IMPROVEMENTS IN RESISTANCE SCALING AT NIST USING CRYOGENIC CURRENT COMPARATORS . Ronald F. Dziuba and Randolph E. Elmquist Nationallnstitue of Standards and Technology Gaithersburg, Md 20899, USA Abstract ) Cryogenic current comparators (CCC's) are being used at NIST to verify Hamon-type resistance scaling techniques from the 1-0 level to the 100-0, I-len, 6453.20-0, and 10-1enresistance levels. MeasurementS comparing the 100/1 ratio of a CCC to that of a Hamtn transfer standard agree to within 0.01 ppm - the practical limit of accuracy for a Hamon standard. The higher ratio accuracies and higher sensitivjpes of CCC bridges will make it possible to lower the uncertainties associated with resistance scaling at NIST by a factor of two or more. Introduction Since January 1, 1990, the unit of resistance has been based on the quantum Hall effect in which a resistance is related to the von Klitzing constant, h/e2, divided by an integer of the quantum Hall state [I). For precision measurements, the integer is usually chosen to be either 2 or 4 resulting in quantized Hall resistances (QHR's) of 12,906.4 0 or 6,453.20 0 (2). These quantized Hall resistances are used to assign values to standard resistors of nominal decade values, directly or indirectly through some scaling process. Traditionally, this scaling process has been accomplished through the use of series-parallel transfer standards or Hamon boxes (3). In recent years, the application of cryogenic current comparators has resulted in the development of systems characterized by high accuracies and high sensitivities in the measurement of resistance ratios (4). This paper describes resistance scaling techniques at NIST using special Hamon transfer standards and the latest CCC resistance bridges featuring isolated ramping current sources. Resistance Scaling Hamon Standards Several Hamon transfer standards have been built at NIST using series connected card-type Evanohm resistors sealed in aluminum boxes filled with mineral oil. The Hamon standard, designated HQHA, shown in Fig. 1 is used to scale from the quantized Hall resistance of 6453.200 to the 100-0 level. It consists of nine series-connected resistors. The first eight resistors have a nominal value of 800 0 each and the ninth has a value of 53.20 0 to make the total resistance equal to 6453.20O. The total resistance of HQHA is compared to 6453.20-0reference resistors whose values are based on the quantum Hall effect. The eight 800 0 resistors of HQHA can be connected in a paralIel configuration to equal 100 0 and then compared to the series configurationof a 10 x 100 Hamon standard, H10. The 53.20-0 section is compared to H10 using an automatic NIST resistance thermometer bridge. The 53.20-0 measurement is not very critical since it only represents SI:$ 0.82% of the total resistance of HQHA. Hamon H10 in its parallel configuration is then compared to tlie NIST bank of 1-0 working standards to complete this scaling process. IOS Fig. 1 Resistance Scaling from QHR to 1 O. F1gUI'e2 is a block diagram indicating how the resistance scaling at NIST is extended to the 1-1en and 10-1eO resistance levels using Hamon standards. Hamon HUe, a 10 x 1kO transfer standard, is compared in its parallel configuration to the series configuration of Hamon H10. This assigns a value to H1k based on the QHR. Then Hamon H1k can be used in a series-parallel conf1gUI'ation to measure 1-1enstandard resistors, or it can be connected in a series configuration to measure 10-1eO standards. CCC Resistance Bridges At NIST two CCC's are in use for scaling from the 1-0 level to the 100-0, I-leO,6453.20-0,and 10-kOresistance levels. The first CCC was constructed in 1985 and is an overlapped-tube type with a commercial rfSQUID sensor. It contains 12 windings of 1, 1, 1, 1, 2, 4, 8, 16, 17, 32, 32, and 64; where, the unit winding contains 32 turns. Ratios available with this CCC include 1/1, 10/1, 100/1 and 64.532/1. The second CCC was constructed in 1990 and is also an overlapped-tube type; however, it uses a dc QUANTIZED BQRA HALL RESISTANCE H:J 13.!n (IUSU!I) Ie.: 4 II!I(»p B BIOS BI0 BlOoP t NJBT OHM o !I)