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Asked to think of an animal that can fly, most people don't picture a frog. Nonetheless, in April 1997, a team of British and Dutch researchers announced success in lavitating a live frog by using a powerful magnet. According to one of the human observers, the frog emerged from the flight unharmed and "happily joined" his fellow frogs in a biology department.
The amusing video image of the frog hovering in midair circulated widely and captured many people's fancy.... [snip]
The seeming ability to defy gravity is what delights most people, but the demonstration highlights a more subtle idea that is often overlooked in everyday life - that many objects considered nonmagnetic do, in fact, possess magnetic properties. The water, proteins, and organic molecules that make up frogs and other living things are diamagnetic, which means that in the presence of a magnetic field, they become weakly magnetized in such a way as to oppose the applied field.
Diamagnetism is what allowed the researchers to float the frog. Scientists are now looking into this phenomenon to simulate zero gravity and thus provide a low-cost substitute for experiments now possible only in outer space. They plan to tease out how the absence of gravity affects biological systems, especially developing embryos.
With the advent of high-temperature superconductors, the magnetic levitation of nonmagnetic material became and easy tabletop demonstration too. A chunk of superconductor can hover above an ordinary refrigerator magnet when cooled to liquid nitrogen temperatures or lower. A superconductor acts as a perfect diamagnet and excludes an applied magnetic field, says Simon Foner, former associate director of the Massachusetts Institute of Technology's Francis Bitter Magnet Laboratory. In effect, electrons within the superconductor move in a way that generates a field equal and opposite to the applied field. Because superconductors are such good diamagnets, a relatively weak magnetic field is enough to make them float.
Frogs are much poorer diamagnets. In the presence of a magnetic field, the electrons orbiting a frog's atoms generate an opposing field that has only a tiny fraction of the applied field's strength. It therefore takes a stronger applied field combined with a change in magnetic field or gradient, to create enough repulsion to support a frog's weight.
To perform their trick, the researchers - from the University of Nijmegen in the Netherlands, the University of Sao Carlos in Brazil, and the University of Nottingham in England - used a powerful water-cooled solenoid magnet, a cylindrical coil wound from a few hundred turns of wire. Current passing through the wire creates a field whose north-south axis lies along the center of the coil and whose strength varies along the axis.
Placed in the hollow core of the coil, a vertical tube a few inches in diameter, the frog generates a diamagnetic field that could theoretically be detected by a compass, says Nijmegen's Andre Geim.
When the frog is in an area of the magnet where it experiences a large combined effect between the gradient in the applied magnetic field and the field strength, a repulsive force pushes the frog up. At the point where magnetic repulsion and gravity exactly counterbalance each other, the frog floats.
Even though the magnetic field needed to levitate the frog is much larger than that of a household magnet, it's still low enough to be reproduced easily in a laboratory. "It takes only 100 times higher fields" than for the superconductor demonstration, says Geim. The relative ease of levitating a frog "appeared to be counterintuitive for many, including myself and my colleagues.
Geim doesn't consider the floating frogs a "scientific discovery", since the physics that explains it has been known for years. The demonstration does show, however, that scientists may not need to leave the ground to see their ideas fly.
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