About Superconductivity

How would you like to board a Maglev train and then speed off to your destination at more than 300 miles per hour? 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. This effect is called "superconductivity." The table lists the everyday metals that exhibit superconductivity and the temperature below which electrical resistance disappears. These elements require cooling by liquid helium to become superconductors. Such materials are called "low-temperature superconductors."

Much later, in the 1980s, physicists discovered ceramic compounds that exhibit superconductivity at temperatures as high as -145º Celsius. This temperature is high enough that the materials need be cooled only with liquid nitrogen, which is far less expensive to do than with liquid helium.

Japanese Maglev train prototype (photo courtesy of Railway Technical Research Institute, Maglev Systems Development Dept.)