

The difference is only 0.1x10(-8) ohm per meter, so the ohm meter must be of high sensitivity, and the surface of the coins would have to be scrubbed for pure metal contact rather than some minute corrosion ( whoops, toning) on the surface or as Jeff says ~press hard.

 

Why would you press the meter leads into the coin? Have you heard of the edge? You just touch the metal, you do not press the point of the probe into the metal ever. you will have to set the meter too.

 

A basic meter will read the continuity of any conductive material as the same. You'd have to have a very expensive piece of equipment to read a difference in the short distance of a coin.

 

Drawn out to a wire with a uniform 1 sq. mm cross-section, a silver dollar would be 2.736 m long. The resistance of copper would be 46 milliohms, while silver would be 43.5 milliohms. There's a $500 meter on Amazon that should be able to detect this.

 

https://www.metalsupermarkets.com/which-metals-conduct-electricity/

I do not own a conductivity meter for metals or liquids.

 

Have you ever tried this? You'll find that if you "just touch" the coin with the probes, you'll be touching surface contaminants (dirt, oil, oxides, sulfides). If you think you can distinguish silver from copper or nickel this way, you're kidding yourself.

I just bought a fairly spiffy new meter (well, new old stock); I'll see if I can cobble together a demonstration. Maybe I can even put the coin on a scale -- a kitchen scale, not a coin scale -- to show how hard you have to press to get a stable reading. (And I say "stable", not "accurate", because I'm pretty sure the actual resistance of the metal in the coin will be dwarfed by other effects.)

 

While it seems like a workable idea at first glance, it would be very difficult to get useful results (and pretty much impossible to do without hurting the coin). As others have said, any oxide or sulfide film will greatly impede the flow of electrons.

I got to experience this first hand with an engineering class project last fall by comparing the current that would flow through a sample of freshly polished lead foil vs. lead foil that had a lead phosphate film on it. The sample with the lead phosphate film had a significantly smaller current (~40% less after IIRC giving the phosphate film 24 hours to form). I assume that a coin that's been developing a sulfide or oxide layer for years would similarly resist an electric current.

 

You could clamp a coin with something resembling cable ends like found on jumper cables and preferably machined to fit the reeds/radius.

One jaw would be + and the other -.

Comparing resistances of silver, brass, steel, powder metals, copper, etc is going to show up pretty much continuous using a DVOM or analog meter.

Measuring Seimens with a conductivity meter, unless I am thinking about this upside down, would require enough current flow to most likely heat the coin sufficiently enough to color it.

I'm thinking to measure the conductivity, you're not going to do it with uA or mA.

Sound right?

If not, and you have a conductivity meter, ain't nothing but a thing to get a damage free connection to flow a few mA.

 

Well, lets up the sophistication and spend close to a million to get a combo that can really tell a silver or gold coin from an imitation. First we need a fault indicator such as these
https://www.olympus-ims.com/en/applications/ut-testing-gold-bars/
Either procedure 1 or 2
using ultrasound to find faults/flaws ( inconsistencies ) in layers of the coin to be sure it is the same continuous ( non-clad metal) and then use a XRF device ( same company) to analyze it if it passes the first part to see if its really gold or silver, as different proportions of metals ( such as gold karat level or silver % ).

I think a charge of $ 20-50,000 /coin should be acceptable so we can all rest securely at night.

 

"Unc Details, ultrasound transducer gel residue".

 

Bingo. The resistance of a silver coin will be somewhere in the micro-ohm range. That means that if you push a full amp through the coin, you'll be trying to accurately measure a voltage drop of microvolts, which is extremely challenging (read "expensive").

The real killer, though, is contact resistance. There's no way you'll reduce that below the coin's own resistance without plating or soldering large contacts directly onto the coin's metal. Even driving a spike into the coin probably wouldn't be good enough.

Actual electrical testers work by inducing current in the coin (like a transformer) and measuring the induced current's behavior. I'm still skeptical about those testers being reliable in practice, but they're apparently good enough, or at least convincing enough, to maintain a market.