An Interpolation Gas Thermometer (IGT) is defined by the International Temperature Scale of 1990 (ITS-90) as an interpolation instrument in the temperature range from 3 K to 24.6 K. This paper reports the construction and calibration of such an instrument. The cell volume and the dead volume of the IGT are about 433 cm~(3) and 11 cm~(3), respectively. The constant volume cell has 3.6 kg of oxygen-free-high-conductivity copper. Located on both sides at the top and bottom of the cell are four thermometer pockets for installation of a capsule-type thermometer. The temperature of the cell was regulated to ±0.1 mK during measurements. The temperature difference between the upper and lower parts of the cell was measured to be less than 1 mK. The IGT was calibrated at three temperatures using a rhodium-iron thermometer that was calibrated at the National Physical Laboratory (NPL), U. K. The temperature scale of the calibrated Korea Research Institute of Standards and Science (KRISS) IGT was transferred to the rhodium-iron thermometer, and was then compared with the temperature scale of the NPL instrument. The values agreed within 0.6 mK for temperatures between 3 K and 24.6 K.
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