Balancing Magnetic Coins
posted on 3 Mar 2013 by guy
last changed 22 May 2015
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ages: 8 to 99 yrs
budget: $2.00 to $20.00
prep time: 10 to 30 min
class time: 15 to 60 min
This lesson describes a magic trick with magnetic coins (or washers) that demonstrates some important principles in magnetism. It's a good lead-in to a conversation on magnetic materials and magnetization.
optional equipment: magic penny kit
subjects: Engineering, Physics
keywords: magnetism, coins, magic, trick, spinning
balancing and spinning coins
A balancing act with magnetic coins from Patagonia, by MagneticCoins.
The practice of balancing and spinning magnetic coins has been around since at least the early 1990's, but the most beautiful demo I've seen is in the video above from 2013.
To begin, you'll need to find some magnetic coins containing iron or nickel. Sadly, U.S. coins are not magnetic, except for the 1943 penny, which was made of steel. Even the U.S. nickel, which is currently made of a 75/25 blend of copper and nickel,1 is not measurably magnetic. A partial list of coins that ARE magnetic has been compiled by the helpful folks at http://www.magneticcoins.info/. In the absence of magnetic coins, steel washers may be substituted, but the edges are not usually as flat as on coins and they tend not to balance as well.
To prepare the stage, lay a ruler across two stacks of books, or two bottles as in the video. Put a strong magnet on top of the center of the ruler, with the poles aligned vertically: north pole up and south pole down, or vice versa. You may also stick another magnet under the ruler if you need more field strength. To get the right field strength for your coin stack, you will want to be able to adjust the height of the magnet, or alternatively adjust the height of the platform that the coins rest on. Resting the ruler on stacks of books of a chosen height works well for this purpose.
Take the first coin and attach it to the underside of the magnet, letting it hang down vertically. Bring up more coins and attach each coin to the other coins that are already in place. When your coin sculpture is complete, gently grab hold of the topmost coin and pull it downward away from the magnet until the stack of coins is resting on table underneath. If the coin stack jumps back up to the magnet when you let go, your magnet is too strong for the stack — make a heavier stack or raise the magnet. If the coin stack falls apart before you can lower it all the way to the table, the magnet is not strong enough — use a stronger magnet, or bring the magnet lower and use a smaller stack of coins.
Once you have your coin stack balanced on the table, you can start it spinning by blowing gently on one side. A straw is helpful for directing the stream of air. You may have to time your breaths to match the rocking motion of the coins, but eventually you should be able to set the coins spinning. A single vertical tower of coins can be made to spin quite rapidly.
what's going on?
Certain ferromagnetic elements, notably iron and nickel, can become magnetized when placed in an external magnetic field. See our lesson on Iron and Magnets for further discussion of how magnetization works. Many steel or nickel-based coins fall into this category. When these magnetic coins become magnetized, they can stick to each other to form elaborate towers.
By suspending the coin tower underneath a strong magnet, the coins remain magnetized and continue to stick to each other, and the magnet helps support the stack against the force of gravity. The magnet pulls hardest on the topmost coin, which is closest, and helps stretch out the coin stack to keep it from folding.
questions to ponder
- Why doesn't the topmost coin jump up to the magnet, which is stronger, rather than stick to the magnetized coins below it, which are weaker?
The magnet above is stronger, but it is farther away. As long as the top coin is touching the coins below, it will stick to them.
- Why does the coin stack tend to orient in a plane along the ruler? Why does it prefer that direction rather than other directions?
As long as you are using an asymmetric magnet, like a block magnet, the field is likely to be spread out along the long axis of the magnet, which you probably oriented along the ruler. The coin stack is also laid out in a plane, and the field from the coin stack is also spread out in the direction of the plane. Those two planes will tend to pull on each other until they are aligned. If you replace with asymmetric magnet with a perfectly cylindrical magnet, the coin stack will no longer have a preferred direction.
The folks at the Magic Penny Trust (http://www.magicpenny.org/) have been balancing and spinning coins for many years. Check out their many wonderful artworks for inspiration.