THREE COINS IN THE P.I. FOUNTAIN

[Ferric Toes]

What will the answer be…?

I have 3 identical coins which are British Crowns made of cupro-nickel alloy. Diameter 38.5mm. thickness 3mm, and weight 28gm. Electrical conductivity is 5.5 as measured on a Sigma 2000 conductivity meter. That is 5.5% of the conductivity of pure annealed copper (100%). True TC is 45uS.

Using a small solenoid coil (35mm D) I put one coin on axis, spaced such that no saturation is taking place in the RX circuit. On a scope I now see a clean decay curve which merges with the noise round about 100uS. Plotting the curve on log linear scales between 20 and 100uS, I get a nice straight line, so it is a classic exponential decay.

I now place a second identical coin on top of the first with a thin insulating layer between, and the results plotted.

The last part of the experiment is to place the third coin on top of the other two, again with the insulating layer and a plot completed.

Comparing the results and also the scope traces, it wasn’t quite what I expected. What do you think I saw? Good question for the theoreticians, and for those with practical gear to do a test.

What’s the relevance? Cache hunting where considerable numbers of coins are together, but not necessarily in electrical contact due to oxidation.

Eric.

[WM6]

Gues that traces are pretty the same in all cases.

But can you repeat this test with oxidated (instead of isulated) coins.

So called "oxidation" can be in some cases conductive enough to get "cache" response.

[bernte_one]

i think there will be a raise in the line but all together smaller as you would multipli the raise from the first coin 3times

[Ferric Toes]

Gues that traces are pretty the same in all cases.

But can you repeat this test with oxidated (instead of isulated) coins.

So called "oxidation" can be in some cases conductive enough to get "cache" response.

I also did the test without the insulation, so there was metal to metal contact and the results were exactly the same. The coins are two of 1965 and one 1977 and are in clean condition. Even holding them tightly together did not make a difference.

Eric.

[Carl-NC]

I would expect the TC to increase somewhat for the 2nd coin, and a lesser amount for the 3rd.

[green]

What Carl said. I stacked nickels, 2,3,4,5 touching and insulated in a four inch coil. Five nickels more than doubled the TC with less than a 10% increase in amplitude

[Skippy]

I'm guessing that a lot of masking is taking place, and the second and third coins don't contribute much. But...I reckon there may be an effect where the first coin excites the second one, and subsequently the third one, giving a doubly and triply delayed response. The second and third responses will be weak, but visible?

[green]

I tried the test with 5 quarters. Got different results. The single coin had a slightly lower voltage at the start but a higher voltage at longer decay times. I still don't have my preamp done and am looking at coil voltage after the decay with a scope. Subtracting a no coin reference signal from the target signal in Excel. The coil is laying on the bench with the coins stacked on the bench in the middle of the coil. Should I expect result to be a lot different measuring the coins at a distance 1,2 and 3 times the coil radius with a preamp and integrator?

[Overtheedge]

Admittedly I've still got a lot to learn, but here is my thoughts.

Amplitude reduction with an extension in the time delay.

My reasoning is the eddy currents in each coin affects the other coin's emitted signal. You are stacking emitters whose phase delays are not in synch.

Effectively you get a smeared return.

But what do I know? Not all that much until I have tested and confirmed for myself.
eric

It's not what you know that gets you in trouble, it's what you know that ain't so.

[bbsailor]

Eric,

After scratching my head a few times I came to the conclusion that the three coins must act like the links in a fine gold chain. The TC of the individual link (or coin) governs it's detectability not the mass of all the poorly electrically coupled links (or coins).

bbsailor

[Ferric Toes]

Here is a plot for the three coins. Lower curve is 1 coin and show a good single exponential. The starting amplitude is set to 1000 for each of the 3 curves, although the exponential fit for the second curve (2 coins) was at some variance, particularly at the start. For 3 coins (top curve) it is better. It demonstrates that as coins are added to the stack the TC increases even though they are insulated from one another. I am not clear as to why.

Tomorrow I will post a different graph set showing how the starting amplitude varies as coins are added.

Eric.

[ODM]

Mutual inductance rising more rapidly than resistance falls? The contacts between coins are likely to be a different magnitude of resistance compared to the skin of a single coin, too?

[WM6]

It demonstrates that as coins are added to the stack the TC increases even though they are insulated from one another. I am not clear as to why.

Probably you need to add some EM shielding from side of stacked coins. Could be that some of composed pulse spectrum penetrate insulation.

[green]

4 inch coil, coins centered in coil, not stacked coins were touching, not stacked nickels about 1 inch above coil center, stacked coins were touching. Plotted difference of no coin reference and target. Noisy but still shows trend.

[Ferric Toes]

4 inch coil, coins centered in coil, not stacked coins were touching, not stacked nickels about 1 inch above coil center, stacked coins were touching. Plotted difference of no coin reference and target. Noisy but still shows trend.

Nice plots . I expect the results will be rather different for other coins, particularly coins that are made of a more conductive metal e.g. silver or copper. Each coin in turn is attenuating the high frequency components of the TX waveform, giving less excitation to the next coin. Lower frequencies still get through to give the enhancement at late times. Star = 1 coin, Circle = 2 coin, Triangle = 3 coin.

Here is a further plot for my coins where you see the initial amplitude reduced for the next coin in line, but the enhancement at late times. I am hoping that some brave person will try a Spice simulation - if it is possible.

Eric.