New source of superconductor measurement misinterpretation discovered. (News Briefs).As part of NIST's program to develop standard measurement techniques for superconductors, two NIST (National Institute of Standards & Technology, Washington, DC, www.nist.gov) The standards-defining agency of the U.S. government, formerly the National Bureau of Standards. It is one of three agencies that fall under the Technology Administration (www.technology. scientists have identified and studied a new source of misinterpretation in critical current measurements of superconductors. The critical current is the maximum current a conductor can carry before a quench quench, v to cool a hot object rapidly by plunging it into water or oil. quench to put out, extinguish, or suppress; to cool (as hot metal) by immersing in water. , when it reverts to the normal resistive resistive /re·sis·tive/ (re-zis´tiv) pertaining to or characterized by resistance. state. Researchers can tell when the critical current is reached by measuring the resistive voltage on pairs of voltage taps soldered to the superconductor A material that has little resistance to the flow of electricity. Traditional superconductors operate at absolute zero (-459.67 degrees Fahrenheit or -273.15 degrees Celsius). Experiments in the 1980s raised the temperature to -321 degrees Fahrenheit. wires. However, the NIST scientists discovered that anomalous inductive inductive 1. eliciting a reaction within an organism. 2. inductive heating a form of radiofrequency hyperthermia that selectively heats muscle, blood and proteinaceous tissue, sparing fat and air-containing tissues. voltages can be induced in the loop formed by the voltage taps. The inductive voltages vary systematically with current, current sweep direction (increasing or decreasing), applied magnetic field, and whether the specimen was driven into the normal state in an immediately preceding measurement. Furthermore, the decay time of the inductive voltage signal after ending the current ramp is longer near the onset of the resistive transition. These decay times are even longer during a superconductors first current sweep after a quench. Many superconductor applications now require higher current densities, larger wire diameters, and less copper stabilizer stabilizer: see airplane. , all of which result in marginally stable conductors with high critical currents above 1000 A. Variable induced voltages and long decay times become a concern when currents or current ramp rates are high or when voltage curves need to be extrapolated for measurements on marginally stable conductors. The resulting data can be mistakenly attributed to a bad conductor, a damaged specimen, an electrical ground loop, a low critical current, or specimen motion in the background magnetic field. To avoid anomalous induced voltages, the NIST scientists recommend cycling the current before acquiring data after a quench, avoiding data acquisition while the current is being ramped, and allowing 3 seconds of settling time The introduction to this article provides insufficient context for those unfamiliar with the subject matter. Please help [ improve the introduction] to meet Wikipedia's layout standards. You can discuss the issue on the talk page. after current levels are changed before measurements are made near the critical current. CONTACT: Loren Goodrich, (301) 975-3143; goodrich @boulder.nist.gov. |
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