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Superconductivity
When electrical current travels through an object, some of the electrical power normally leaks away, a phenomenon known as electrical resistance. However, some types of metal and ceramics can lose all of their electrical resistance if they drop below a critical temperature. When this happens, they become superconductors, able to carry electrical energy freely for more than a billion years without losing any of it. The change to the superconductor state is both instanteous and complete once the critical temperature has been passed, leading scientists to speculate that superconductors are actually a different type of matter.
Superconductive Discs
The critical temperature for zero electrical resistance is so cold that it is usually expressed in terms of the Kelvin scale, which uses Absolute Zero as its base. Absolute Zero would be a temperature so cold that all of the molecules in the object stopped moving. With 0 degrees Kelvin being Absolute Zero, aluminum loses its electrical resistance at 1.1 degrees Kelvin, and lead at 7.2. Some types of metal and ceramic have a critical temperature of around 120 degrees Kelvin. It is possible to create and sustain such a temperature using liquid nitrogen, so a disc of this type can be suspended in liquid nitrogen and transformed into a superconductor.
The Meissner Effect
One of the properties of a superconductor is that it repels any magnetic object with which it comes into contact. When the object in question drops below the critical temperature, it begins to give off a magnetic field that will repel any other magnetic field. This is often demonstrated by placing a superconductor disc into liquid nitrogen and then putting a small magnet on the disc. As the temperature of the conductive disc drops past the critical point in contact with the liquid nitrogen, the magnet will begin to levitate. This effect is caused by the "diamagnetic" or magnet-repelling ability of the superconductor.
Experiments
The Meissner Effect is often demonstrated through experiment, by placing a small magnet on a superconductive dish in liquid nitrogen. However, the experiment will only work if the conditions are right. The diamagnetic force of the superconductive disc must be greater than the weight of the magnet. In addition, a superconductor disc made primarily of bismuth will be able to sustain the effect longer if removed from the liquid nitrogen than a superconductor disc made primarily of yttrium.