In Physics in Action we take you to a frontier area of physics every week. After a short introduction to the basic physics involved, we give you a taste of the current reseach in the field. The magnets that levitate these trains are an application of superconductivity .
Metals are good conductors of electric current.
That is, they have very low electrical resistance, but this resistance is not zero. A voltage difference is still required to generate the current in the metal, and the metal heats up while the current is flowing. The electrical resistance of an object depends on its temperature and declines slowly as the temperature falls. Early in the last century, however, a Dutch physicist discovered that a sample of mercury, when cooled below a certain temperature close to absolute zero, loses all electrical resistance. When the mercury is in this state, an electric current flows indefinitely, even in the absence of any applied voltage.