Johnson Noise Thermometry is an electronic approach to measuring temperature. For several years, NIST has been developing a switching-correlator-type Johnson-noise thermometer that uses a quantized voltage noise source as an accurate voltage reference. When this method is used to measure resistors at the triple-point of water, the system creates a direct electronic method for determining the ratio of the Boltzmann constant k to the Planck constant h. In 2010, NIST optimized the JNT system for use with 100 : sense resistors and produced a determination for k with a relative standard uncertainty of 12u10 -6 . In order to further validate and improve the measurement method, we modified the system to operate with a 200 : resistor source instead of the 100 : source. In this paper, we summarize the technical challenges and achievements to date and project what is achievable in the near future.
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