"A squid turns a change in a magnetic field, which is something very hard to measure, into a change in voltage which is something that is very easy to measure," says Clarke.

For application purposes, squids are almost always coupled to auxiliary components. In the case of an instrument called a magnetometer, for example, the squid is connected to a "flux transformer," a device consisting of a relatively large loop of superconducting material and a much smaller multiturn coil. A flux transformer functions as a sort of "hearing aid" because its large loop picks up a magnetic field over a much greater area than the squid. With the help of a flux transformer, the sensitivity of a squid to changes in the strength of the magnetic field being measured can be boosted a hundredfold.

Another valuable instrument is a squid-based "gradiometer" in which two connected pick-up loops are wound in opposite directions in order to simultaneously measure a magnetic field at two different points in space. Under this arrangement, a flux appears in the squid only if the field is not the same at the two points of measurement, making it sensitive to changes in non-uniform fields.

For nearly 20 years, Clarke, like everyone else, fashioned his squids out of superconductors made from niobium, a metal whose Tc is 9.5 kelvin (-264 degrees Celsius). Chilling any material to such a low temperature requires the use of liquid helium, which is expensive and difficult to transport. For this reason, squids have largely remained an important but somewhat exotic tool of scientific and engineering research.

That may be changing, however, as a result of the discovery in 1986 of a new class of ceramic oxide materials that can sustain superconductivity at much higher temperatures. Some of these new compounds now routinely sustain superconductivity at temperatures well over 100 kelvin (-173 degrees Celsius). Though still quite cold by any conventional understanding of the term, these ceramic oxides are designated high-Tc superconductors to distinguish them from earlier low-Tc superconductors.

The initial excitement over high-Tc superconductors generated by the popular press (readers may recall promises of magnetically levitated high-speed trains and dirt-cheap electrical power) stemmed largely from the fact that these materials can be chilled with liquid nitrogen which is much less expensive and much easier to work with than liquid helium. The cost difference between the two, for example, has been likened to the cost difference between milk and Scotch.

Despite the expectations that high-Tc materials would immediately expand the use of superconducting technology into a bevy of new arenas, this has so far not been the case with a few notable exceptions. One of these notable exceptions is squids. Clarke and members of his research team have been a driving force for the burgeoning application of high-Tc superconductivity to squids, and their rising prominence on the high-technology landscape.

In the spring of 1991, a collaboration involving the Clarke group, and researchers at Conductus, Inc., a company based in Sunnyvale, California, announced a significant technological breakthrough at the annual meeting of the American Physical Society. The collaboration had developed what was then the most sophisticated device ever made out of the new high-Tc superconductors.