I think there is a misunderstanding regarding the magnet slide test

[Widsith]

Dear All,

I often see comments about silver's diamagnetic nature when discussing using the magnet slide test.  In my opinion, this is a mistake. 

Silver is a conductive material.  It is its conductivity that 'slows it down'.

When a magnetic field moves through a conductive material (like when sliding across a conductive coin) small currents of electrical current are formed in the coin. These are called 'eddy currents'. They are in a closed loop of current flow in the metal. These currents, in tern, create a magnetic field in opposition to the field that produced them (from the magnet). This results in a counter force that 'slows down the' magnet on its slide journey due to gravity.

How much current that flows in the eddy currents are due to the conductivity of the material (based on 'identical' coin geometries). Copper has a lower conductivity and so the eddy currents will be smaller and the retarding force will also be smaller.  So the magnet will experience a lesser retarding force and will slide faster. The same sort of thing will happen with other metals that are diamagnetic. The diamagnetic aspect just means that the magnet will not 'stick' to non-ferromagnetic materials.  However, there are non-magnetic stainless steels and so the same argument (above) will apply there.  

Here is a vid that may help:  

 

[HawkHybrid]

silver needs to be diamagnetic so that the silver is not stuck on the closest magnet when sliding.

it needs to be more conductive than copper for the energy required for the slide to be noticeable

given a fixed magnet set up.

so I'm guessing it needs both.

 

HH

[Bumble]

You are quite right. The slowing up effect is an instance of Lenz's law, and is in effect a form of magnetic braking. It has nothing to do with diamagnetism. What the test is measuring is the electrical conductance of the metal. Lead, for example, has less than a tenth of the conducitivity of silver, so the difference would be very marked. Copper on the other hand is almost as conductive as silver, so the magnet test would be no good on a copper coin or bar that was silver plated.

[Bumble]

For information, the following table shows the electrical conductivity of some common metals. The densities are also noteworthy. Lead is similar in density to silver, and tungsten is very similar to gold. The magnetic susceptibility indicates whether the metal is paramagnetic (+ve) or diamagnetic (-ve). Tungsten is paramagnetic, so it will be attracted by a permanent magnet, while gold, silver and copper are diamagnetic.

[Widsith]

Here is a vid that may help:  

 

[Widsith]

If you have tubes of plastic, copper and aluminium ( as I had once) it is easy to see the different rates of fall due to Lenz's law. 

[Widsith]

3 hours ago, Bumble said:

You are quite right. The slowing up effect is an instance of Lenz's law, and is in effect a form of magnetic braking. It has nothing to do with diamagnetism. What the test is measuring is the electrical conductance of the metal. Lead, for example, has less than a tenth of the conducitivity of silver, so the difference would be very marked. Copper on the other hand is almost as conductive as silver, so the magnet test would be no good on a copper coin or bar that was silver plated.

I suspect you are an engineer. Do I get my 'no prize' for guessing this?

[Bumble]

I used to be a scientist when I was younger.

[GoldDiggerDave]

The new Sheldon Scale?

 

 

[zxtm99]

I like the magnetic slide test. I spent about £10 on 10 rectangular N52 magnets, enough for a decent sized slide. It is really easy to tell solid silver from silver plated brass - I bought an iffy 1 oz bar on eBay which fell like a stone. A quick Google confirmed it was a fake and I quickly received a refund.

 

However, as pointed out above, copper and silver have quite similar properties. Has anyone tried copper vs. silver to see if it is possible to spot the difference? I have some Sterling silver (a double-florin), and it can be distinuguished from 999 so I am quite hopeful.

 

To test this, I've just ordered a small 999 copper bar on eBay and will report back, but interested if anyone else has been able to tell silver from copper? It just such a quick test, and harmless to the silver if you cover the slide with a thick cloth or paper.

 

[swanky]

3 hours ago, zxtm99 said:

However, as pointed out above, copper and silver have quite similar properties.

The density difference is enough for a weight test to differentiate.

 

Edited July 16, 2021 by swanky

[zxtm99]

12 hours ago, swanky said:

The density difference is enough for a weight test to differentiate.

 

Indeed - quite lucky with silver as there is niot much of the same density. I guess a lead/tin alloy for example could be made to match, but that would fail the magnet test.

I think the weight test is the 'gold standard' (bad choice of term) but the magnetic sliide is really quick and easy and has no possibility of harming the bullion.

FWIW, I have ordered a 1 oz (avdp) .999 copper bar. I already have a silver coated brass 1 oz bar that faills the magnet test. Between those two and a genuine siilver bar, I hope to get a feel for how easy it is to spot the real thing. It seems most fakes are brass plated silver, but sincce N52 rare earth magnets are so cheap and available, I wonder if the fakers might start using copper. If so, then it would have to be the density test plus the magnet test in case of a lead alloy.

