Superconductivity was discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes when he put a bar of mercury into liquid helium. Once the bar was immersed, it lost virtually all resistance to electricity. When he connected a battery to a superconducting coil through a pair of switches, he found that current in the coil continued to flow, even after the battery was taken out of the circuit.
In one of Onnes's early superconductor experiments, the current flowed in a superconducting coil with no measurable change for more than two years. It stopped only because the experimenters ran out of helium to keep the superconductor cool. In theory, that current could have flowed for hundreds of billions of years and more.
Onnes's experiment highlights some of the traits that make superconducting electromagnets such attractive tools: Their systems need far less energy than ordinary electromagnet systems and as a result they can be made much more compact.
For example, to generate a magnetic field of 3 to 4 tesla (about 100,000 times as strong as Earth's magnetic field), an iron-core electromagnet would require some 100 kilowatts of power and its core alone would weigh several hundred tons.
The entire system for a superconducting magnet of similar strength would need at most about 17 kilowatts, largely for keeping the magnets cold.
As for size, a 7.5-tesla superconducting magnet would be on the order of a foot tall, roughly 4 or 5 inches in diameter.
The fact that the current in Onnes's magnet kept flowing after the battery was disconnected leads to one potential use for these devices: as ``storage tanks'' for electricity. A properly designed power plant might use superconducting electromagnets to store electricity during off-peak hours for use during time of high demand.
In fact, Dr. Robert Lloyd, project manager for energy storage research and development at Bechtel Corporation, says that if this technology can be made cost effective, it can, in effect, add about 20 percent to the total US generating capacity.
It's conceivable that electricity released from superconducting magnets will travel from power plants via superconducting power lines. Currently, some 40 percent of the electricity pumped into power lines is lost as heat before it gets to its destination. Holding on to that 40 percent could potentially save billions of dollars.
Other potential uses for superconducting electromagnets become more exotic, including the prospect of using them for power storage and generation in space.