NIST researches improve high-resistance measurement capability. (General Developments).A 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. scientist has developed a new technique that enables more accurate scaling from the primary NIST quantum Hall resistance standard to high resistance levels of 1 M[ohm] and above. The technique exploits a unique property of the quantum Hall effect The quantum Hall effect is a quantum-mechanical version of the Hall effect, observed in two-dimensional electron systems subjected to low temperatures and strong magnetic fields, in which the Hall conductance which allows a quantized quan·tize tr.v. quan·tized, quan·tiz·ing, quan·tiz·es Physics 1. To limit the possible values of (a magnitude or quantity) to a discrete set of values by quantum mechanical rules. 2. Hall resistance standard to serve as a near-ideal two-terminal resistor whose resistance, 12 906.403 5 W, is essentially independent of the resistance of the room-temperature connecting leads. This will allow calibration of NIST 1 M[ohm] working standards with a relative uncertainty of better than 1 X [10.sup.-7], an improvement of about an order of magnitude A change in quantity or volume as measured by the decimal point. For example, from tens to hundreds is one order of magnitude. Tens to thousands is two orders of magnitude; tens to millions is three orders of magnitude, etc. . The new measurement system was recently used to calibrate a set of four 1 M[ohm] resistance standards for an NCSLI NCSLI National Conference of Standards Laboratories International interlaboratory comparison. The relative uncertainty for these calibrations is about 1 X [10.sup.-6], a factor of three improvement over the present NIST calibration service. The property exploited by the new measurement system is unique to the quantum Hall effect and determines exactly how current flows in device leads when more than two connections are made to a single QHE device. The new NIST bridge incorporates a cryogenic current comparator The Cryogenic Current Comparator (CCC) is used in the electrical precision measurements to compare electric currents and/or to determine electric current ratios with highest accuracy. , relies on superconducting leads for many connections, and is compatible with conventional guarding techniques. The ultimate result is a bridge that is significantly less susceptible to leakage errors than the series-parallel resistance networks (Hamon devices) previously used for high-resistance scaling. It may be possible to extend the range of this direct scaling measurement up to 10 [ohm][Image] or even 100 [ohm][Image] by using larger turns-ratio current comparators. CONTACT: Michael Kelley, (301) 975-3722; michael.kelley@nist.gov. |
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