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. 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.