[Widsith]

17 hours ago, zxtm99 said:

I like the magnetic slide test. I spent about £10 on 10 rectangular N52 magnets, enough for a decent sized slide. It is really easy to tell solid silver from silver plated brass - I bought an iffy 1 oz bar on eBay which fell like a stone. A quick Google confirmed it was a fake and I quickly received a refund.

 

However, as pointed out above, copper and silver have quite similar properties. Has anyone tried copper vs. silver to see if it is possible to spot the difference? I have some Sterling silver (a double-florin), and it can be distinuguished from 999 so I am quite hopeful.

 

To test this, I've just ordered a small 999 copper bar on eBay and will report back, but interested if anyone else has been able to tell silver from copper? It just such a quick test, and harmless to the silver if you cover the slide with a thick cloth or paper.

 

I have been thinking about making a slide with optical sensors to detect when a coin passes. It may be enough to detect the difference although it may only be good for coins with the same friction due to the embossing.  I will thike about it further.

[zxtm99]

A bit more research, and I am fairly sure that copper and silver will be indistinguible on the magnetic slide. I was initially hopeful, as I found I could tell .925 silver from .999 using the slide. But it turns out that .925 is considerablely less conductive than silver (or pure copper) - if copper is considered to be 100% (it's the standard for wiring), pure silver is 105% but sterling is only 88%. Brass is more like 25% so stands out very easily on the slide.

 

However, there is quite a big difference in magnetic susceptability between copper and silver, so a different test may be possible, exploiting this, rather than the eddy currents/conductivity used in the slide. If the test article is placed on a piece of expanded polystyrene and floated in a bowl of water, then due to diamagnetism, it should be repelled by a magnet and drift away. Copper should do the same (also diamagnetic) but this effect should be 3x for silver vs. copper, but I've never heard of this being used to test PMs.

 

When I receive my pure copper bar, I willl give it a go, along with my free (refunded) silver plated brass bar!

 

ps. 'Old hands' won't need this info, but for any fellow newbies out there, virtually all the Pan American 1 oz silver bars on eBay are fake. Once you know, you can tell from a photo (no need for a test) but a quick trawl on eBay revealed about 90% fake. Funny thing is, many sellers are using stock photos so could have disguised this, but they are stock photos of a fake!

[stackerp5]

1 hour ago, Widsith said:

I have been thinking about making a slide with optical sensors to detect when a coin passes. It may be enough to detect the difference although it may only be good for coins with the same friction due to the embossing.  I will thike about it further.

That's an interesting idea. I can see two solutions for this:

1. Have many coins tested (at least 3-10 of each and possible fakes) to get a good idea of what to expect. OR;
2. Add something to the surface of the coins (thin translucent parchment/cooking paper) that would probably help level out the surface difference from a kangaroo to a brit (for example), nothing high releif though.

Not sure this is logically right but just my 2p, thats a good idea you got there!

[zxtm99]

On 17/07/2021 at 12:44, zxtm99 said:

When I receive my pure copper bar, I willl give it a go, along with my free (refunded) silver plated brass bar!

I obtained a 1 oz (avdp) 999 copper bar of silimar dimensions to some silver ones I have. As expected and feared, while it is very easy to tell silver plated brass from solid silver, I can see no difference on the magnetic slide beween pure silver and pure copper. In theory, copper should slide just a few percent more slowly, but due to friction effects blurring the test, there's no way I could detect copper. I would therefore guess that silver plated copper would pass the slide test, so no option but to carefully measure density.

Does anyone know why most fakes seem to be plated brass, as copper is so much harder to detect? Is brass easier to work with or something? Or perhaps the silver plated copper ones just don't get detected as often.....

There is one last thing I want to try. If the bars were floated on some expanded polystsrene in a bowl of water, the diagmagnetism means it will slowly drift way from a magnet. Silver is 3x as diamagnetic as copper, so may drift noticeably more quickly. This video is where I got the idea. I just don't have any polysterene to hand tonight!

Edited July 28, 2021 by zxtm99

[SilverStorm]

The only comment I can offer on this topic is this: never rely on any one method of testing to confirm the validity of your PM.  Always use several testing methods (using a scale to weigh, caliper to measure the diameter, etc, etc) to validate your findings.  When you package several testing methods together to prove your PM is legitimate, you can be more confident that you made a good purchase.

[stackerp5]

34 minutes ago, SilverStorm said:

The only comment I can offer on this topic is this: never rely on any one method of testing to confirm the validity of your PM.  Always use several testing methods (using a scale to weigh, caliper to measure the diameter, etc, etc) to validate your findings.  When you package several testing methods together to prove your PM is legitimate, you can be more confident that you made a good purchase.

Agreed, might I add specific gravity tests to the list. There are tutorial videos by BackyardBullion on YouTube.