Most of the pioneers of low-temperature physics expected gases to liquefy, but none of the

m predicted superconductivity. This phenomenon was discovered in 1911 by Onnes while he was studying frozen mercury.

More than 40 years passed before physicists were able to offer an explanation for superconductivity. The accepted theory, developed in the 1950s, holds that the fundamental behavior. of electrons changes at very low temperatures because of the effects of quantum mechanics. Electrons are tiny particles that make up the outer part of an atom, circling rapidly around the nucleus of the atom. In a regular conductor—a metal that conducts an electric current—the outermost electrons are not bound tightly to the atoms, and so they move around relatively freely. The flow of these electrons is an electric current.

At normal temperatures, a conductor's electrons cannot move completely freely through the metal because they are "bumped around" by the metal's atoms. But according to the leading theory of superconductivity, when a metal is very cold, electrons form. pairs. Then, like couples maneuvering on a crowded dance floor but never colliding, the paired electrons are able to move unimpeded through the metal. In pairing up, it seems, the electrons are able to "blend together" and move in unison without resistance. This explanation seems to account for superconductivity at extremely low temperatures, but in 1986 scientists in Switzerland found that some metal-containing ceramics are superconductors at much higher temperatures. By 1992, scientists had developed ceramics that become superconducting at - 297'F, and some researchers speculated that room-temperature superconductors may be possible. Scientists are still trying to formulate a theory for high-temperature superconductivity.

The new ceramic materials can be maintained at their superconducting temperatures, with relatively inexpensive liquid nitrogen rather than the much colder and much more costly liquid helium required by metal superconductors. The cost difference could make superconductivity practical for many new technologies. For example, magnetically levitated trains, which require superconducting electromagnets, would be much cheaper to build than they are now. Superconducting devices might also be used for advanced power transmission lines and in new types of compact, ultrafast computers. But for the time being, superconductivity is finding application mostly in scientific research and in some kinds of medical imaging devices.

The flow of an electric current in a regular conductor is made possible by the fact that______.

A．electrons circle rapidly around the atom

B．the outermost electron move relatively freely around the atom

C．the innermost electrons stick to the atom

D．the outermost electrons are bound tightly to the inner ones