We take electricity so much for granted. Imagine life without electrical conductivity — the ability of electrons to move unimpeded through the thicket of atoms that comprise a chunk of metal! A lot of electricity is wasted, though, overcoming the resistance of various substances used to conduct it — nearly 20 per cent during transmission. So scientists try to tap the exciting possibilities offered by ‘superconductors’ — solid materials that conduct electricity without resistance when they are cooled to sub-zero temperatures. Current travels through these without losing energy, reducing energy consumption and environmental damage.
Dutch scientist Heike Kamerlingh Onnes discovered superconductivity in 1908 while experimenting with cryogenics (extremely low temperatures). He managed to liquefy helium, which has a boiling point a little over -269 degree C. That’s just four degrees above absolute zero — the lowest temperature possible (at which all molecular motion stops) — and is usually referred to as ‘4 Kelvin’. Onnes used liquid helium to study the effects of cryogenic temperatures on various materials, and he noticed that electrical resistance in mercury and other metals virtually disappeared below 4.15 K. An electrical current introduced into the circuit of such a super-cold metal would flow for hours, or even days. It had become a superconductor. The current’s electrons pair at low temperatures, requiring almost no energy to move.
Superconductors can repel magnetic fields, so are used to make super-powerful magnets. This is the basis for magnetic levitation (maglev) trains, where superconductors in the track repel superconductor-based magnets in the train, making the train float above the track. Maglev trains could replace noisy, polluting jetliners. Superconducting magnets are also used in medical scanners, research instruments like particle accelerators, and frictionless bearings for gyroscopes that keep spacecraft and satellites oriented. When two superconductors are brought close together, but not allowed to touch, electrons jump the gap and current flows as if the two conductors were touching. The current across this junction is very sensitive to electro-magnetic fields and can be used as a precise sensor or an incredibly tiny and fast electronic ‘on-off’ switch for next-generation supercomputers.
Superconductors exist at higher temperatures too. Ceramics, for instance, can superconduct at 98 K (nothing more exotic than ordinary dry ice keeps things cool). But the search for the Holy Grail — superconductors at room temperature — continues.