Built on Facts

Shrinking Quarters

Tom at Swans on Tea (You do read Swans on Tea, right? You’re missing out if you don’t.) points out one of the more impressive physics demos out there. It’s quarter shrinking, in slow motion.

You take a $0.25 coin, stick it in the middle of a coil of wire, and dump an tremendous amount of current through the wire all at once. The following sequence of events then happens in a tiny fraction of a second: the current very rapidly generates a magnetic field, in accordance with Ampere’s law. This rapidly increasing magnetic field generates an electric field, in accordance with Faraday’s law. This electric field generates a current in the coin. And this current itself produces a magnetic field. Lenz’s law dictates that this magnetic field will point in the opposite direction from the original field. These opposing field compress the coin into a much smaller distorted disc.

What’s especially snazzy about the particular instance in the link is that they’ve filmed it with high-speed cameras operating at a hundred thousand frames per second. It’s pretty amazing to see in action, though even at these high speeds the compression only takes a very few frames.

Trivia question: in the first video, an experimenter reminds everyone to look away before he pulls out the pin to discharge the remaining energy in the capacitor. Why?


  1. #1 Benjamin Franz
    June 18, 2009

    So you don’t get a retinal flash burn.

  2. #2 Rhett
    June 18, 2009

    Answer: He says to look away so you won’t see the other guy come out and replace the quarter with a smaller one while another guy makes the boom sound.

  3. #3 Stephanie Chasteen
    June 18, 2009

    I like Rhett’s answer…

  4. #4 Uncle Al
    June 18, 2009

    It begs replacement of the coin by a goldfish then a disk of molten lithium-6 deuteride. Follow with massive Federal subsidies for humane capital punishment and electricity generation too cheap to meter (hand in hand as it were). Then take the complex conjugate and publish a paper on theoretical modeling of black hole tidal distortion when passing through an event horizon.

  5. #5 rob
    June 18, 2009

    ahhh…but are goldfish diamagnetic enough to form a strong opposing magnetic field that will shrink them? or will it just end up being an electromagetic goldfish gun?

  6. #6 CCPhysicist
    June 20, 2009

    Nice work, but bad physics in the commentary:

    “300 microFarads of charge”. Shudder.

    “we need to discharge all of the current from the capacitor”. Shudder squared.

    But I really like that you can see that the radial compression of the quarter is complete before the wire vaporizes and you get that wonderful plasma explosion.

    Uncle Al, what is the resistivity of molten Li6D? Does the D even stay in there at those temperatures? And what makes you think you can get the relevant densities, cold fusion?

  7. #7 Uncle Al
    June 20, 2009

    Molten LiD is an ionic conductor SOP. Place in a spherical metal membrane container under a couple of atmospheres of deuterium gas to prevent dissociation and buffer phase change expansion. Phase diagrams and tables in “COMPILATION OF THE PROPERTIES OF LITHIUM HYDRIDE”, NTIS N7274680 re “Technical Memorandum X-483″ 1963, declassified.

    The induced field obviously substitutes for a palladium lattice in lowering D-D tunneling barrier. When things are cooking, neutrons fission Li-6 to tritium to spice it up. Borrow the Z-Machine’s power supply for quick and dirty proof of concept runs.