100 Ohm transfer standard
A new 100 /spl Omega/ transfer standard has been built and placed in service at VTT Automation. The temperature of the resistor is maintained at 30.72/spl deg/C where the temperature coefficient is zero. The daily variation in temperature is less than /spl plusmn/0.005/spl deg/C. The temperature during transportation is about 0.06/spl deg/C less than the nominal value and it can be hold for about 40 hours. The behavior of the standard was verified in the resistance comparison between VTT (Finland), PTB (Germany), BIPM (International) and BNM-LCIE (France) in November-December 1998. The result indicates that the variation of the standard during the transportation is less than 6.10/sup -9/.
Comparison of four QHR systems within one month using a temperature and pressure stabilized 100-Ω resistor
We have carried out a resistance comparison of four QHR systems using a 100-/spl Omega/ resistor in a temperature-controlled and hermetically sealed enclosure, the comparison was performed in November and December 1998, the participants were Bureau International des Poids et Mesures (BIPM), Bureau National de Metrology-Laboratoire Central des Industries Electroniques (BNM-LCIE) France, Physikalisch-Technische Bundesanstalt (PTB) Germany, and Valtion Teknillinen Tutkimuskeskus (VTT Automation) Finland, acting as a pilot laboratory. The comparison results agree within 3 parts in 10/sup 9/ with a relative combined standard uncertainty of /spl plusmn/1/spl times/10/sup -8/.
An AC-bridge for low-frequency measurements of the quantized Hall resistance
A simple circuit which makes possible the use of a resistance-ratio bridge based on a cryogenic current comparator (CCC) with both AC and DC is described. The different sources of uncertainty associated with the use of AC in a CCC bridge are discussed. It is shown that they should have an effect which does not exceed a few parts in 10/sup 9/ of the resistance-ratio being measured, if the frequency is limited to a few hertz. This analysis is confirmed by experimental results of resistance-ratio measurements between the quantized Hall resistance (QHR) and a 100 Omega resistance standard carried out at DC, 1, 2, and 4 Hz. These measurements are, to the author's knowledge, the first accurate DC measurements of the QHR. They demonstrate that the quantization of the Hall resistance, observed with AC and for the frequency range studied here, remains complete to within a few parts in 10/sup 9/ or better. >