Dot Physics

Some time ago I was looking for materials to possibly build a foucault pendulum. Of course the first step is google. There was a site that suggested two old issues of Scientific American, and it happens that we have tons of old Scientific Americans in the store room. I found the two that I needed. I will talk about foucault pendulum in second, but let me show this picture.


This is a device to prevent elliptical motion of the pendulum. Part of it has a ring of nickel. Now for the quote:

“The nickel ring, perhaps appropriately, should have the dimensions of a U.S. five-cent piece, with a 3/16-inch hole exactly in the center. (Do not make the part from a coin; it is illegal to deface U.S. currency.)”

Pure awesome. But, it turns out that this is NOT illegal. This was addressed in a Make Magazine article on making rings from coins. Here is a Lifehacker post on essentially the same thing with all the notes about the legalness of destroying coins. Essentially, it is illegal to deface the coins to make money (i.e. cut off a little of each coin to make money from the metal cut off). Clearly it is not illegal – you see these “smash a penny” machines at museums and stuff.

Finally, here are some Foucault pendulum links:

There was a whole bunch of other cool stuff I saw in this magazine. Material for a future post.


  1. #1 Rich
    April 22, 2009

    Very interesting…. it also makes me think, after the change in composition for coins some years ago (whereby pennies went from mostly copper and a little zinc, to mostly zinc and a little copper), if the nickel is now mostly NOT nickel (25% nickel, 75% copper according to the U.S. Mint website) would this still suit the specs for the Foucault pendulum? I guess I’m wondering if the nickel cladding wore off the coin, would it still work.

    And then THAT got me wondering, if the nickel is now mostly copper, yet the penny is mostly zinc, why didn’t they just go to the (presumably cheaper) zinc for the core of the nickel too? That is, if copper was too expensive to serve as the primary metal of pennies, why then would we use it for the larger nickels? I do realize that there must be fewer nickels than pennies that are minted.

  2. #2 Rhett
    April 22, 2009


    I am not really sure about the purpose of the nickel and if it has to be nickel at all.

    The answer to your second question is clearly beyond the realm of physics and in the realm of politics. Who knows why they make some of the decisions they make.

  3. #3 Rob
    April 22, 2009

    That reminds me of the days when Scientific American was an excellent magazine with The Amateur Scientist column, Martin Gardner’s column, etc. While I still get and read it, it’s a pale imitation of its former self (imho, ymmv, usual disclaimers).

  4. #4 Fran
    April 22, 2009

    I agree that not all the changes in Scientific American have been good, but I enjoy understanding more of the articles than I used to. This is a case where making content more accessible has been a good thing. (imho, etc, too)

  5. #5 Uncle Al
    April 23, 2009

    If you will build a fine Foucault pendulum you can also be very clever with its placement versus solar eclipse paths,

    Tungsten carbide bob, 15.6 g/cm^3? Depleted uranium bob (nickel-plated), 19 g/cm^3?

  6. #6 Dave
    April 23, 2009

    Note that Canadian nickels retained more Nickel content until fairly recently (Plus, given the current exchange rate, they’re even cheaper than American nickels!). 🙂 Note that some Canadian coins were magnetic (well, up until a few years ago), showing the ferromagnetic properties of nickel.


  7. #7 Rhett
    April 25, 2009

    So, it appears that nickels (coin) have always been only 25% nickel – except for the wartime nickel which had no nickel at all (because the nickel was used in stainless steel).

  8. #8 Bill
    April 29, 2009

    I think the reason the nickel’s composition hasn’t been changed to include zinc is that it is accepted by vending machines which use the coin’s weight as one of the factors for determining coin type. The penny on the other hand is really only accepted by bubble gum machines which only check size.

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