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One way to check for counterfeit silver/gold coins is to "slide" them down a ramp made of neodymium magnets. You can see an example here, or just look up "silver slide". I have seen this work for various coins and it is claimed that this is due to "eddy current breaking" which occurs when a diamagnetic material is exposed to a changing magnetic field.

Pretty much the only coins I have tried that did not slide noticeably slower are modern nickles (75% Cu, 25% Ni) and American Gold Eagles (91.67% Au, 3% Ag, 5.33% Cu). The slowest coin was a modern dime (91.67% Cu, 8.33% Ni). I have quantitative data on all this but not sure if it adds much to figure out how to include a table here... but I saw a factor of ~6 difference.

I have read that bismuth is the most diamagnetic element, eg is about 8x more diamagnetic than silver.

So if I cast a round (ie, a coin) about the size of a quarter out of 99.99% bismuth should it slide slowly down a magnetic ramp?

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  • $\begingroup$ This may be related: physics.stackexchange.com/questions/193501/… $\endgroup$
    – Livid
    Commented Jul 15, 2019 at 19:25
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    $\begingroup$ that's none of your bismuth! This is a physics joke. you may laugh now. $\endgroup$
    – niels nielsen
    Commented Jul 16, 2019 at 2:32
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    $\begingroup$ I don't think this has anything to do with the diamagnetic or paramagnetic properties of a material. I doubt even the diamagnetism of bismuth would matter. What matters is the electrical conductivity and probably the uniformity of the coin. Larger eddy currents will be induced in a more conductive coin, and the magnets will apply a greater force on it to oppose its motion down the slide. $\endgroup$
    – Puk
    Commented Jul 17, 2019 at 19:39
  • $\begingroup$ @Puk would it help if I took the time to include a table of the speeds I collected for various coins? $\endgroup$
    – Livid
    Commented Jul 17, 2019 at 20:58

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No. The eddy current breaking effect is very weak for bismuth because of its high electrical resistivity. The effect is strongest for pure silver and oxygen-free copper which have the lowest electrical resistivity. The effect is not caused by diamagnetism. This can be demonstrated using aluminum which is paramagnetic. Dropping a magnet into a thick aluminum pipe will slow the descent of the magnet just as in the more dramatic demonstration commonly done with copper (video comparison). The magnet falls more quickly in an aluminum pipe than in a copper one because aluminum has a higher electrical resistivity. If the pipe were made of bismuth, the effect would be minuscule.

A light diamagnetic substance such as pyrolytic graphite will levitate above the slide without contact friction. Bismuth is too dense to levitate itself over a magnet and its weak diamagnetism will not make a significant contribution to the sliding speed. See this video for a demonstration.

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